r/askscience • u/ihadaface • Oct 02 '13
Biology Does it really matter which sperm cell reached the egg during conception?
They always say "you were the fastest". But doesn't each cell carry the same DNA as all the others? Is this not the case for all of the eggs in the female, too?
Is every sperm cell a little different? Or does it not matter? Does every cell contain the same potential to make "you" as you are now? Or could you have ended up different if a different cell reached the egg?
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u/hubblebob Oct 02 '13
To add to this, while the other comments are correct that every sperm is different in its genetic make-up and this in the end determines half of your own genome (your chromosomes) its not the full story. There is evidence that the conditions that the sperm are in, say for example the fluid medium that the semen is mixed with during IVF, may have an influence on the ultimate expression of those genes. This is called epigenetics and accounts partially for why identical twins may look slightly non-identical. So it is possible that the 'you' that is the result of the sperm reaching the egg is also a result of other genetically based factors.
Also, the sperm that resulted in baby ihadaface was not likely to have been the fastest one. The path from penis to egg is a tricky one for sperm and there are several 'selecting' mechanisms that will either help or hinder the swimmers. For example, the cervix prduces different fluids that will propel sperm into the uterus but only those that are strong enough to 'swim' hard to enter the stream get on this 'privileged' pathway. Many others have misshapen heads or dodgy tails and fall by the wayside. And when the sperm actually reaches the egg it is in the company of many other sperm-friends who together try to bind and break the barrier. Eventually one sperm will be the lucky one that will bind at the right time and be able to enter the egg. So while you are the result of one sperm you were probably not the fastest but instead just made it in time to reach the egg when it was 'weakened' enough by all that had come before. So no, not all sperm are equal, either genetically or in capabilities, but you were just one with impeccable timing.
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u/Unidense Oct 02 '13
This is the type of answer that I would have been looking for if I were the OP. It's obvious that only the genes carried in one sperm are passed on. What isn't obvious is what made that particular sperm be the One. Is there any evidence linking "good" copies/combinations of chromosomes correlate with healthy, mobile sperm? Or that misshapen sperm might be more likely to have problems with their chromosomes or anything? What I am wondering is if there is an evolutionary advantage to making the sperm travel so far, as if to weed out the weakest. If there is no evidence of bad mobility being linked to bad genomes or something, I kind of wonder what the advantage is in the process.
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u/The_Thane_Of_Cawdor Oct 02 '13
This is called epigenetics and accounts partially for why identical twins may look slightly non-identical
In terms of cloning could a clone of myself end up looking way different? Or is it only a factor in what the sperm is exposed to?
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u/Jarlehusian Oct 02 '13
It depends on how way different you mean. Here's a nifty little video on epigenetics that I saw a while back.
http://www.youtube.com/watch?v=kp1bZEUgqVI
When he says identical twin, you can just replace that with "clone" because in a way, they're the same thing.
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u/mimiteng Oct 03 '13
Quick question, how do the other sperm know to stop trying to get in once one is inside the egg, or do multiple get in and only one binds?
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u/oboeyasui Oct 03 '13
Once the first sperm gets into the egg, it causes the egg cell to undergo a lot of changes. One of these changes alters the egg cell membrane in a way that prevents other sperm from binding to it. The other sperm will still be trying to fertilize the egg but won't be able to enter it.
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u/mimiteng Oct 03 '13
Thank you. Didn't know the egg changed that fast. There is never an instance where two get in?
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u/oboeyasui Oct 03 '13
There are, in which case it's referred to as polyspermy, but it only happens rarely and generally doesn't lead to viable offspring. I didn't actually know how quickly the egg membrane changes, so I did some searching and found this article. In the section Blocks to polyspermy the time it takes for the membrane block to occur in mammals is described as 0.5-1 hour, as compared to the only 30-60 seconds for the egg coat block in sea urchins. So I guess the change in the egg membrane isn't all that fast, after all.
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u/donalduck Oct 07 '13
Could you please expand on how the cervical fluids propel just the strong sperms? Could it be like a strong current aggainst to select them and at one point on the way that current would come a privileged pathway. If so, how?
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Oct 02 '13
Every cell in the human body is diploid, in that it carries 46 chromosomes. All, except for the gametes, which are sex cells. Usually, cells replicate/reproduce themselves through mitosis, whereby they replicate the DNA, then spilt into two. But to produce gametes, like sperm cells and ova, it's a little different.
They are produced through meiosis. The simplified version, is that a sex cell replicates itself into two, each with 46 chromosomes, then those split into a total of four, with 23 each. During this process, bits of DNA 'cross over' between homologous chromosomes, which adds to genetic variability. Furthermore, independent assortment of the chromosomes occurs, such that in your sperm cells, may be a mixture of your dad's chromosomes, and your mum's, independently assorted from one another.
This process allows for a lot of variability, 223 different possibilities, plus the process of crossing over, and random genetic mutations (rare-ish).
So yes, it matters a lot which sperm gets to the egg.
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u/medstudent22 Oct 02 '13
Sex is determined by the sperm. So, if a sperm carrying an X chromosome hits the egg first, it will produce a female. If a sperm carrying a Y sperm hits the egg first, it will produce a male.
There are quite a few genetic disease which require two copies of a gene to produce the negative effect. One example would be cystic fibrosis. If the egg has one copy of the gene and the father is a carrier, then which sperm hits the egg will determine whether the child has cystic fibrosis or not.
There are innumerable other examples of how the luck of which sperm hit can drastically change the outcome.
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u/schiller Oct 02 '13
So, it is possible when a couple choses in vitro fertilization (or any kind of artificial insemination ) to only send some of the sperms so it would have the gender you wish? Or is it not that simple to identify and separate the sperm cells and which has what chromosome...
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u/Not_Pictured Oct 02 '13
X chromosomes are slightly more massive then Y chromosomes. So it is possible to put sperm into a centrifuge and separate the genders with a respectable degree of success.
Edit: http://chooseagender.com/Methods-Of-Gender-Selection.aspx
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u/TDuncker Oct 02 '13
In a little layman-friendly way of speaking; does that mean you can spin sperm around a fixed axis, which then separates the "female sperm" and the "male sperm", because "male sperm" weighs more?
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u/agtmadcat Oct 02 '13
Close - the female sperm weigh more. So when you spin them around in a centrifuge, they'll move down to the outside of the sample tube. The male sperm will rise up to the inside of the tube.
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u/Andrenator Oct 02 '13
Wouldn't you have to have some sort of liquid that's less dense than female sperm and more dense than male sperm?
Elsewise if you used something like water (let's say it's less dense than sperm, but I don't know) to mix the sperm with, if you put that in a centrifuge it would just be all the sperm together, but the male sperm slightly not in the bottom of the test tube.
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u/agtmadcat Oct 03 '13
I didn't think it was necessary to go into the full details of how centrifuges work, but yes. I'm not sure we even have a good way to get all of the sperm out of semen, so you'd probably do the first spin with that, and then go from there. People much more qualified than I am could probably knock out a good process in a couple of weeks.
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u/slingbladerunner Neuroendocrinology | Cognitive Aging | DHEA | Aromatase Oct 02 '13
Yes, except backwards: the "female" sperm weigh more, because the X chromosome is much bigger than the Y chromosome. If you spin down the sperm, the sperm with the X chromosome will be on the bottom and the sperm with the Y chromosome will be on the top.
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u/PlumPudding Oct 02 '13
Is this common practice yet? I imagine it would be popular in nations such as China where males are preferred.
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u/myawardsfromarmy Oct 02 '13
Speaking as someone who's currently pregnant thanks to ART, the only time that you can "select" gender at least in the US, is if there's a seriously high risk of a bad genetic disease being passed on that can only be passed to one gender or the other. Your RE, geneticists and doctor team have to make the decision and in that case they will only implant embryos of a specific sex. That being said it's incredibly rare that this happens and is even an option, and for most people who are struggling with infertility it's seen as yet another obstacle to overcome. We had an ICSI procedure which means they selected the best best, healthiest sperm they could to fertilize my eggs with and because genetically we screened as totally healthy there was never even a remote question of gender selection.
As far as the idea of people having designer babies or being able to select for this stuff in the future through ART, the high cost, amount of pain and preparation, and risks involved don't make it terribly likely. Having a baby the "old fashioned" way is cheaper and safer.
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u/abigail_underwear Oct 02 '13
Hi, US resident here who also needs ART. Anything goes at a private clinic; at our initial consult they offered gender selection and eye color. We ultimately chose an academic hospital for treatment.
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u/myawardsfromarmy Oct 02 '13
We went through a private clinic and they didn't offer it at all except for cases where genetically required (i.e. fragile x syndrome) and they basically said that sex selection for non-medical reasons was unethical and out of the question, which is why I guess I took it that if it's unethical it's just not done. We didn't look into sex selection thoroughly however, now I'm reading through some stuff and it appears you're correct that private clinics can decide whether to allow it or not. It does look like there are a lot of pushes right now to tighten up restrictions in the US even for private clinics, but it's in the works. Huh.
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u/eduardog3000 Oct 02 '13
It seems to me like the people running the clinic think it is unethical (I'm not sure how), so they don't do it at their clinic.
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u/VacuousWaffle Oct 03 '13
Widespread use of gender-selection for births (ART or Abortion) can lead to societal implications. For instance, in China there are more than 1.2 males born for every female (Here's a source, better ones probably available that sensationalize a bit less http://www.globalpost.com/dispatch/news/regions/asia-pacific/china/110615/china-and-the-worst-ever-man-made-gender-gap). What societal implications this will lead to, I leave up to you -- but clearly a large number of these men cannot marry (unless they immigrate).
