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Father's genes determine gender of child
June 11, 2018, 11:24 am

A new study will help prospective parents work out whether they are likely to have sons or daughters.

Researchers at New Castle University in the UK have shown that men inherit a tendency to have more sons or more daughters from their parents. 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 research involved a study of 927 family trees containing information on 556,387 people from North America and Europe going back to 1600.

Men determine the sex of a baby depending on whether their sperm is carrying an X or Y chromosome. 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). The Newcastle University 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.

A gene consists of two parts, known as alleles, one inherited from each parent. The research team at Newcastle say that it 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.

Men with the first combination, known as mm, produce more Y sperm and have more sons.

The second, known as mf, produce a roughly equal number of X and Y sperm and have an approximately equal number of sons and daughters. The third, known as ff produce more X sperm and have more daughters.

In many of the countries that fought in the World Wars, there was a sudden increase in the number of boys born afterwards. The so far undiscovered gene, could explain why this happened.

As the odds were in favor of men with more sons seeing a son return from the war, those sons were more likely to father boys themselves because they inherited that tendency from their fathers. In contrast, men with more daughters may have lost their only sons in the war and those sons would have been more likely to father girls. This would explain why the men that survived the war were more likely to have male children, which resulted in the boy-baby boom.

The figure (above) illustrates how the gene works. It is a simplified example, in which men either have only sons, only daughters, or equal numbers of each, though in reality it is less clear cut. It shows that although the gene has no effect in females, they also carry the gene and pass it to their children.

In the first family tree (A) the grandfather is mm, so all his children are male. He only passes on the m allele, so his children are more likely to have the mm combination of alleles themselves. As a result, those sons may also have only sons (as shown). The grandsons have the mf combination of alleles, because they inherited an m from their father and an f from their mother. As a result, they have an equal number of sons and daughters (the great grandchildren).

In the second tree (B) the grandfather is ff, so all his children are female, they have the ff combination of alleles because their father and mother were both ff. One of the female children has her own children with a male who has the mm combination of alleles. That male determines the sex of the children, so the grandchildren are all male. The grandsons have the mf combination of alleles, because they inherited an m from their father and f from their mother. As a result, they have an equal number of sons and daughters (the great-grandchildren).


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