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I've heard that with the distribution of our genetic code women have less variation on the bell curve than men.

Is there any basis for this? It was my understanding that women have more genetic variation than men due to having two X chromosomes and there being more variation on gene expression within the X chromosomes.

I've also seen a theory that with traits linked to genes on the X-chromosome in women being averaged across the genetic variants found on each of a woman's two X-chromosomes, this would reduce the likelihood of extreme traits (men have just one X-chromosome, precluding this averaging process).

Is there any biological possibility for men to have more genetic variation than women, and if so what would be a potential reason for it?

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This question is very interesting and I would love to have a reference to an article that provides such evidence.

Decomposing your question:

  1. Is it true that there is more genetic variance among males than among females (in humans I guess)

  2. If the answer to (1) is yes then: Why is it so?

I am trying to address these two points below

1. Is it true that there is more genetic variance among males than among females ?

There are different ways of understanding your question 1. Do you want to talk about…

  • Allele richness

  • Heterozygosity

  • Polymorphism

  • Part of the genetic variance in phenotypic variance (heritability). I don't think it is what you meant though!

Therefore, the question is slightly unclear and I don't know of any paper who address any of these questions anyway. So... I don't know!

2. Assuming (1) is true, then Why is it so?

There are many possible explanations for such observation (assuming this pattern has been observed). Below I give some hypothesis I can think of.

  • There might have a higher selection (survival differential) among females than males. In a consequence, genetic variance among young females is higher than among older females. In total, genetic variance among females would be lower to that of males.

  • There might have a very high variance on the Y-chromosom but not on the X-chromosom. This might for example be a consequence of the arrest of recombination that occur on the Y-chromosom. As a consequence of this arrest of recombination, selection hardly purge deleterious mutations (see Muller's Ratchet). See Shigeta's answer for a more detailed overview of this point. See shigeta's answer for more information.

  • Males might migrate more. If one look at the sex-specific genetic variance in a population (and not on the world-wide metapopulation), this variance might be greater for the sex that migrate more because different genetic background might be brought from other neighbor populations.

  • Higher selection within mitochondria in females very early in life (in the egg for example) resulting in a decreased mitochondrial genetic variance in females. This is maybe a bit far-fetched.

  • There might have other hypotheses for species for which sex of an individual is not determined by sex-chromosomes. (see Sex Determination System)

  • Depending on what you mean in your question, I guess we might find many other hypotheses when thinking of migration, different fitness landscape (different optimum), different selection intensity.

  • We might as well think of genetic variance of gender-specific gametes (especially for haplon-diplontic species for example)

  • This evidence (assuming it has been observed) was a False Positive!

Note: @terdon provided a very good explanation (which was deleted) of why males might cause most of the genetic diversity but as @terdon and I discussed (in the comments that were also deleted) this is not an explanation for greater diversity among males. The reason is that the variation is created via mutation in the testis and the ovary. The fact that most mutations happen in males does not influence the sex-specific diversity because males sire as many male offsprings than female offsprings (so do females!).

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This is an interesting question and we should probably have a statistical geneticist around, so further answers may show up better than this one. So I'm also assuming the question is: Is the rate of variation on the X,Y and autosomal chromosomes different? Also assuming XY male heterogamy as humans have - different sex chromosomes will give different results between male/female.

The main distinction being - if males absorb more mutations on X as well as Y, then it would be nigh on impossible to see this phenomenon statistically since those chromosomes get passed on to females immediately.

Its important to remember that some of the X and Y regions do recombine even in men in the pseudoautosomal regions. The Y also maintains itself with a self-recombination event in meiosis. So the message here applies to the SDR (sex determining regions) of the X and Y chromosomes - other regions are quite possibly not different than the other chromosomes.

Anyway I've found this modeling paper "Patterns of Neutral Genetic Variation on Recombining Sex Chromosomes" which estimates and compares the relative coalescence time for variations on the autosomal, X and Y regions.

In any case the models in the paper says 'yes they are different and Y non-autosomal regions accumulate variants faster'. The coalescence time is the amount of time for a variation to spread into the general population on one of these 3 types of regions.

In cases where there is neutral selectional pressure "Close to the SDR, expected coalescence times are shorter on Y chromosomes and longer on X chromosomes than for autosomal sites."

Which is to say speeds for incorporating variations are Y > X > autosomal

Note that the paper goes on to say that when there are more than one competing versions of the Y chromosome in a population (which seems likely to be the case), then the spread of variations on the Y goes to a normal autosomal rate.

So its likely that this effect is small overall.

It has been documented that some Y chromosomes are very competitive though.

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