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In his paper, Mr Gellatly demonstrates that it is likely men carry two different types of allele, which results in three possible combinations in a gene that controls the ratio of X and Y sperm On a larger scale, the number of men with more X sperm compared to the number of men with more Y sperm affects the sex ratio of children born each year.Ī gene consists of two parts, known as alleles, one inherited from each parent. The study suggests that an as-yet undiscovered gene controls whether a man’s sperm contains more X or more Y chromosomes, which affects the sex of his children. An X chromosome combines with the mother’s X chromosome to make a baby girl (XX) and a Y chromosome will combine with the mother’s to make a boy (XY). Men determine the sex of a baby depending on whether their sperm is carrying an X or Y chromosome. However, in women, you just can’t predict it," Mr Gellatly explains. We now know that men are more likely to have sons if they have more brothers but are more likely to have daughters if they have more sisters. "The family tree study showed that whether you’re likely to have a boy or a girl is inherited. The research, published online today by the journal Evolutionary Biology, involved a study of 927 family trees containing information on 556,387 people from North America and Europe going back to 1600. This means that a man with many brothers is more likely to have sons, while a man with many sisters is more likely to have daughters. The work by Corry Gellatly, a research scientist at the university, has shown that men inherit a tendency to have more sons or more daughters from their parents. A Newcastle University study involving thousands of families is helping prospective parents work out whether they are likely to have sons or daughters.