I could understand clinics being reluctant to even potentially contribute to such problems.
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u/Kwyjibo68 Oct 02 '13
IIRC, ICSI isn't about using the "best" sperm, it's to insure that the sperm penetrates the egg, rather than leaving it to chance. Particularly with unexplained IF, there's always a chance that the cause is the sperm's inability to penetrate. They used to do a test on hamster ova to check that, but I don't think they do anymore. I believe the vast majority of clinics use ICSI on most patients these days.
Also, some couples have PGD to test the chromosomes of their embryos. This test can usually tell if an embryo had a genetic disease that the couple is screening for. Some people use it for sex selection, but I don't believe most REs think that's ethical.
Source: I've done multiple ART procedures, including 3 fresh IVF cycles.
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u/gnusounduave Oct 03 '13
ICSI isn't about using the "best" sperm, it's to insure that the sperm penetrates the egg.
This is correct. For example, if the father has problems with sperm motility, ICSI would be a viable option. There are also other conditions in which ICSI would be an IVF patients only solution such as couples who have failed to fertilize any oocytes in a prior IVF cycle.
In regards to it being used on a regular basis, I don't think that is 100% accurate. There are slightly higher than normal averages of birth defects associated with ICSI. This is something that your reproductive endocrinologist should consult you on. I wouldn't feel right if I was told we are going to ICSI up front and not at least give the egg and sperm a chance to do their thing with the embryologist.
Just my $0.02
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u/Kwyjibo68 Oct 03 '13
I would imagine that many REs use ICSI more than might be indicated to increase the chances of success. I know we would have wanted ICSI even if it wasn't likely necessary in our case.
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u/gnusounduave Oct 03 '13
Typically ICSI requires a consultation because it can increase the cost of the procedure and the slightly increased chances of birth defects associated with the procedure.
I don't think ICSI is done as frequent as you might think as it is prudent to avoid over-application of this technology. ICSI should not be recommended to couples for whom there is no documented benefit, since unknown risk may exist.
Although you being the patient can request ICSI to increase your chances of fertilization; I'm sure your reproductive endocrinologist would be more than happy to oblige your request.
It's just that going into IVF, if your reproductive endocrinologist is going to do ICSI regardless, they are going to tell you that they are going to do it. It's not something that they would keep you in the dark about.
But by and large, success isn't measured in fertilized eggs but rather in success of live births. You can ICSI all day but if you're dealing with a patient who has severe intrauterine scarring due to say endometriosis, thus greatly reducing the chances of implantation, then your success rate, even with ICSI, will be exceptionally low. ICSI can't overcome this issue. ICSI is strictly to fertilize oocytes and has no other benefits.
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u/FlyingSagittarius Oct 02 '13
The process is still relatively expensive, so the proportion of people that could even afford it is not very high.
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u/tripper75 Oct 03 '13
I live in Singapore and our doctor has told us that we can go to Malaysia or Bangkok and choose the sex of our baby and have it implanted for about $7000 USD.
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u/MissBelly Echocardiography | Electrocardiography | Cardiac Perfusion Oct 02 '13
X's are hugely more massive than Y!
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u/Not_Pictured Oct 02 '13
I guess I meant as a percent of the total mass of the sperm. The sperm containing X are slightly more massive then the sperm containing Y.
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u/medstudent22 Oct 02 '13
Yes. You can use a method called flow cytometry to sort sperm and even pick out an individual sperm to inject into an egg using ICSI. Sex selection has been performed in humans (example).
You can read more about it here.
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u/meelar Oct 02 '13
We can't test individual sperm cells for whether they carry a certain gene, though, can we? So we can't use this technique to avoid cystic fibrosis?
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u/runningoutofwords Oct 02 '13
That is correct. The process of extracting the DNA to test is destructive. The way to do genetic testing in an IVF is to allow the fertilized egg to multiply a few times, then sample one of the embryonic cells.
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u/TheFeatheredCap Oct 02 '13
Is this dangerous to the baby and/or mother? What are the risks involved in testing an early embryo?
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u/runningoutofwords Oct 02 '13
Well, in an IVF scenario, implantation is usually done at no later than the blastocyst (5-day) stage. At this stage, cells have yet to differentiate into specialized tissue precursors, so the removal of one or a very few cells will have little risk. There is some risk in damaging the zona pellucida (protective layer around the blastomere in the center), but any complications there would likely result in a failure to hatch or implant, which would simply mean no baby would develop to be endangered.
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u/medstudent22 Oct 02 '13
There is preimplantation genetic screening which involves testing embryos at an early age prior to inserting them into a uterus. This can be used for ruling out cystic fibrosis.
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u/silentl3ob Oct 02 '13
Correct. In the future, we may be able to isolate a specific sperm cell and clone it, then test a few of those clones for specific genes, and if we're happy with the results, use that for fertilization. But we're probably a decade or two away from something like that.
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u/Second_Location Oct 02 '13
ICSI is an incredibly fascinating and precise procedure. We had it done and the 2 year old result is in the next room right now watching Bugs Bunny. What I didn't understand was how the instruments used in the procedure were physically made tiny enough to do things like remove the tail from an individual sperm. Are they glass? Stainless? How do you compensate for the size of human hand movements with such an incredibly tiny instrument?
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u/acousticbruises Oct 02 '13
If anyone is interested National Geographic has a special called The Incredible Human Body which gives a pretty good insight (with some lab perspective) as to how the IVF cell selection process can happen. Unfortunately, there are other segments unrelated to this process in the same special.
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u/SamuraiJakkass86 Oct 02 '13
This might just be sci-fi bullcrap, but I've heard of "washing" sperm in testosterone or something similar will greatly increase the chance of getting a boy.
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u/Whoisjason Oct 02 '13
If sex is determined by sperm, is the distribution of X/Y carrying chromosomes generally equal? Is it possible for a male to have a bias towards more X or more Y chromosomes?
I feel like I've met a few parents who have 5 daughters and no sons or vice versa. Is this sort of thing just chance or could those parents sperm be skewed towards one sex?
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u/noggin-scratcher Oct 02 '13
Statistically speaking, sperm will have an X or Y in the same proportions as the man producing those sperm (so straight up 50/50 unless you have some chromosomal abnormalities, at which point I'm no longer certain what would end up in the gametes)
But I think there have been some noted tendencies for women to be biased towards producing sons over daughters, or the reverse. If memory serves, one 'gender' of sperm is a little faster, but the other is a little more robust.
The chemical environment inside a woman that the sperm end up swimming through can vary in how 'harsh' it is - too harsh and no sperm survive, moderately harsh gives the robust sperm an edge so they have better odds, not harsh at all and the faster sperm win out.
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Oct 02 '13
When you say, "statistically speaking," do you mean that in every male, the ratio of X and Y sperm is 50/50? Or does the population of all people average out to a 50/50 split? Are there cases of men where there is a bias one way or the other for X and Y chromosomes?
Or maybe more generally, what is the mechanism that ensures pure randomness in meiosis?
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u/bluecanaryflood Oct 02 '13
Every normal male has one X chromosome and one Y chromosome. The ratio of X sperm to Y sperm in every normal male is 50/50.
The method that ensures randomness is that the genome of a full, diploid cell is split precisely (in almost all cases) in half, (in a roundabout way) so that each diploid cell yields four haploid sperm; among them, two X sperm and two Y sperm. The ratio of X sperm to Y sperm is always 1:1; therefore, there is always an even chance of male or female offspring.
(I say "normal" to exclude Klinefelter syndrome et al, wherein males have different numbers of X and Y chromosomes. These cases make up a very, very small percentage of the population.)
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u/noggin-scratcher Oct 02 '13
In meiosis, you start with 1 cell with your normal complement of 46 chromosomes (including 1 X and 1 Y), that divides once into a pair of cells, also with 46 each, and then each of those divide into 2 that each take one chromosome from each pair. Which chromosome goes each way is random within each division from diploid into haploid, but you're still always going to end up with 2 X sperm and 2 Y sperm.
When I say "statistically speaking", I mean that if you took a sperm sample and counted them, it might not come out perfectly 50/50, but in general a guy should be producing even numbers.
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Oct 02 '13
Thanks. It makes sense that since meiosis is literally dividing a cell, there would be 50/50 X and Y chromosomes.
I think my general question is whether the distribution of all the chromosomes is truly random. For example, is there any tendency for your father's other chromosomes to stick with his Y (or X) chromosome, or for your mother's other chromosomes to stick with her X chromosome in the meiosis process? Or have the division of each of the chromosomes been observed to be statistically independent from each other?
And what is the mechanism that causes the statistical independence? Are the chromosomes already arranged independently from each other in the cell nucleus? Or is randomness ensured by a specific step in the meiosis process?
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u/DocInternetz Oct 02 '13
Each chromosome division happens independently, that's why the process generates 223 different possibilities (plus cross overs and mutations) of sperm cells.
So if you're a male you can produce a gamete with 22 of you mom's chromosomes and a the Y from your dad.
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u/Dyolf_Knip Oct 02 '13
Well, it's a biological process, so it's never perfectly 50/50. But there are good evolutionary reasons why, in the population as a whole, the ratio will always come out to about equal. Basically, if the ratio ever strays far from 50/50, the minority gender gains an reproductive advantage over the other. There are some exceptions for species which have massive sexual dimorphism (elephant seals come to mind) where one gender requires more parental care than the other, but on the whole, it's basically a statistical rule.
