For many couples giving birth to a healthy baby boy or girl is a blessing regardless of the gender. However, some parent would like to know what determines the ability to conceive a boy or girl. Now new research suggests that there is an underlying genetic factor that could influence the sex of children. So what makes you have a boy or girl?
Father’s genetics hold the key
The Newcastle University conducted a study assessing the family tree of thousands of people. Lead by research scientist Corry Gellatly, the study investigated 927 family trees with information pertaining to 556, 387 people from Europe and North America documented since 1600.
The researchers found that men have a tendency to have more sons or daughters based on their parent’s reproductive history. In other words, a man with lots of sisters is more likely to father a daughter, while a man with lots of brothers is more likely to father a son. This suggests that the likelihood of having a boy or girl is inherited. The researchers found no correlation between having a boy or girl based on the mother’s family tree.
Men are responsible for determining the sex of a baby based on whether the fertilising sperm is carrying a Y or X chromosome. If a Y chromosome combines with the X chromosome from the mother a boy (YX) will be conceived. Conversely, if a X chromosome from the father combines with the X chromosome from the mother, a girl (XX) will be conceived.
This new research suggests that there is an undefined genetic control that influences the amount of X or Y chromosomes present within men’s sperm. On a wider scale, this study indicates that the sex ratio of children born annually is determined by the percentage of men with more X sperm compared with the percentage of men with more Y sperm.
Are you more likely to father a boy or girl?
Genes are made up of two alleles, with one inherited from each parent. Gellatly’s research suggests that men may carry two variations of alleles. In this situation, there would be three different gene combinations that control the ratio of Y and X sperm. These have been defined as:
mm – produce a higher percentage of Y sperm and therefore have more sons
mf – produce a similar number of Y and X sperm and therefore have roughly equal chance of fathering daughters and sons
ff – produce a higher percentage of X sperm and therefore have more daughters
Gellatly theorises that the genes inherited from a man’s parents will determine if he is likely to father more daughters or more sons. This may explain the roughly equal ratio of men and women in the population, with slight shifts over generations.
For example, in a situation where men outnumber women in the population, women will find a mate more easily. Consequently men who are genetically predisposed to having daughters will be able to pass on more of their genes, increasing the number of daughters born in the next generation.
Genetics and war time sex ratios
History shows that in countries that fought in the World Wars experienced an increase in male births in the following years. In the UK after World War I for every 100 girls born and extra two boys were born. This shift in sex ratios may be due to the underlying gene diversity revealed in Gellatly’s research.
During this war time, men with more sons were likely to see a son return from battle. Based on the inherited genes, these sons were more likely to father boys in the subsequent generation. Conversely men with more daughters were more likely to have lost their son(s) during the war, and these sons would have has a higher chance of fathering daughters.
Thus, men who survived the war had a higher probability of fathering sons, leading to the increased male birth rate following the end of World War I. This pattern has been repeated in other countries for as long as demographic statistics have been recorded.
Simplifying the genetics
A simplified diagram showing men either having only sons, or daughters, or equal numbers of each sex. In reality it isn’t as definitive.
Credit: Image from Newcastle University
This illustration is a simplified explanation of how the gene works. It shows a situation where men either have only daughters, only sons, or an equal number of sons and daughters. In reality it is not this clearly defined.
In family tree A, the grandfather carries the mm gene, fathering all male children. As he only has the m allele to pass on his sons are also more likely to carry the mm gene and father boys (as shown on the family tree). The grandsons will have the mf allele combination because they inherited a m allele from their father and a f allele from their mother. Therefore the great grandchildren will be an even mix of boys and girls.
In family tree B, the grandfather carries the ff gene, fathering all female children. These daughters also carry the ff allele combination because their mother and father were both ff. When one of the daughters has children with a man with the mm allele combination all the grandchildren are male. This is because the male determines the sex of the children. These grandsons with the mf allele combination will then have an equal number of daughters and sons (the great grandchildren).
This example shows that although the proposed gene described by Gellatly’s research has no effect in females, they do carry the gene and their children will inherit it.
The research by Gellately suggests that there is a gene that potentially controls whether a man’s sperm has more Y or X chromosomes. This will then influence the sex of his children. Therefore, based on this study the likelihood of having a boy or a girl is inherently determined. Men with lots of brothers are more likely to have sons, while men with lots of sisters are likely to have more daughters. However, it’s not possible to predict with accuracy in women whether they are more inclined to have a boy or girl.
Gellatly, C. (2008). Trends in Population Sex Ratios May be Explained by Changes in the Frequencies of Polymorphic Alleles of a Sex Ratio Gene. Evolutionary Biology. Volume 36, Issue 2, (pp. 190-200).