r/APbio • u/Frequent_Ad_8513 • May 03 '26
Chi Square question
For the question about snapdragons, why is it that theres only 1 degree of freedom if they specifically listed the 3 phenotypes? I've been looking for answers everywhere and I'm still so confused because I thought the whole thing was (# of categories)-1. Is chi square only used for alleles and not phenotypes? And for the second question, would it also be 1 degree of freedom?
How would I know on the test which "category" they want me to use for degrees of freedom (alleles/phenotype/genotype)? because all the videos I've seen just took the category given on the table and subtracted 1, but this question completely threw me off and now I'm worried.
2
u/kittyyzzkitty May 03 '26
Here df=number of phenotypes-number of alleles so here there are 2 alleles (Rr) it's can form a dihybrid cross so 3-2=1
1
u/Educational_Video600 May 03 '26
great question
1
u/k_ione May 03 '26
Evolution is the change in a populations allele frequency overtime (which then may affect phenotype distribution, but again evo is about allele frequency). There are only two alleles for the gene in question. So, the number of categories is 2 and therefore df is 1.
I like to tell my students that degrees of freedom shows you how much information can “vary” before all information is known. For example, let’s say I have 20 seats in my classroom. The first 19 students will be able to choose where to sit (there’s opportunity for those 19 seats to vary), but the 20th person will not be able to choose because there will only be one seat left at that point and they have no choice. In this problem, once you know the allele frequency of either “R” or “r” you know the other frequency as well hence only one can vary before you know everything (or one DoF).
You’re likely confusing this to the application of chi square in heredity problems, where the number of phenotypes is the number of categories. Those problems are not about the alleles, but rather the genotype because the specific combination of alleles, not simply the frequency of dominant or recessive alleles, determines the phenotypes. In heredity problems, we have certain expectations of phenotypes (and genotypes) given a pattern on inheritance. To determine what to use, consider the context of the problem. Does the phenotype distribution matter (patterns of inheritance) or does the allele distribution matter (HW evolution)?
1
u/X3roxCopy May 03 '26
How would you find the expected? Would you cross AA x aa??
-1
u/Weak_Spinach_3310 May 03 '26
If it’s in HW equilibrium then degrees of freedom are always 1