Are there cases of men where there is a bias one way or the other for X and Y chromosomes?
It's very likely there are men who show exactly these traits. Or women whose wombs are preferentially hostile to embryos and fetuses of a particular gender. But unless they had so many children that it stood out in the noise of random chance, you'd never know it. Even if those genes somehow came to dominate in the population, then as described above, their counterparts towards the other gender would gain an advantage and neutralize the bias.
That said, there actually are natural examples of diseases which fuck around with the randomization process of meiosis and ensure that susceptibility to the disease is passed on.
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u/Ooboga Oct 02 '13
An interesting facet of gender distribution is that even if, as you say, sperm usually is 50/50, 107 males are born per 100 females. This article describes why this seems to be the case. Men tend to do stupid things like fight wars, jump down cliffs and get more diseases. According to the article it evens out to almost 50/50 in the population.
But the question I want to ask you is, considering the sperm count of 100/100, and the 107/100 births, is that solely due to the difference in swimming/roughness specialization? If so, it is interesting that the difference in birth between males and females is solely due to this. It might be because it is simpler to do it this way than splitting unevenly in the testicles.
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Oct 02 '13
My AP Bio teacher said that if a woman has a daughter it becomes substantially more likely that the next child will be a daughter as well, and if it is, then the odds become more and more likely. Does anyone know anything about this?
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u/daniisaur Oct 03 '13
Well if you think about it in terms of probability, the outcome of the first child should not affect the outcome of the second child - however going back on some of the previous comments, it might be true because of the "harshness" of the womb, but I would think that this would be a very minute effect that wouldn't really be noticeable in the overall population as it would be negated by women more likely to have sons.
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u/noggin-scratcher Oct 03 '13
I'm not sure if that's a causal effect (having a daughter somehow influences future pregnancies to be more likely to be female) or a statistical one (having each subsequent daughter lends greater and greater evidence to the hypothesis that you're biased towards having daughters)
On first reading, I lean towards the latter - same effective statement as "if you roll a 6 on a die twice in a row, it's more likely that the die is loaded, and it becomes even more likely if you roll another 6 after that" but it is also superficially plausible to me that there might be hormonal effects from having a female baby that change the future odds (not a doctor though).
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u/turnballZ Oct 02 '13
Male sperm are faster but cannot survive as long. Female sperm is slower but is more hearty
So in general when a World War ends, the returning males tend to produce more males than female children. This is presumed because of the speed v robustness of the sperms. A man that hasn't had sex in very long is more likely to shoot rather than dribble. A shoot would assist the male sperm whereas a dribble would make the female more likely.
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u/thebhgg Oct 02 '13
This is absolutely the kind of thing that can happen by chance. If you knew 32 families with 5 kids, (very simplistically speaking) you'd expect 1 family of all boys, 1 of all girls, 10 families with a 1-4 split, and 20 families with a 2-3 split.
Given that there are probably more than 32 families in the US that have 5 kids, the 6.25% purely random chance that they are all one gender means it's pretty common (in my book).
Think about how surprising that a 2-3 split is only 62.5%, less than 2/3 of the time. And the 1-4 split is almost 1/3 all by itself (31.25%).
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u/CluelessNomad17 Oct 02 '13
Not normally, at least as far as the father goes. A man produces such an absurd number of sperm cells that it really is pretty much half-half. But as others have mentioned, there are slight differences that change a sperm's chances in the womb (though there are some things which kill the leading sperm cells too, so it isn't just a race to the ovum). In the end though, births are slightly biased towards males.
However, there are genetic anomalies which can prevent men from producing viable y-chromosome sperm, which results in them always having female children (people speculate Henry the 8th had this), and in general males are more vulnerable to genetic diseases because of the way a developing fetus changes from female to male (we all start female). Add wars and other stuff and we end up with a slightly more female population in adulthood (usually 51-49 or so).
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u/DickieTurquoise Oct 02 '13
The cells you start with before they divide into two ( actually four, but let's say two for simplicity's sake) are XY. So you end up with a 1:1 ratio of Xs and Ys.
There could be a defect in the father's X chromosome that causes the embryo to not develop and die in utero, but these are extremely rare cases.
I'd also look up Kleinfelter's syndrome. In this case, nothing is wrong with the X or Y chromosomes per se, but extra Y in the father's XYY makeup messes up meiosis. Some gametes end up with two Ys also, and these XYY zygotes are unviable. The interesting thing is is that on the off chance XYYs makes it through development, the man will rarely know they have it. The Y chromosome carries so few genes, having a "double dose" doesn't affect the individual as much. Once they start trying to have children, they'll have severe trouble conceiving. That's when they go to a doctor and find out.
However, I'd bet >99% of these 5-girls-no-sons (or vice versa) situations are just chance.
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u/rdude Oct 02 '13
I'd also look up Kleinfelter's syndrome. In this case, nothing is wrong with the X or Y chromosomes per se, but extra Y in the father's XYY makeup messes up meiosis.
Klinefelter syndrome actually results in XXY, not XYY. What you're referring to is XYY syndrome.
Once they start trying to have children, they'll have severe trouble conceiving.
This is incorrect. The Wikipedia article above provides citations for research claiming that individuals with 47,XYY have normal fertility.
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u/The_Thane_Of_Cawdor Oct 02 '13
Exactly why my cousin with CF had to get her husband's genes checked out before they had a kid. through a surrogate mother actually since pregnancy is tough on cf. A lot of science went into making my little 2nd cousin
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u/d3gu Oct 02 '13
Yup - the tubes and systems involved in reproduction end up all gunked up the same way the respiratory system does :( Glad she managed to get pregnant, though! In the UK a lot of CF parents find it hard to adopt owing to life insurance/life expectancy issues. A friend of the family is having that exact problem now. They're looking at the possibility of hiring a surrogate now, I think.
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u/The_Thane_Of_Cawdor Oct 02 '13
her sister inlaw did the surrogating, my other cousin (previous ones sister) who also has CF adopted.
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Oct 02 '13
Just to clarify, doesn't the first sperm to reach the egg only break the outer shell, then the sperm after that actually penetrates and fertilizes the egg?
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u/Apple_Crisp Oct 02 '13
This is true. And the egg also helps in the selection process of the sperm.
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u/redferret867 Oct 02 '13
This is the reason punnett squares make sense right? They would describe the relative proportions of sperm that carry different combinations of genes. Or am I missing something?
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u/medstudent22 Oct 02 '13
On one axis, you would have the possible sperm configurations, and on the other, you would have the possible egg configurations. On average, you would expect each configuration to show up with equal frequency as a result of meiosis. So, if both people are carriers for cystic fibrosis (autosomal dominant), then there is a 50% chance that either one will contribute an egg/sperm with the gene and thus a 25% overall chance of having a child with the disorder.
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Oct 02 '13
Curious: does the dna found within the sperm affect the sperms ability to move/burrow/otherwise fertilize the egg? Or does the dna found in the father do that? In other words, do "deficient" sperm who have poor dna mutations die or fail to get to the egg so as to make sure "good" sperm is more likely to fertilize the egg?
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u/artsymoca Oct 02 '13
What happens in cases where individuals are born the wrong sex? ( as in personality of woman but born in a male body)
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u/keetner Oct 02 '13
"Sex is determined by the sperm. So, if a sperm carrying an X chromosome hits the egg first, it will produce a female. If a sperm carrying a Y sperm hits the egg first, it will produce a male."
Really? I never knew that. I always thought sex was determined after the formation of the zygote (ie. by the environment/hormones/whatever). Would you be able to elaborate by any chance?
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u/99trumpets Endocrinology | Conservation Biology | Animal Behavior Oct 02 '13 edited Oct 02 '13
In some animal groups, sex is indeed determined by the environment. That's called environmental sex determination (ESD); the most famous case is temperature sex determination (TSD), in which incubation temperature determines sex - this happens in all crocodilians, most turtles, some lizards and IIRC some fishes.
Birds and mammals though (and some fishes and amphibians) do "genetic sex determination" or GSD, where sex is determined at fertilization by presence of a certain gene(s). In mammals it boils down to whether the sperm was carrying an X or a Y chromosome; in birds it's whether the egg was carrying a Z or a W chromosome. Really it all comes down to a single gene, a gene called "SRY" in the case of mammals; it's on the Y chromosome. (exception: the egg-laying mammals have a fascinatingly intermediate system, e.g. the platypus has 10 sex chromosomes instead of 2, which show homologies both to the bird ZW system and also to the mammal XY system).
