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Regarding "men tend to have more variance in everything due to having only one X chromosome": So, there are lots of recessive genetic conditions: for a particul

Regarding "men tend to have more variance in everything due to having only one X chromosome": So, there are lots of recessive genetic conditions: for a particular gene, if you have two copies of the common A allele, you're normal; if you have one copy of A and one of the rare lowercase "a"—written as Aa—then you're normal; but if you have "aa", then you have the rare genetic condition. Then, if we suppose that 99% of people's copies of the gene are A and 1% are a, and that there's totally random distribution, then you can calculate that 98.01% of the population has AA, 1.98% has Aa, and 0.01% has aa. Which means 99.99% will appear "normal" and 0.01% will exhibit the condition. Now, what happens when the gene is on the X chromosome? Women will have two copies, men have one. What happens when a man has "A" or has "a"? I think the mechanism for recessive genes is often something like, "Allele A codes for an enzyme/protein, while allele "a" codes for a version of it that does nothing; if your body is producing the normal amount of that enzyme or half of it, it still has the normal result, but if you produce no working copies of that enzyme, then you get the unusual result." Or something like it. So in that case, a man who has "A" will appear normal, and a man with "a" will have the condition. The result would be that, for an "X-linked" condition, 99.99% of women will appear "normal" (1.98% of which will be "carriers" who are Aa) and 0.01% will have the condition, while 99% of men will be "A" and appear normal and 1% will have "a" and have the condition. Thus, men are enormously more likely to get unusual recessive genetic conditions when the gene is on the X chromosome. Colorblindness is one such condition, which comes to mind: https://en.wikipedia.org/wiki/Color_blindness https://en.wikipedia.org/wiki/Color_blindness So, if we hypothesize that there might be lots of recessive genes that have "some" effect on behavior or personality or whatever we care about... If the genes are on chromosomes other than the X, then men and women will have equal variation. If the genes are on the X chromosome, then men will exhibit more variance, because more of them exhibit the rare condition (while a similar number of women are merely carriers of it). So, if we suppose that there are some important recessive genes on the X chromosome, then we would expect to see higher male variance due to that.

ckosidows 5y ago
I thought every gene needed a pair of alleles? I don't understand much about genetics, so please excuse my ignorance. But I remember making Punnett squares in school. Does your comment imply that some genes don't have Punnett squares and are only affected by one parent's DNA? Would that mean that some genes are _only_ inherited from our mothers? I remember reading something about how some aspect of our blood is only passed down from our mothers; is it because they always give us an X and our fathers give us a Y? And then genes that are missing from Y always come from the X? Can some genes only be passed from the Y? Since women don't have a Y chromosome how would that work?
- waterhouse 5y ago
  Most genes are on the 22 "homologous" chromosomes, which you have 2 of. For those, you do have 2 copies of the gene (one per chromosome), one of which comes from the mother, one from the father—and which copy you get is effectively random (each parent does a process called "crossing over" that sort of mixes their two copies to produce the new chromosome they pass to the child). Punnett squares can be used for these genes. That leaves the "sex" chromosomes, X and Y. Females have two X chromosomes and no Y, males have one X and one Y. (The Y chromosome is very small, has very few genes, and doesn't change much.) A mother passes down one X chromosome, via "crossing over", to a child of either sex. A father passes down either his full X chromosome, making the child a daughter, or his Y chromosome, making a son. (So, yes, it is impossible for a father to pass on X-linked conditions to his sons, while his daughters will always carry a copy. Meanwhile, if a male has an X-linked condition, then his mother must have at least one copy, so she's either a carrier or has the condition herself.) I think most genes follow a dominant/recessive pattern, where AA and Aa behave the same (by convention the uppercase letter is used for the dominant allele) while "aa" behaves different. There are certainly exceptions to that, though. See https://en.wikipedia.org/wiki/Dominance_(genetics) https://en.wikipedia.org/wiki/Dominance_(genetics) (in particular "incomplete dominance", "codominance"). When you have one copy of a gene (usually because it's on the X chromosome—"X-linked"), if it is of the dominant/recessive pattern, then I would expect "A" to behave like "AA" and "a" to behave like "aa". Note that, since crossing over never happens on the Y chromosome, it's passed down fully intact from father to son, and the only changes that can happen to it are mutations—a very slow process, compared to the mixing that occurs on the other chromosomes. Evolution on the Y is thus rather slow. Also, (according to Wiki) the Y is estimated to have only 63 protein-coding genes, while the X has 804. I think the Y mostly just serves to make the child male.
  - ckosidows 5y ago
    This is a helpful, detailed explanation. Thank you!
deleted 5y ago
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ckosidows 5y ago

I thought every gene needed a pair of alleles? I don't understand much about genetics, so please excuse my ignorance. But I remember making Punnett squares in school. Does your comment imply that some genes don't have Punnett squares and are only affected by one parent's DNA? Would that mean that some genes are _only_ inherited from our mothers? I remember reading something about how some aspect of our blood is only passed down from our mothers; is it because they always give us an X and our fathers give us a Y? And then genes that are missing from Y always come from the X? Can some genes only be passed from the Y? Since women don't have a Y chromosome how would that work?

waterhouse 5y ago
Most genes are on the 22 "homologous" chromosomes, which you have 2 of. For those, you do have 2 copies of the gene (one per chromosome), one of which comes from the mother, one from the father—and which copy you get is effectively random (each parent does a process called "crossing over" that sort of mixes their two copies to produce the new chromosome they pass to the child). Punnett squares can be used for these genes. That leaves the "sex" chromosomes, X and Y. Females have two X chromosomes and no Y, males have one X and one Y. (The Y chromosome is very small, has very few genes, and doesn't change much.) A mother passes down one X chromosome, via "crossing over", to a child of either sex. A father passes down either his full X chromosome, making the child a daughter, or his Y chromosome, making a son. (So, yes, it is impossible for a father to pass on X-linked conditions to his sons, while his daughters will always carry a copy. Meanwhile, if a male has an X-linked condition, then his mother must have at least one copy, so she's either a carrier or has the condition herself.) I think most genes follow a dominant/recessive pattern, where AA and Aa behave the same (by convention the uppercase letter is used for the dominant allele) while "aa" behaves different. There are certainly exceptions to that, though. See https://en.wikipedia.org/wiki/Dominance_(genetics) https://en.wikipedia.org/wiki/Dominance_(genetics) (in particular "incomplete dominance", "codominance"). When you have one copy of a gene (usually because it's on the X chromosome—"X-linked"), if it is of the dominant/recessive pattern, then I would expect "A" to behave like "AA" and "a" to behave like "aa". Note that, since crossing over never happens on the Y chromosome, it's passed down fully intact from father to son, and the only changes that can happen to it are mutations—a very slow process, compared to the mixing that occurs on the other chromosomes. Evolution on the Y is thus rather slow. Also, (according to Wiki) the Y is estimated to have only 63 protein-coding genes, while the X has 804. I think the Y mostly just serves to make the child male.
- ckosidows 5y ago
  This is a helpful, detailed explanation. Thank you!

deleted 5y ago

[deleted]