Scientists explain how your partner’s genes determine the gender of your kids

There are so many old wives’ tales out there on how you can determine how many sons and daughters you will have, but scientists say there is a more accurate way to find out for sure – look at your partner’s siblings.

Researchers at a Newcastle University in the U.K. studied 927 family trees containing information on 556,387 people from North America and Europe going back to 1600.

Here’s what they found out…

You partner’s genes will determine what gender your children will be

This study, which spanned back hundreds of years, suggests that a man’s genes play an influencing role in whether or not he will have sons or daughters.

It turns out men inherit a tendency to have more sons or daughters from their parents. Therefore, a man with lots of brothers is more likely to have sons, while a man with many sisters is more likely to have daughters.

Men determine the gender of a baby, as he is the one carrying the X or Y chromosome in his sperm, which determines the sex. Men carry XY and women carry two X chromosomes in their ovaries. When the sperm and ovary meet, they either create XX female offspring or XY male offspring.

“The family tree study showed that whether you’re likely to have a boy or a girl is inherited,” says Corry Gellatly, a research scientist at the university.

“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. However, in women, you just can’t predict it.”

How genes play a part in whether you will have sons or daughters

Genes contain two parts, known as alleles, which is inherited from each parents.

There are three combinations that control the ratio of X and Y sperm a man carries.

1. The first, known as mm, produce more Y sperm and men with this combination have more sons.
2. The second, known as mf, produce the same amount of X and Y sperm and they men with this combination have an equal number of sons and daughters.
3. The third, known as ff, produce more X sperm and men with this combination have more daughters.

“The gene that is passed on from both parents, which causes some men to have more sons and some to have more daughters, may explain why we see the number of men and women roughly balanced in a population,” says Corry.

“If there are too many males in the population, for example, females will more easily find a mate, so men who have more daughters will pass on more of their genes, causing more females to be born in later generations.”

The boy baby boom after war

In the past, men tended to fight in wars and researchers say this may go on to explain why so many boy babies were born after the first World War – an extra two boys were born for every 100 girls in the U.K. compared to the year before the war started.

They explain that the odds were in favour of men with more sons seeing a son return from war and they were likely to go on to have a son themselves. Whereas, men with more daughters may have lost their only sons in war. This may explain why the men that survived were more likely to have male children.

Very interesting stuff! Using this research, are you more likely to have sons or daughters?

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