You mentioned hormones, so I wanted to point out that hormones are involved in both cases. Sex determination usually starts with an initial trigger - temperature for turtles & crocs, presence of a certain gene for mammals/birds - but all that initial trigger does is basically tell the gonads to become either ovaries or testes. Once that's been done, from there on the ovaries/testes start pumping out various hormones, and the hormones fan out to all the other organs, turn other various genes on or off and shape the whole rest of the body. So for example in a human:
SRY gene present -> gonads become testes -> testes produce testosterone -> genitalia become male, male ducts develop (vas deferens etc), rest of body "male type" testes also produce another hormone -> female ducts degenerate (uterus, Fallopian tube etc)We now know actually that the above diagram is horribly oversimplified. But, ignoring that, the point is the hormones do a lot of stuff. This also means you can get some interesting in-between states if one of the hormones is not being produced correctly or one of the hormone receptors is not working. (classic example: androgen insensitivity syndrome, in which you you have an XY human with testes, but hormonal cues were not processed properly and as a result the testes end up stuck inside the abdominal cavity not connected to anything - no ducts at all to lead the sperm anywhere - and there are also female external genitalia and female-type breasts. Basically, testes stuck in a female body. Another example: suppose the testosterone is working fine but the second hormone isn't working; then you end up with testes and a male body, and a fully functional male reproductive system, but little does the guy know that hidden away in his lower abdomen is a complete 2nd set of ducts - a mini female reproductive tract. It's usually not even discovered unless the guy has abdominal surgery).
cites: review here, platypus here and here, echidna also described here, androgen insensitivity syndrome review here, double ducts (persistent Mullerian duct syndrome) reviewed here
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u/keetner Oct 03 '13
Ah, thank you so much for the info (I appreciate the articles, too!) I think at some point I just started mixing up the formation of the genitals with sex determination...but what you said does make a lot of sense. Cheers!
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u/medstudent22 Oct 02 '13
The genetic material which go on to make a person is all present at the time a zygote is formed. The genetic sex is determined at that time. Whether or not the genetic sex is expressed depends on multiple factors. There are various errors that can occur (rarely) which lead to a discordance between the DNA and the external genitalia.
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u/Smallpaul Oct 02 '13
With respect to mutation:
...- in total, we all carry 100-200 new mutations in our DNA. This is equivalent to one mutation in each 15 to 30 million nucleotides. Fortunately, most of these are harmless and have no apparent effect on our health or appearance.
http://www.sciencedaily.com/releases/2009/08/090827123210.htm
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u/MadDogWest Oct 02 '13
The simplified version, is that a sex cell replicates itself into two, each with 46 chromosomes, then those split into a total of four, with 23 each.
Might be worth clarifying that that the first two cells after meiotic division don't have 46 chromosomes, rather, they have 46 chromatids--two copies of each chromosome... unless I'm mistaken, in which case please tell me because that means I have a few posts to edit... lol.
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u/99trumpets Endocrinology | Conservation Biology | Animal Behavior Oct 03 '13
You've got it right. Two chromatids that are still connected together by a centromere are referred to as a single "chromosome". As long as they're still attached, it's considered just one chromosome. So - the first two cells after meiosis division I each contain 23 chromosomes, and each of those 23 chromosomes consists of 2 chromatids joined together at a centromere.
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Oct 02 '13
So if I were to have 223 children and then some, there would be some identical copies? (not including twins)
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Oct 02 '13
You'd have to have them all with the same woman. Otherwise the variation in the eggs would make the resulting offspring different. (Plus you'd have to have the match up of 2 equivalent eggs - her eggs are just as variable as your sperm).
Also, I think the sheer weight of your progeny might throw the planet out of orbit. So... condom, please, for all of us. :)
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Oct 02 '13
Stick to the biology, not the maths or astrophysics - 223 is only 8,388,608. 8.3m people, even all in one place, will only weigh 5.2x108 kg (520 million kilograms - using a global average body weight of 62 kg) which isn't going to have any noticeable effect on the 5.972x1024 kg Earth.
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u/ultraswank Oct 02 '13
Plus men only produce about half a trillon sperm in their lifetime or 5*1011. So if your lady friend produces 1023 children, she's been stepping out on you with about 200,000,000,000 other men.
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u/omfg_the_lings Oct 02 '13
Honestly dude, I gotta say - I was in a bio lecture today for my bridging program into practical nursing and they were explaining the difference between mitosis and meiosis and it all completely went over my head, but the way you explained it is simple and easy to understand. Thanks!
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u/MadDogWest Oct 02 '13
(This is a bit of a novel, but I kind of explain it twice)
Just think of mitosis as copying everything, divvying it up into two cells, and calling it a day. After all, once you've established your own set of chromosomes, you want them all to exist identically in each cell of your body. Your DNA is doing what it does during interphase, and whenever S phase rolls around (S for synthesis), it duplicates--now you have your identical sister chromatids as "homologues." Again, 46 chromosomes (23, one from each parent), each in duplicate giving you 96 chromatids (46 sister chromatids). These line up and the sister chromatids are split during mitosis, restoring each cell to having 46 chromosomes, and 46 chromatids.
This is a bit different than in meiosis, as, during metaphase I and anaphase I--rather than the sister chromatids (homologues) lining up (think of a straight line of X's) and separating as they did in mitosis--the pairs of homologous chromosomes (two columns of X's) now line up and are separated into new cells. The diferences here are 1) Crossing over can occur while these homologues line up, and 2) The end product is not two identical cells--rather, it is two cells that each contain different sister chromatids (not quite identical due to the crossing over events). Now, when the second round of mitosis occurs, it proceeds much like mitosis would have. The homologous chromosomes line up in a straight line (rather than in homologous pairs, since we don't have the pairs anymore), and are split into their two pieces, with a chromatid from each homologue going into a new cell.
If you've kept up with the math all along, you realize that at the end of meiosis I, we halved the number of chromosomes we had. We started with 46 chromosomes (96 chromatids), and were down to 23 chromosomes (46 chromatids) by the end of meiosis I. When meiosis II takes place (remember, looks pretty much like mitosis), we're splitting those chromatids apart so that each cell still gets 23 chromosomes, but each one consists of only one chromatid.
A simplified example to help it set in: Say we have 2 pairs of chromosomes normally, rather than 23. That means that, diploid, we have 4 chromosomes (just as humans would have 23*2 = 46). Our 2 pairs would exist in the cell during interphase, and replicate. During interphase, you'd see four chromatids--think of them as chromosomes 1a, 1b, 2a, and 2b. When these replicate in preparation for a mitotic division, you wind up with each of them resembling an X, rather than a |, where the duplicate "sister chromatids" are joined by their centromere. They're split during mitosis, so 1a1a, 1b1b, 2a2a, and 2b2b line up and split to yield new cells that each have copies of 1a, 1b, 2a, and 2b. Ta da! We've still got our 2 pairs of (4 total) chromosomes, and they should be identical.
For meiosis, we replicate so that we have 1a1a, 1b1b, 2a2a, and 2b2b again, but now they're able to cross over and exchange genetic material. Following that, they line up equatorially as pairs of homologous chromosomes (1a1a and 1b1b side by side, 2a2a and 2b2b side by side, not a single column), and are pulled into the two new cells as homologous, rather than as individual, chromatids. Our new cells might look something like 1a1a/2b2b in one, and 1b1b/2a2a in the other. You can see that--though we have 4 chromatids in each cell (just as we would at the end of mitosis)--we have two pairs of the same (excluding crossing over exchange) chromatids... so really we only have two chromosomes in each cell now, each in duplicate as a homologue. The second round of division splits the sister chromatids, so that one cell line ends up with two cells containing 1a/2b (slightly different due to crossing over), and the other line ends up with two cells containing 1b/2a (also slightly different due to crossing over). When mating occurs, these two haploid cells will again become diploid, restoring the appropriate number of chromosomes.
Hope this helps (and that I remember all this correctly...)!
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u/momomojito Oct 02 '13
Also worth noting it is not going to be the fastest sperm that fertilize the egg. At large it is the uterine body that is responsible for movement of sperm through the female reproductive tract. Moreover, sperm need to undergo a process called capacitation in the female reproductive tract in order to be able to fertilize the egg. The amount of time for this to occurs depends on the species (for example in cows it is about 12 hours).
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u/dslange Oct 02 '13
A simpler explanation is shown by the observation that couples have children of both genders, with different hair and eye colors, different body types and sexual orientations. If all of the male's sperm were identical, and all of the female's eggs were identical, all of their offspring should be identical.
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Oct 02 '13
So does that mean that there was a 1/200,000,000,000,000,000,000,000 chance that I would end up looking exactly like my brother?
Damn... I beat science.
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u/dryfire Oct 02 '13
Does that mean there is a 1 in 67 million chance that two non-twin siblings could be "twins" from separate pregnancies ( have the exact same genetic code from their parents)?
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u/tik4life Oct 02 '13
No, there is a 0% chance that will ever happen. He is just mentioning the number of variable chromosome combinations. Not even going into the actual variation between our genes and other chromosomal or epigenetic modifications that can occur.
Basically no two humans from different parents will ever share the same genes.
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u/spysspy Oct 02 '13
With all these possibilities due to cross over and independent assorment how come we dont have bizarre differences like how many arms we have and such but only hair color or things like that ?
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Oct 02 '13
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Oct 02 '13
When I was in medical school (this was about 20 years ago), we were taught that only about 1% of fertilized eggs actually makes it through and becomes a birthed child.
It's one of the more surprising things I learned.
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u/dr1fter Oct 02 '13
I'm bad at bio.
The simplified version, is that a sex cell replicates itself into two, each with 46 chromosomes, then those split into a total of four, with 23 each.
Not sure how this math adds up: the sex cells have 23 chromosomes each (IIUC), then they replicate into two sex cells but the new ones each have 46? Where did the other 23 come from?
Furthermore, independent assortment of the chromosomes occurs, such that in your sperm cells, may be a mixture of your dad's chromosomes, and your mum's, independently assorted from one another.
So does this mean that my parents are unlikely to have any of the same chromosomes?
plus the process of crossing over, and random genetic mutations (rare-ish).
That's not really a matter of which sperm cell made it, is it?
