\n<\/p>\n
Babies tend to have the same probability to be born biologically male or or female \u2014 but family genetics could skew those odds in either direction. <\/span>Credit: Waltraud Grubitzsch\/dpa via Alamy<\/span><\/p>\n<\/figcaption><\/picture><\/figure>\n By sifting through an anonymized genealogy database, researchers have discovered a Utah family that has been having twice as many boys as girls for seven generations. It is the first clear evidence that humans might have \u2018selfish genes\u2019 that distort the sex ratio of offspring from roughly 50:50, the researchers argue in a preprint posted on bioRxiv earlier this month1<\/a><\/sup>. The findings have not been peer-reviewed.<\/p>\n Such sex \u2018distorters\u2019 have been discovered \u2014 and studied in great depth \u2014 in laboratory animals such as mice and flies, in which their effects can be detected through selective breeding. \u201cIf you look, more often than not, you find them,\u201d says Nitin Phadnis, an evolutionary geneticist at the University of Utah in Salt Lake City, who co-led the study.<\/p>\n Theoretical predictions suggest that sex distorters probably do exist in people as well, and that they could produce excesses of biological boys or girls at birth. But humans\u2019 long generation times and low birth rates as well as ethical issues have made such genes \u2014 and other \u2018selfish<\/a>\u2019 genetic elements , meaning that they bias their own transmission to future generations whether or not they improve an individual\u2019s biological fitness \u2014 difficult to spot.<\/p>\n To overcome such issues, Phadnis and his colleagues looked to the Utah Population Database, which contains genealogical, health and other data for people from the late eighteenth century through to the present.<\/p>\n In humans, biological sex is determined by the sex chromosome that fathers pass onto their offspring: each sperm cell typically carries either a Y or an X chromosome, but not both. The mother\u2019s egg cell, by contrast, usually carries a single X. Therefore, when sperm cells fertilize an egg cell, those with a Y chromosome<\/a> give rise to biological male offspring (people who have both an X and a Y) and those with an X chromosome<\/a> create biological female offspring (people with two Xs).<\/p>\n The researchers focused their search on families with a bias towards male offspring that could carry a sex distorter on the Y chromosome, because they are easier to detect in genealogical data. Other factors, such as deadly recessive mutations on the X chromosome, can lead to female-heavy families.<\/p>\n Using data on the recorded sex of 76,445 individuals in the Utah database, the researchers applied two statistical tests to identify families that were unlikely to be male-biased just by chance. Of more than 26,000 paternal lineages, just one family passed both tests. Across seven generations, the database documented 60 male offspring and 29 female \u2014 a ratio of more than 2:1.<\/p>\n War, famine and other extreme hardships can lead to male-skewed families, as can female infanticide. But there is no historical record of such drivers in Utah, says Phadnis. Misattributed births, for instance due to extramarital affairs, or polygamy \u2014 which was once common in Utah \u2014 are also unlikely to have affected the results, says co-author James Baldwin-Brown, also an evolutionary geneticist at the University of Utah. \u201cThe signal in this family is very strong.\u201d<\/p>\n Polly Campbell, an evolutionary biologist at the University of California, Riverside, says that the researchers make a good case that the Utah family\u2019s pattern of births is caused by a sex-distortion gene. But she isn\u2019t surprised to see one found in humans, because similar genes have been found in other well-studied animals. \u201cIt\u2019s really cool to see it,\u201d she says. \u201cWe don\u2019t usually expect sex-ratio distortion to be visible in natural populations.\u201d<\/p>\n However, Wynn Meyer, a population geneticist at Lehigh University in Bethlehem, Pennsylvania, isn\u2019t convinced that other explanations have been ruled out. Humans can, for instance, influence the sex of offspring by using preimplantation genetic testing on embryos conceived through in vitro<\/i> fertilization or through culturally transmitted practices, Meyer adds. <\/p>\n<\/div>\nBoys boys boys<\/h2>\n
Cultural relics<\/h2>\n