German geneticists have shown that mothers have consistently outnumbered fathers, suggesting that more females have contributed their DNA to the human gene pool than men.

Now, at first blush this conclusion might seem a bit counter-intuitive. Logic would dictate that the proportion of male and female DNA should be about equal. But the new analysis, which appears in Investigative Genetics, shows that more women than men have passed on their DNA since the first humans walked out of Africa some 70,000 years ago.

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According to the researchers there are two likely and non-mutually exclusive explanations. The first is polygyny — when one man has multiple wives (in such cases, only one male is passing on his genetic information, whereas multiple females pass on theirs). The other has to do with reproductive customs and the (apparent) fact that women tend to relocate when moving with their husbands (e.g. women likely traveled for marriages, leaving their hometowns to move in with their husbands). Taken together, these factors may explain why females have made a greater genetic contribution to the global population than males.

Comparing NRY and mtDNA

For the study, Mark Stoneking and his colleagues from the Max Planck Institute for Evolutionary Anthropology compared the paternally-inherited non-recombining Y chromosome (NRY) with maternally inherited mitochondrial DNA (mtDNA) of 623 males from 51 different populations, including Australian, European, and American populations.

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They devised a new protocol to more thoroughly analyze genetic variation within the male Y-chromosome. By looking at a single part of the Y-chromosome, they could see all of the genetic variants, or slight differences in the order of the DNA's "letters" within that region.

A Guardian article explains more:

"Imagine a population of 100 females and 100 males," said [Stoneking]. "If all the females but only one of the males reproduced, then while the males and females contribute 50:50 to the next generation, the male contribution is all from just one male." The next generation would all have the same Y chromosome but 100 different sets of mitochondrial DNA, which is passed solely down the maternal line...

...Using computer models, the researchers showed that the differences in genetic diversity arose if more women than men were breeding throughout human history. According to their simulation, an ancestral population of 60 women and 30 men were breeding in Africa before humans left the continent. The numbers fell to around 25 women and 15 men breeding at the time of the first migration of Homo sapiens, around 70,000 years ago. The whole population would have been larger, but the extras were not contributing to the gene pool.

Stoneking had this to say in a LiveScience article:

[Historically] more of the women were reproducing than the men. This often happens in human societies, because not all men are able to afford wives, or sometimes a few men will have many wives.

And indeed, results showed that genetic differences between human populations were almost always larger for the Y chromosome than for mitochondrial DNA, except in East Asia for some reason.

Caveats Galore

"However, there are several reasons why this conclusion should be viewed as tentative," conclude the authors in the study.

First, the sample sizes of some of the regional groups were quite low. Second, there are drawbacks to focusing exclusively on NRV versus mtDNA. Third, the models used in the simulations are obviously a very simplified version of reality. And finally, a more detailed investigation is warranted into the relative importance of background selection versus purely demographic processes in influencing regional variations.

That said, and in the words of Stoneking: "What we've found is that there are significant differences in the history of human males and females in different parts of the world. Understanding why that's the case and what are the social historical processes that led to those differences are what we want to investigate now."

Read the entire study at Investigative Genetics: "Human paternal and maternal demographic histories: insights from high-resolution Y chromosome and mtDNA sequences". Supplementary sources: Guardian, LiveScience, Science 2.0.