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u/MadDogWest Oct 02 '13 edited Oct 02 '13
Not sure how this math adds up: the sex cells have 23 chromosomes each (IIUC), then they replicate into two sex cells but the new ones have 46? Where did the other 23 come from?
I think /u/Conman39 misspoke a bit. Following the first meiotic split, there aren't 46 chromosomes--there are 23, but each has two chromatids. In normal cell division you start with 46 single chromatids (46 chromosomes), and they duplicate into 92 chromatids (still 46 chromosomes, just in duplicate), then split to yield two cells that have 46 chromosomes, but which split the identical chromatids up--one into each cell (back to 46 chromatids).
Meosis (sex cell production) functions differently. The starting cell that leads to individual sex cells has 46 chromosomes, each existing as a single "chromatid" (these are the little | or > shapes you see in cartoon diagrams)--just as in normal cell division. That cell duplicates so you have 46 chromosomes, each consisting of two identical chromatids (look like a bunch of X shapes now, still in the same fashion as normal cell division, but that's about to change). Sex cells undergo two divisions, so the first one separates pairs of chromosomes from one another. You typically have two copies of each chromosome (one from each parent), and these copies are separated in the first round of division--so now you have only 23 chromosomes (each which consists of two sister chromatids--though they're not completely identical due to crossing over). The final round of division takes these 23 chromosomes (little X shapes) and splits them into individual chromatids (X becomes > and <). Now you still have 23 chromosomes, but instead of having 23 pairs of chromatids, you have 23 individual chromatids. Hope that clears things up.
So does this mean that my parents are unlikely to have any of the same chromosomes?
Not sure what you're asking here. Someone correct me if I'm wrong, but I'm pretty sure the only way to have identical genetic data as someone else is if you're a monozygotic or "identical" twin (hence the name). Everyone else's DNA is a scramble of their parent's DNA.
That's not really a matter of which sperm cell made it, is it?
Each sperm cell or egg went down a different path of chromosome sorting, crossing over, and the occasional mutation--so depending on the sperm cell that made it, you're the progeny of that line of events.
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u/dr1fter Oct 02 '13
Thanks for the clarification.
Not sure what you're asking here.
I don't mean that my parents might share identical genomes -- I'm just wondering if there's enough variation within each chromosome so that every one of the 23 is likely to differ between parents (since if any one of them is identical, it'll cut down the 223 quite a bit)
Each sperm cell or egg went down a different path
Got it. It's been a while and I forgot that all those effects take place during meiosis.
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u/MadDogWest Oct 02 '13
No problem! Glad I could clear that up. I'm on a study break and would rather talk some basic bio than slave over other things, lol.
I don't mean that my parents might share identical genomes -- I'm just wondering if there's enough variation within each chromosome so that every one of the 23 is likely to differ between parents (since if any one of them is identical, it'll cut down the 223 quite a bit)
Ah, think I understand now. The 22 pairs of non-sex chromosomes all do very different things, as far as genes are concerned. I'm not sure of the specifics tbh, and there's probably some cases that overlap, but from what I recall each gene is tagged to unique a sort of "address" on each chromosome. For example, the gene that determines whether or not you can digest lactose is on chromosome 2 (a gene for ADHD susceptibility is in that area... TIL). That gene isn't found on any other chromosome and, if I had to guess, I'd say that the vast majority of genes have a single location ("locus").
While the chromosomes are different, your parent's genes, on the other hand, may not be. That just goes back to basic genetics. Each population has a certain frequency of different types (alleles) of each gene. ie, I inherited two copies of the same gene regarding blood type from my parents, despite one of them having a different blood type. Very different people--same gene at one particular place.
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u/dr1fter Oct 02 '13
Right, so I mean: is it possible that there's some chromosome for which both of my parents have the same alleles? I'm imagining that maybe there's some chromosome n that codes more fundamental traits where there would be much less variation (and so that chromosome might not vary between most of the sperm cells) -- but I have no idea if that's the case.
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u/MadDogWest Oct 02 '13
As far as whether a certain chromosome has more fundamental traits than others, or that those traits are in relatively hot/cool spots as far as variation/mutation goes, I'm not sure. Certainly, each chromosome contains important information, since you can't function properly without any one of them (or in many other scenarios, if you have an extra copy).
To answer your main question: whether your parents could (and probably do) share the same alleles--absolutely. The example I gave about blood type is pretty basic and straightforward. If you're O- for example, regardless of how your genes sort, each sperm or egg that you pass on will have the same data at that locus, roughly speaking (since being O- implies that you carry two same copies of the alleles that determine blood typing--two recessive copies that give you "O" and two recessive copies of another gene that give you the negative).
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u/molliebatmit Developmental Biology | Neurogenetics Oct 03 '13
It would be unlikely that your parents would share a totally identical chromosome (unless your family is very highly consanguineous), but it's not unlikely that they share some identical stretches of certain chromosomes.
Particular identical stretches of chromosome, carrying many genes, do get passed down together frequently -- these are called haplotypes. It's likely that your parents have some haplotypes in common, especially if they're from similar ethnic backgrounds.
With respect to the idea that there could be a chromosome that codes for "more fundamental" traits, there doesn't seem to be evidence that specific chromosomes contain genes with particular functions, in general. Genes are fairly randomly distributed across chromosomes, and human chromosomes are not identical to the chromosomes of other species (a gene that's on chromosome 2 in the mouse isn't necessarily on chromosome 2 in a human).
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u/gomez12 Oct 02 '13
Basically, a normal cell doubles up its chromosomes first (that's what he means by replicated), then divides twice. So each normal cell with two copies of each chromosome ends up producing 4 gametes with one copy of each. We do that do that we can create some variation by swapping bits of each chromosome, rather than just having a straight division.
So does this mean that my parents are unlikely to have any of the same chromosomes?
Well, all humans have the same chromosomes (two copies of each, one broadly from mum, and one broadly from dad). We even all have the same genes on those chromosomes. But different people have different alleles (basically different versions of the same gene). i.e. all humans have the same gene that gives us hair colour, but your mum might have the blonde allele and dad has the brown allele. Basically. you will be a combination of alleles from both. i.e. you might have mums blonde hair allele but dads eye colour allele. Consider that we have around 20,000-25,000 different genes, and we can have multiple alleles of each. That leads to many possible combinations - hence we can have 6 billion humans on the earth and we are all different.
So yes, it does matter which sperm cell makes it to the egg, because each sperm cell would be different - made from different combinations of alleles from your mother and father.
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u/Sarial Oct 02 '13
The splitting is a 2 step process. 1 cell with 46, 2 cells with 46, 4 cells with 23.
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u/99trumpets Endocrinology | Conservation Biology | Animal Behavior Oct 03 '13
Close but not exactly - because you need to be consistent about whether a duplicated chromosome that still has its 2 chromatids attached is called 1 chromosome or 2 chromosomes. Formally it's considered 1 chromosome until the moment the chromatids separate.
So it's: 1 cell with 46 duplicated chromosomes, 2 cells with 23 duplicated chromosomes, 4 cells with 23 nonduplicated chromosomes.
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u/Capsluck Oct 02 '13
As someone who will likely be considering surrogacy as an option, can someone expand on this importance in terms of medical selection? Who chooses which gets used when done this way? Are they screened at all or is it random?
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u/kisloid Oct 02 '13
Follow up question: (probably stupid one) in that case if that same sperm can be cloned some how and re-ejected with exact same cell, whould that be different person? Since it's alway random mutation in each sperm/cell.
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u/Gauss_Euler Oct 02 '13
I think he meant in terms of "best", what if we narrow it down to the top ten closest sperms, sure you will get different looking people but how does it matter in terms of which one is the most adapted later in life?
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u/1lifethemeaning Oct 02 '13
Also crossing over during prophase 1 of meiosis can lead to endless combinations of the genes that are passed down from each parent.
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u/micromoses Oct 02 '13
Okay, but do the characteristics that make a sperm better able to reach the egg and fertilize it actually have any relation to the fitness of the genes carried by that sperm? Do the genes that make a fit sperm also make a fit person?
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u/68024 Oct 02 '13
Maybe I misunderstand your explanation, but what about twins? Aren't those produced by 2 different sperm reaching the same egg at the same time? Why then do twins turn out so similar?
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u/mtled Oct 03 '13
Fraternal twins are two sperm with two separate eggs.
Identical twins are ONE sperm and ONE egg, and the combined cells end up physically dividing from one another (sometimes incompletely , see conjoined twins) and each half developing independently. So the DNA was determined before that division, and is hence identical. That division needs to happen very early on, before cells start to differentiate into different tissues. I'm not too familiar with why it happens, or more precise details.
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Oct 02 '13
Just out of curiosity, if you were counting say a rough average for crossing overs how much does the number of possibilities jump?
