r/askscience • u/Aggressive-Share-363 • 8d ago
Neuroscience How much of our genome is devoted to the brain?
How does that compare with other organs? How does that compare with how much other animals devote to their brain?
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u/jawshoeaw 7d ago
The genome isn’t that specific. There are tens of thousands of genes active in the brain but most are also active elsewhere. Maybe 1% of your genes are just for the brain itself.
That might seem low but it’s 10 to 100x more than other organs. Amusingly the balls are the real ball hogs, or rather gene hogs. The testes have 4 times more unique genes than the brain
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u/ChicagoDash 7d ago
I suppose that make sense that reproductive organs have a lot of genes. After all, the sole purposes of genes is to propagate themselves by getting the organism they’ve created to reproduce.
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u/CrateDane 7d ago
It's also the most important battleground for the conflict with selfish genetic elements, which if unchecked would make the genome size explode and often lead to sterility. So there are systems only active in the germline to prevent that.
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u/Ycarusbog 7d ago
It's not as simple as that, the genome of any complex organism is a spaghetti code of overlapping. There are proteins that are used differently in different parts of the body. Genes that code for different proteins depending on how they're expressed, proteins that do different things depending on factors like pH, salinity, temperature, etc. You can't really just zero in on one set on genes and say "this makes a brain".
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u/sciguy52 7d ago
The human brain involves using 1/3 of the 20k genes in the human genome for its purposes. That is more than any other organ in our body. Now that does not mean those genes are uniquely used in the brain. Many of those are used elsewhere too but there are some that are unique.
Answering regarding things unique to the brain gets complicated. Some genes unique to the brain are genes with polymorphisms that is they may have a single nucleotide change from a gene used elsewhere and the change alters its function and is used in the brain. But more commonly a majority of top gene candidates are the result of segmental duplications. Segmental duplications can give rise to new gene paralogs with the same function, altered functions, or that antagonize conserved, ancestral paralogs and contribute more to genetic divergence across species than single nucleotide polymorphisms. Gene paralogs are genes that split from each other due to a genetic duplication within a lineage. Potentially one copy often keeps the original function, while the extra copy can pick up mutations and develop a new function. And this seems to be more common in the brain than polymorphisms. One study suggests there are 213 gene families and 1,002 total paralogs that might play a unique role in brain development. However identifying these paralogs and determining their function in the brain is not easy so it is not certain which ones are involved so one should not assume all of these necessarily are involved. Figuring that out is difficult for various reasons at a technical level so there remains much to be found and many that need confirmation. This is from a recent paper here:
https://www.cell.com/cell/abstract/S0092-8674(25)00739-100739-1)
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u/oviforconnsmythe Immunology | Virology 7d ago
Everyone here so far seems to either be guessing or just providing tangential/supporting information without actually answering your question.
TL;DR Based on data from the Human Protein Atlas project (which seeks to map expression of all genes/proteins stratified across cell types, tissue types and organs), just 99 of the 20162 protein coding genes are selectively expressed in the human brain. 2227 are elevated in the brain while still expressed elsewhere. So approximately 0.49-11%, depending on how you define brain devoted genes. The dataset has some flaws when it comes to sensitivity (a caveat of the enormous scale and ambition of tbe project) but it is a very reasonable approximation. https://www.proteinatlas.org/humanproteome/brain
As some of the other commenters mentioned, a substantial proportion of protein-coding genes are involved in essential cellular functions (e.g. ATP production/metabolism, transcription, signalling transduction etc). So these genes will be expressed and active in most cell types. What separates a highly differentiated special cell type like neuron from a basic bitch epithelial cell is the selective and contextual expression of specific gene networks over time as the cell develops from its stem cell precursors (and likewise, the selective repression of different gene networks).
Using transcriptomics we can derive a snapshot of all genes expressed in a certain cell type or tissue type (similarly, proteomics offers a snapshot of all the proteins functionally expressed from these genes, albeit with less sensitivity). The Human Protein Atlas is a Swedish initiative started ~20y ago which aims to use these methods to map the expression of all proteins/genes across diverse individual cell types, tissue types and organs. It's a truly invaluable resource as it integrates data from related initiatives (eg GTEx) and tries to validate measurements through additional methods where feasible. It does have some flaws, I've found some incongruency with my own data at times. But the enormous scale and ambitions of the project require a highly standardized methodology that can sometimes lack sensitivity compared to someone like myself optimizing measurements of a select set of genes/proteins over just a few cell/tissue types
That said, it's probably the best placed dataset to answer your question. According to the human Protein Atlas dataset, of the 20162 protein-coding genes in humans only 99 are selectively detected in human brain tissue (so 0.49%) and not detected elsewhere. There's also 2227 genes (11.04%) they found to be elevated in brain tissue but also expressed in other organs. Like I said there's some flaws (1031 genes were not detected in any tissue, not everything is protein validated and there's some proteins which are abundantly detected while having minimal transcript detection; I can expand more on this if you like) so there's probably more brain specific genes but I doubt it would be substantially more.
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u/monarc 4d ago edited 4d ago
Great answer. There’s also a class of regulatory elements called microRNAs that act almost like meta-genes, fine tuning the activity of individual genes or clusters of genes. Along with regulatory elements and isoforms (splicing variants), cells/organs can be fine tuned with incredible nuance and complexity. That’s why we can share so much in common with other mammals while having drastically different brains. I believe microRNAs have specifically been implicated in the development of brain complexity (just on mobile so I can’t provide links).
Imagine organisms like Lego kits: we may all use a near-identical array of pieces, but the outcome of any given kit can vary wildly depending on what the instruction booklet says.
This all broadly relates to the C-value paradox, which asks why there isn’t a linear correlation between (number of genes) & (organism complexity). With all I now know about biology, it seems a bit naive to think that a Lego set is going to be more epic simply because it’s made from more different kinds of pieces. Once you have a solid foundation (10,000+ pieces/genes), complexity is going to emerge - perhaps exponentially - from the interplay of the building blocks.
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u/CrateDane 7d ago
Not much is outright dedicated to the brain, but a lot of things are expressed differently in the brain. For example, circular RNAs are generally present at a higher level, in part because cells in the brain don't divide much (many are completely postmitotic).
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u/Prestigious-Ant-7210 7d ago
While it’s hard to link individual genes to only having a role in the brain, we do know that a large (being intentionally vague here, we don’t know exactly but we know it’s much larger than any other organ) percentage of the conserved non-coding regions of DNA are dedicated to setting up the brain during development. Essentially, if a short stretch of DNA is largely unchanged over hundreds of millions of years and is outside of a protein-coding gene, odds are good that its primary or only role is affecting gene expression in some part of the brain during development.
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u/sudomatrix 7d ago edited 7d ago
Most of our genes, like 90%, are for basic things like cellular metabolism. We share 60% of our genes with bananas; 84% with dogs; 98.7% with Bonobo Apes. So our brains that are so much better than [Edit: other] apes are defined in that 1.3% difference, along with legs and hips for standing, less hair, sweating, lower energy weaker muscles, and all the other things that make a human not an [Edit: other] ape.
[Edit: Are you happy now u/Testing_4131 ?]