Y marks the spot: a genetic explanation to why you have more brothers than sisters (or vice versa)

Have you every wondered why some families have more boys than girls or why more boys (or girls) are born on certain years? Recently we came a cross this study that provides a genetic link between the sons to daughter ratio in a family to genes passed on by the dad. 

Is it a boy or a girl?

Here’s an age-old question that expectant parents ask or get asked.  Most doctors will give you the standard answer – it’s a 50/50 chance because of the following. We all inherit an X chromosome from our moms. So the sex of the baby depends on the sex chromosome passed on by the dad. If the dad passes on a Y chromosome to the baby the sex will be male (XY), or if it’s another X chromosome the sex of the baby will be female (XX).

In theory, 50% of sperm carry an X chromosome and the other half a Y chromosome, making the chance of having a son or a daughter equally likely. But we all know biology is not always perfect. If you look at the sex ratio (ratio of male to female births) during specific years or in different populations, you will find that it can sometimes be biased towards one gender or another. For instance, years following world war II more boys were born than girls, suggesting that external factors (war in this case) can have an effect on the sex ratio.

Shifting the boy to girl ratio

Over the years researchers have shown associations between this bias in sex ratio and certain external factors like differences in lifestyle. However, the effects they were able to identify weren’t very large. This is why we find this particularly quite interesting, because it looks for a genetic explanation for the change in boy to girl ratio we see through history.

The goal of this study was to test the hypothesis that other genetic factors can influence the number of sperm that carry X versus Y chromosome. This would most certainly affect the ratio of boys versus girls.

They looked at the simplest scenario where the X to Y sperm ratio was affected by a single gene, with two different versions (alleles) designated m and f. The ideas was if a dad inherited genem they were likely to make more sperm with the Y chromosome, whereas if they had gene f they would likely produce more sperm with the X chromosome.

So, as you see in these hypothetical family trees, a dad who has the m gene (producing more Y sperm) will have more sons. These sons will inherit gene m, which means they will also have more sons, so on and so forth. On the flip side, if a dad has the f gene, they will have more daughters and pass on the f gene.

Following your dad’s genes

To prove their hypotheses the researchers made use of genealogical data and looked at 927 family trees (with information on over a ½ million people) from North America and Europe that dates back to 1600s.

Their results indeed showed that the likelihood of having a boy or a girl was indeed inherited through the dad’s genes. There was a link between men who had more brothers and the chance of having sons and vice versa (if a man has more sisters they are more likely to have daughters). Interestingly, there was no link between the sex ratio and the genetics of the mom.

Explaining the trends

They tested this theory even further using computer models to test observed trends. For example, when they artificially removed men form the family tree data before they had a chance to have children (mimicking a war time condition) there was in increase in the number of boys born the next generation. This fit in with their model of gene mcontrolling the X/Y sperm ratio.

It even explains why you would see an increase in the number of boys born after times of war. The idea is that more sons a family has the higher the chance of one (or more) returning from war. And when these men have children like their fathers are more likely to have sons. This would temporarily bias the sex ratio towards boys.

Are you a prospective parent curious whether your baby will more likely to be a girl or a boy? You may get a big clue by simply looking at your family tree.

Trends in Population Sex Ratios May be Explained by Changes in the Frequencies of Polymorphic Alleles of a Sex Ratio Gene

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