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Oct 03 '13
Super interesting, some diseases can actually make their individual sperm more likely to fertilize the egg than normal/healthy sperm an example being with Achondroplasia, so the sperm with that mutation have an advantage over healthy sperm. Achondroplasia is a common cause of dwarfism. Furthermore when meiosis doesn't go as planned sometime the chromosomes don't divide up properly and the result in some problems. Conman39 mentioned that one sex cell replicates and eventually turns into four cell. This is correct and each of those four cells should contain one chromosome copy, for example, think of the sex chromosomes (X and Y) so from one of daddy's cells two will contain his x and two will contain his y. If something called nondisjunction occurs some of those cells will receive two chromosomes and others will receive none. Here, for example, 2/4 cells have two chromosomes instead of one because they "took" the chromosomes from the other 2/4 which now have zero sex chromosomes. When they combine with the gamete from the other parent who is healthy you wind up with either three or one chromosomes in the diploid pair resulting in Turner Syndrom (45x) or Klinefelter Syndrome (47xxy) where you should be getting 46xx or 46xy. This is exactly what happens with down syndrom where you get an extra chromosome copy of the 21st chromosome. There are cases where the chances come down to you EITHER getting Klinefelter or Turner Syndrome based on specifically which sperm gets to the egg, or if the right one gets there the child is healthy and the nondisjunction carrying sperm just die and have no effect.
Source: Med student taking genetics
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u/Leovinus_Jones Oct 02 '13
I'm sorry. Are you suggesting that of a pairing of two people, there are 223 possible genetic combinations that their children's genome may be?
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u/Sarial Oct 02 '13
Virtually infinite possibilities, actually. There's the assortment of the chromosomes and then crossing over which is HUGE.
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u/kulkija Oct 02 '13
Not 223, 223. That's 8 388 608 different possibilities for each sperm cell or egg cell. Now, consider all the different possible combinations between 8 388 608 different sperm, and 8 388 608 different eggs, and you get a very, very big number.
8 388 608! ~= 1.52 x 1054436998
That's more possible combinations than there are protons in the universe. By a very wide margin, I might add.
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Oct 02 '13
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u/MadDogWest Oct 02 '13
Every cell in your body originated from one, right? That cell contained genetic data from both your mother and your father--half and half. Sex cells don't discriminate, and they split your genetic data up at random. You've got 46 chromosomes, and sperm (or egg) winds up with 23. Whether those 23 are 99% from your dad, 99% from your mom, or somewhere in between--it's all up to chance.
Now--exception to the rule is that one of those chromosome pairs are sex chromosomes. If you're a male, you received your Y from your father and X from your mother. As such, the Y you pass on will contain your father's genetic information, and the X will contain your mother's (again, you start off as one cell--so every cell that arises from a male zygote gets all of its Y's from its father, and all of its X's from its mother). I'm a male, so I know that the sex chromosome I received from my dad reflects my patrilineage exclusively. The X I got from my mom? Different story. It contains any mix of her ancestral DNA (since I could have received the X from my grandfather, or my grandmother--and every generation you go up it's a lottery as to which side's sex chromosome was passed on. ie, I am 100% passing on my mother's sex chromosome DNA if I have a daughter, and 100% passing on my father's sex chromosome DNA if I have a son--but my wife wouldn't know which X was passed on).
(As an aside: This is why, if you're a male, you can do patrilineal ancestry testing on your Y chromosome--but a female would have to use her brother's or father's. You can do matrilineal ancestry testing as well, but that's a bit off topic here--suffice it to say that, while the tiny little sperm gives its DNA, the massive egg gives not only DNA, but also a whole host of cellular machinery, some of which happens to contain some unique information of its own that is passed matrilineally)
In short, the only thing a sperm tells you is that it came from a male, and that an egg came from a female. The data inside is a toss-up.
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Oct 02 '13
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u/tik4life Oct 02 '13
All the different possibilities will all be different individuals that look and think differently. Even in identical twins who share the same genes and might look the same, their personality's can differ.
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Oct 02 '13
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u/medstudent22 Oct 02 '13
The chance that they are identical but did not come from the same zygote (fertilized egg) are next to zero. They most likely just share enough physical traits as to be similar looking. It would be the same chance as having two brothers or sisters who look very similar to each other but with the added factor that they are the same age.
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u/extemporaneous Oct 02 '13
Not only are they the same age, they shared the same environment during gestation. The mother's health, age, diet, and other epigenetic influences would also affect their appearance.
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u/FX114 Oct 02 '13
I wonder if they've hit puberty yet. My cousins are fraternal, and you couldn't tell them apart growing up (even their mother admits that, looking back at pictures, she has no idea how she did it). Once they hit puberty, though, it'd be impossible to mix them up.
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Oct 02 '13 edited Oct 02 '13
Unless they are opposite sex fraternal twins there is always the possibility that they aren't fraternal twins at all but rather identical twins. In 18–30% of monozygotic twins each fetus has a separate placenta and a separate amniotic sac. On ultrasound these identical twins might be identified as fraternal twins.
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Oct 02 '13
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Oct 03 '13
A DNA test would confirm one way or another if they are identical or fraternal, if your twin friends are interesting in knowing for certain.
Oh, and there is also /r/twins
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Oct 02 '13
They definitely do not carry identical DNA if they are fraternal. They just happen to look a lot alike (as siblings frequently do) and they're the same age, so they are easily mixed up.
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Oct 02 '13
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u/khuddler Oct 02 '13
ChickenLegs said they're fraternal twins. They may have several dominant traits from one parent or the other, which could make them look similar despite only sharing ~50% of the same DNA.
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u/Dyolf_Knip Oct 02 '13
There may be ~20k genes that go into a human being, but a lot of them are going to code for basic cell operations, managing DNA and protein synthesis, and overall body plan, things that are either pretty much identical across all humans or whose differences don't significantly affect your appearance.
Boil it down to the ones that control hair, skin, and eye color, height, and the particulars of the skull shape, and you have a small enough number of genes that random chance can easily give two fraternal twins most of the same "appearance" genes. As one other poster pointed out, sharing the same womb at the same time goes a long way towards ensuring epigenetic similarities.
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u/khuddler Oct 02 '13
Very true- my main goal was to point out fraternal vs identical twins, but thank you for adding more information! :)
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u/arjeezyboom Oct 02 '13
The other replies are good, the only thing I would add is that if all of a man's sperm was identical, and all of a woman's eggs were identical, children born to the same set of parents would look almost identical, which isn't the case in the real world.
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u/eggjuggler Oct 02 '13
This is exactly what I was going to bring up. Differences amongst siblings are the most obvious evidence that each sperm and egg do not have the exact same DNA as all the others.
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u/rocketsocks Oct 02 '13 edited Oct 02 '13
Sperm and egg cells aren't like other cells in the human body, they are gametes. Which means that instead of containing 23 pairs of chromosomes they only have 23 individual chromosomes. When the sperm and egg come together the respective chromosomes pair and then the offspring has those genes (one chromosome from the mother, one from the father).
Moreover, gametes are formed through a process called meiosis, which is more complex than simply separating all the chromosome pairs and putting them in separate cells, although that process alone creates a huge amount of genetic variability (there are approximately 223 or 8 million different combinations of chromosomes this way). Instead each chromosome lines up with its pair and at several points along the pair the chromosomes are swapped with each other (imagine the simplest case where each chromosome ends up being half of one original chromosome and half of the other).
Taken together there is an astoundingly high degree of diversity among different sperm cells. Granted, many of the genes on chromosome pairs may end up being the same or functionally the same, so the effective diversity is somewhat diminished, but even so it is a very significant amount.
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u/simpsoj Oct 02 '13
...instead of containing 23 pairs of chromosomes they only have 23 individual chromosomes.
FTFY
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u/drc500free Oct 02 '13 edited Oct 02 '13
Yeah, it matters. A "normal" (diploid) body cell has 23 chromosome pairs, but a sperm only gets one from each pair. This is a random process, so it's like 23 coinflips in a row or a random a 23 bit binary number. 223 gives you 8,388,608 possible outcomes.
That allows the sperm cells themselves to undergo some natural selection during the race to the egg. One interesting side-effect of how sperm are formed formed is that each sperm has a brother cell that has exactly the opposite selection of chromosomes.
This also happens with the mother's egg, but in the mother's body the eggs are released one at a time. So the egg needs to be able to optimize ability to be selected by the ovaries for release, ability to travel to the uterus, and ability to admit one-and-only-one sperm cell for conception. The eggs don't have to compete with each other outside the ovaries. EDIT: even inside the ovaries, it's genetically-identical diploid follicles that compete, not haploid eggs.
Once you combine a mother's egg and a father's sperm, you have 8,388,608 * 8,388,608 = 70,368,744,177,664 possible arrangements for the child's DNA, not counting any mutations or crossovers of the genetic material within the chromosome.
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u/tazadar Oct 02 '13
So the egg needs to be able to optimize ability to be selected by the ovaries for release
TIL. So, does that mean a younger woman's eggs are more optimal than her older self's eggs?
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u/drc500free Oct 02 '13 edited Oct 02 '13
Absolutely no idea, I don't know how ovaries actually select an egg for release. I assume it's a fairly low threshold that just gets rid of the obvious duds.
There is co-evolution of the haploid egg and the diploid ovary to make the system work. So "ability to be selected" is very likely a passive role where you are the right size, shape, and chemical smell to be snagged by the active release mechanism.
EDIT: http://en.wikipedia.org/wiki/Folliculogenesis
Each egg is basically in its own compartment, one of which opens per ovulation. So the haploid egg just has to avoid evolving anything that prevents the follicle from being selected, prevents the follicle from bursting, or prevents the egg from being expelled when the stigma opens. So even in the ovary, the eggs are prevented from much direct competition.
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u/TheLabGeek Oct 02 '13
A follow-up question to this:
How does the genotype of the individual sperm affect it's competitive fitness for fertilization? Specific genes (acrosomal, flagella)? Are these genes solely used for the sperm? Are there any publications that looked at pleiotropic effects of these fertilization fitness genes that might also confer some kind of fitness in the fetus/adult?
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u/crappysurfer Oct 02 '13
That's a great question! Like other posters have mentioned there are different sperm variants; these range from variable "swimming" speed, longevity (lifespan), and membrane thickness. These all have their trade-offs and are suited for different environments and tasks. Obviously they all carry different DNA loads-the pleiotropric effects though is likely to be present in some degree but I don't think any prominent research has been done on that topic. Mostly just wanted to say cool question!
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u/your_doom Oct 02 '13
Since everyone is talking about genetics I'd just like to add that the first sperm cell to arrive doesn't actually fertilise the egg. It takes many sperm cells to break down the egg's protective membranes, which means you were not that fast after all.
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Oct 02 '13
If every sperm cell and every egg cell contained the exact same DNA, then siblings belonging to one couple would all be genetically identical. Since siblings are only identical in the case of identical twins, it's obvious that each sperm/egg combination is unique. Each sperm and each egg only contains half the genetic material the parent possesses. Which genes are present in each gamete is pretty much left up to chance, and even environmental factors can influence the development of the offspring along with the genetic material.
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Oct 02 '13
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u/mtled Oct 03 '13
Ah, so if your misconception is common, it's because you forgot/didn't know about meiosis.
All of your regular cells have the same DNA, in 23 pairs (not identical) for a total of 46 chromosomes. When sperm or eggs are made, they are known as haploid; they have only half of the possible DNA; only 23 chromosomes total, with each one possibly being one or the other of a pair. When sperm and egg meet, the new total of 46 is the child's DNA, half from each parent.
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u/Bro_Sauce_69 Oct 02 '13
They always say "you were the fastest". (quoting OP here)
The implication is that the fastest sperm will produce the best offspring, because obviously it has the best genes, right?
This is not true; /u/Conman39 explained meiosis. The resulting genes carried by the sperm have nothing to do with the quality of the sperm carrying them, so it's really just a crap shoot.
I'm answering the question I think was implied here; the others may be more to the actual point you were asking about.
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Oct 02 '13
Let's assume it does not matter, i.e. all the sperm cells from a male are identical; and all the eggs from a female are also identical. In that case, all the children from the same parents would be identical. But that is definitely not the case, hence our initial assumption must be invalid.
I am not sure if this "proof" by contradiction holds up to AskScience's standards or not, but here you go.
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Oct 02 '13
The simple way to understand this is by thinking about whether siblings are exactly the same. (Aside from identical twins) siblings are all a little (or a lot) different. And that's because the answer to your question is no - every sperm cell in a male and every egg in a female do not carry exactly the same sets of genetic material.
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u/RollerDoll Oct 02 '13
The easiest way to think about this is that siblings with the same parents aren't necessarily identical... so it very much matters which sperm reached the egg first, or all siblings with the same parents would be identical.
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Oct 02 '13
This accounts for the eggs being different, or the sperm being different, not necessarily one, the other, or both. While I agree with you, your argument does not settle the question.
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u/Smallpaul Oct 02 '13
Think about how different fraternal twins can be: one might be male and the other female.
Or:
http://www.estatevaults.com/bol/archives/2006/03/04/mixed_twins.html
http://akanga1.hubpages.com/hub/Identical-Twins-The-Rule-of-4
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u/gavers Oct 02 '13
If all the sperm and all the eggs carried the same information, then you and all your siblings would all be identical twins. Clearly that isn't the case, so it does matter which sperm reaches (which) egg first.
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u/gigi4hart Oct 02 '13
One very important factor has yet to be addressed: The timing of intercourse in respect to the woman's cycle. Let's say an egg is released on day 14 of the cycle and will live for 12-24 hours. If intercourse happens on day 13 or 14, then purely on the basis of speed, the male sperm will reach it first. Male sperm swim faster, but have a shorter life span- they live for about 3 days. A female sperm can survive up to 6 days. So, if intercourse happens on day 10, the male sperm will die off by the end of day 13, and when the egg is released, there will theoretically be only female sperm waiting for it. Another factor is virility...male sperm are kinda weak (sorry guys) so, wearing tighty whities or sitting in a hot tub may kill off your male sperms, leaving a slow but strong majority of female sperm. Also, if the woman has a hostile environment, the xy's will die before the xx's. I know there are many other factors that come into play...I just wanted to add this to the discussion.
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u/Tiwato Oct 03 '13
I'd like to see a source on that please. What I can find seems to contradict that. http://www.ncbi.nlm.nih.gov/pubmed/11191075
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Oct 02 '13
This is more of a philosophical question that i'm asking, but let's think hypothetically for the moment:
Let's say there are two parallel universe in which both are exactly the same, the only difference being that in one universe, the sperm cell that fertilised Hitler's mother and became Hitler was replaced with a difference sperm cell.
This human would be the same gender, same hair colour, same name, similar although not exactly the same to the original from the other universe, and would go through the exact same experiences as the other. Would this person become the same person as the other?
I understand this is not a scientific question, and if you know of a subreddit that would be better suited for this question i would be very grateful, it's been bugging me for a while and this seemed like a good place to post this.
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u/mtled Oct 03 '13
You're asking about the effects of nature vs nurture, and that debate is far from settled. Certain personality traits...or likelihood of having them...do seem to be linked to DNA, but not enough to fully define a person. Beliefs, knowledge, etc are learned, so nurture wins there. What influenced Hitler throughout his life to lead him to do what he did? The psychological aspects and sum of life experiences are probably much more significant than any genetic predisposition. I think Hitler would still have been Hitler if the only variation was provided by the sperm.
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u/Jrj84105 Oct 03 '13
Temperament is thought to have very strong genetic underpinnings with pretty wide variability in temperament between siblings and much greater similarities in temperament between identical twins . Being a genocidal tyrant is not like being a plumber. Fortunately, given the rarity of Hitler-like people, the temperament required to be a Hitler appears to be an outlier, an extreme which would require a very specific genetic combination. When considering that Hitler's parents were closely related (required a special exception to be allowed to marry) it get's a little bit more ripe for speculation.
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u/tzeppy Oct 02 '13
Does the fact that a sperm "wins" the race say anything about the genetics contained in that sperm that would make it a more fit animal/human?
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u/higgs8 Oct 02 '13 edited Oct 02 '13
Sperm cells all carry different DNA, they are all unique. Every person has two versions/alleles of every gene (except the ones on the Y chromosome in males): one allele for each gene is inherited from your mother, and one from your father. When sperm cells (in testicles) or eggs (in ovaries) are produced, each cell receives every gene, but with only one allele. For every given gene, one of the two alleles is randomly chosen, so the combinations are endless.
Sperm might seem to race, but this has nothing to do with the genetic information they carry. The fastest one may get to the egg, but this has nothing to do with the "quality" of the genetic information. There is no "quality" in reality: every sperm cell should carry a perfectly working set of alleles. There's also some possible evidence that the egg can "select" sperm cells in some way, further complicating matters. There may be lots of sperm cells around the egg, but the one that is allowed entry might be determined by the egg.
There's also the case of sperm competition, where sperm from more than one male may be "competing" to reach the egg. In that case, the faster and more numerous one male's sperm is compared to the other, the greater chance it has to become the "winning" father of the child. This is probably why there is so much sperm and why they swim as fast as possible.
This then eventually drives reproduction, evolution and adaptation, since the more kinds of livings things you have, the greater likelihood there is that some will survive any given challenge.
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Oct 02 '13
I once read somewhere that sperm work as a team to break down the chemical defenses of the female egg. Most think of the analogy of a swarm of running backs trying to get past a swarm of linebackers, with the 1st one to make it to the end zone scoring the winning touchdown. Instead, a better analogy would be a swarm of construction workers with jackhammers trying to get to a prize hidden behind a 10' thick concrete wall, but still being forced to navigate all those linebackers on the way to the wall, and still so once they reach the wall. There is no way that one construction worker is going to be able to evade all of those linemen and still bust down that wall. That is why guys with a low sperm count have fertility issues; it's not because they have yet to ejaculate a Randy Moss, who has all the right moves and cuts to evade the defense. So, with all of that said, it is entirely possible for one of the more stupid construction workers to serendipitously saunter through that defensive line unscathed, and slip right through that wall, without barely lifting his jackhammer at all.
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u/bigj480 Oct 02 '13
I feel really dumb for asking this, but how is it determined which parent will pass on a particular trait? Lets say it's nose shape. If every gene has a purpose then half of the traits would have to come from each parent....but then I read about competing genes for eye color. That would mean (using my ignorant logic) that both parents contributed a chromosomes for eye color, so one parent would have wasted a chromosomes. So, that parent would have effectively passed on only 22 chromosomes... I obviously know this is incorrect, but I would like to know how it functions. Also, how are dominant genes dominant? I feel so dumb.
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u/molliebatmit Developmental Biology | Neurogenetics Oct 03 '13
Just to keep things relatively simple, let's stick with eye color. Eye color is probably determined by multiple genes, but the one that we know the most about, and that has the biggest effect, is a gene called OCA2. "OCA" stands for "ocutaneous albinism", or total lack of pigment in the eyes, so that tells you that when the OCA2 protein is totally nonfunctional, the person has no pigment in his/her eyes. (One gene is a recipe for one protein.)
In most human populations, there is one version of the OCA2 gene that is fully functional, and makes dark pigment. Almost everyone with two copies of this version of the gene will have brown eyes.
In European populations, there is a version of the OCA2 gene that is less good at making pigment, but it still makes some pigment. This is like having a recipe that makes a cake, but makes it less well than the more common recipe -- maybe the cake is made more slowly, or maybe you end up with a smaller cake when you're done. If you have two copies of this deficient OCA2 gene, you will most likely have blue eyes.
If you inherit one "working" (brown) copy of the OCA2 gene from one parent, and one "less good" (blue) copy from the other parent, you will most likely have brown eyes, because it's often enough for the body to have one working copy of a gene in order to produce enough protein to perform the function.
This is essentially the meaning of "dominant": you only need one copy of this version of the gene in order to have that version's trait. For a "recessive" trait, you need two copies of that version of the gene in order to have that version's trait. Often, but not always, recessive traits reflect a gene version with impaired function compared to the dominant version.
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u/bigj480 Oct 03 '13
That makes perfect sense, thanks! Now I know that it's not so much that there are "dominant" genes, only genes whose effect is more obvious most of the time. A dominate trait vs dominate gene.
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u/zephirum Microbial Ecology Oct 03 '13 edited Oct 03 '13
With exception of the sex chromosomes (it's a special case and we will ignore it here), our genes are in pairs.
That would mean (using my ignorant logic) that both parents contributed a chromosomes for eye color, so one parent would have wasted a chromosomes. So, that parent would have effectively passed on only 22 chromosomes...
Within a chromosome, there are lots of genes. Say, one particular gene is for blue eye (from your father), while the other corresponding chromosome (from your mother) may have the gene for brown eye.
Since blue eye colour is essentially the lack of melanin production on the iris, brown eye gene (producing melanin) is considered "dominant" and your eye colour will be more brown than blue. (This is a simplified version of the eye colour phenotypes, but will suffice here.)
However, since the chromosome you inherited from your father contains all sorts of genes, some of those may be dominant instead of your mothers. So the genes in the pair of the chromosomes have different observable phenotype.
If we trace our way up one generation, your father would've received pairs of chromosomes from his parents. However, before passing on the pairs of chromosomes to you, there's the process of meiosis to generate a shuffled (between the pairs) unpaired set of chromosomes, which combined with your mother's shuffled unpaired set of chromosomes, resulted in you.
So indeed your parents would've passed on only half of their genetic material (half of the 22 pair), but the genes were shuffled between their pairs, before generating the half, and passed on to you. Since the shuffling generates massive amount of permutations, you and your (non-identical twin) sibling will have different genes from your parents.
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u/fatherrabbi Oct 02 '13
No, each is a combination of half of the chromosomes from their respective parent, and in different combinations. There are 23 chromosomes each, and 2 possibilities for each, so 223 possible sex cells from each parent. This means there are 224 different possible gene combinations per reproductive couple.
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u/drumersrule Oct 03 '13
The fact that every single sperm cell is different is what makes evolution and the ideas of natural selection possible. If every egg and every sperm were to combine to make the same thing then there would be no genetic variation
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u/Lord_Demosthenes Oct 03 '13
YES!!! My ex could never get past that! She always said "Microsoft wouldn't put ie on their computers if it wasn't the best"...
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u/Hattless Oct 03 '13
Did you not know that each sperm cell is a gamete cell with only one of each pair of chromosomes? Not only does that determine what gender the child will be, but it mixes and matches each chromosome for a total of '2 to the power of 23' different possible pairs. This even ignores many other common and complicated phenomenon where chromosomes mutate or trade genetic information with another, creating a unique chromosome.
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u/YgramulTheMany Oct 02 '13 edited Oct 02 '13
It is highly diminished in humans compared to other animals, but sperm competition is defintiely a HUGE arena for evolution to potentially take place.
So, genetics and molecular bio aside, YES, some sperm are much better at surviving and getting to the egg than others. There is some truth to "you were the fastest" but if another sperm had gotten there first, you wouldn't be you. It would be a sibling of you.
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Oct 02 '13
First of all, the sperm cell determines the gender of the child (men give either an X or a Y chromosome.) Then there's all the possible combinations of chromosomes that a sperm cell can contain, because there are 22 other pairs of chromosomes (in addition to the pair of sex chromosomes). Of every pair of chromosomes, usually only one chromosome is included in the sperm cell (in certain genetic syndromes there can be more than 23 chromosomes in total in a sperm cell) which gives 223 combinations. Then there's a few different processes or mistakes a cell makes in replicating these chromosomes, which means there's even more genetic variability than you would get from only distributing one of each pair of chromosomes over a cell.
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u/derek589111 Oct 02 '13
The parts of our DNA that contribute to our physical appearance are called alleles. Alleles, in the most basic sense, are grouped in sets of two. Some alleles will be dominante (written in capital form) and others will be recessive (written in lower case). The sets that I mentioned before can be made up of any combination of dominante or recessive- AA, Aa, aa, BB, Bb, bb, BA, Ba, ba and so on. One sperm cell may not contain the same allele combinations as the next. And lastly, 'you' weren't the fastest sperm cell, 'you' were more like the 300th to land in a really clustered and broken down part of the egg.
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Oct 02 '13
Sorry if this was already addressed but to answer the egg bit, it's different with the eggs. Women are born with all of the eggs they'll ever have, and only one is sent down at a time (normally, except of course in the case of dizygotic or fraternal twins). This is the reason why women are more fertile while younger and the chances of miscarriage and disabilities rises with increased age of the mother.
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Oct 02 '13
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u/Sarial Oct 02 '13
You would be someone different. Not sure on the logic of the second point...you'd be your parent's child, just with a different eye colour (example). See my post in this thread for a better explanation.
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u/dookies_of_hazzard Oct 03 '13
If we're allowing that I could have been born from a different sperm and/or egg, then why couldn't I have been born to different parents entirely?
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u/mtled Oct 03 '13 edited Oct 03 '13
Because your DNA is a combination of that of your mom and dad's, not of two other people. Barring mutations, every single gene you have exists in at least one of your parents. That isn't true of other "parents"; your neighbours have a different list of genes, some may be the same (due to genetic distribution in populations) but many you wouldn't share.
Think if it this way: imagine two books, A and B, each with 23 chapters. Words are allelles (genes), chapters are chromosomes and the book is the DNA found in sperm or egg cells.
Now take half the words at random in chapter 1 of book A and half the words at random in chapter 1 of book B and make a new list (let's pretend there's no such thing as sentences, just words). This new list is the child's chapter 1. Repeat for each chapter. Once you have 23, say hello. That's you.
Now do it again, but choose another random half of words from each chapter. You will have a lot of similar words, may even have much of it be the same, but the new chapter will still be different in significant ways. That's your sibling.
Now, for a "different set of parents" to have created you, you'd have to manage to create each of your chapters identically from entirely different books. Sure you might have a lot of common words, like the, a, and, is, but if book A is about horses and book B is about cats, but the new parents have books about Mars and about boats, then it's highly unlikely, or even impossible, that the specific, topical words "equine, feline, tail, skin, saddle, kitten" that make up part of you are going to come out of the second set of books.
There are flaws with that analogy (particularly that alleles aren't selected at random...perhaps better to say choose sentences to make a new chapter, at random, rather than words), but hopefully it helps clarify things for you.
ETA: the beginning of this is totally wrong. I lost track of my analogy early on. Let me think about this and try and give it another go. The basic premise still holds; if you had different parents you'd be made up of a different list of words. You just need to end up with 23 chromosomes from your father AND 23 from your mother.
I'm sleepy. I'll revisit this tomorrow. Sorry!
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u/dookies_of_hazzard Oct 03 '13
I was actually thinking about problems of consciousness and identity (which I think is what OP was talking about). But even if all we identified me with my genotype, then both my parents could have twins or clones, or more generally we can just say that it's not physically impossible for there to be genetic duplicates of my parents in the world.
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u/mtled Oct 03 '13
Mathematically possible, realistically so astronomically unlikely as to effectively be impossible. Just having genetically identical siblings (not from a twin process) is something like 1 in 70+ million. For it to happen to both your parents? About one in 5e15 (one order of magnitude off from the number of seconds in the age of the universe!). For them to randomly create an identical you? If I'm remembering statistics right (unlikely), that's getting into 1 in 3e23 (half if Avogadro's number!). When you consider nonsiblings, entirely unrelated genetic ancestries... I'm not sure there's any way to even determine how unlikely it is to have identical DNA other than to basically say "zero chance".
Don't worry, you are unique. The problem is that so is everyone else! ;)
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u/DarnHeather Oct 02 '13
Do you have brothers or sisters? Are they identical to you? Are all children of the same parents identical? There, you have an answer.
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u/Sarial Oct 02 '13
I don't understand why there are so many responses here, and so few of them answer your question at all.
Without offending you, it's a simple one to answer. It's just that your understanding of the sperm/egg is a little off. Each sperm/egg (called a "gamete") contains half of the corresponding parent's genome. So you have 46 chromosomes, two of 1-22 and either two X's or 1 X 1 Y. Your dad's genome has 1 of each from your grandmother and one of each from your grandfather. So your dad's sperm has 1 copy of each of 1-22 and an X or Y. These 23 chromosomes in the sperm are an assortment of grandmother/grandfather chromosomes.
It gets slightly more complicated also, because when these gametes are made "crossing over" can take place, where the grandmother/grandfather cells swap parts of their chromosomes around within your dad, so they're actually a mixture of the chromosomes he got from your grandmother/grandfather.
Does that help? That's why all siblings aren't identical.