Do you want a baby boy or girl? These genetic factors could determine your offspring's gender

Having a son or a daughter isn't a game of chance, it seems.

Thinking of having a baby, or are you already pregnant? It seems the gender game doesn't depend on chance or luck, after all. A research study of hundreds of years of family trees found that a man's genes play a role in his production of male or female-producing sperm, and that men inherit a tendency to have more sons or daughters from their own parents. The long and short of it? A man with many brothers is likely to produce many sons, while a man with many sisters is likely to have more daughters.

Photo by Michal Bar Haim on Unsplash

The study by research scientist Corry Gellatly of Newcastle University involved thousands of families, and clearly displayed that men inherit a tendency to have more sons or daughters from their parents. Over 900 family trees containing information on over half a million people from North America and Europe going all the way back to the year 1600 were studied, showing remarkable results.

"The family tree study showed that whether you’re likely to have a boy or a girl is inherited. 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," Mr Gellatly explained.

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That's because men (or sperm donors in some cases) 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 study proves that yet-to-be discovered tertiary gene controls whether a man's sperm contains more X or Y chromosomes, which in turn affects the gendered outcome of his children.

In his research paper, Mr Gellatly demonstrates that it is likely men carry two different types of alleles, 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 than daughters.
• The second combination, 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 produce 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. 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,” details Newcastle University researcher Mr Gellatly.

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And guess what? More boys are born after wars.

Statistics also show that in many of the countries that fought in the World Wars, there was a sudden increase in the number of boys born afterwards. This is known as The Returning Soldier Effect, and is well-documented. The year after World War I ended, an extra two boys were born for every 100 girls in the United Kingdom compared to the year before the war started -- that uptick wasn't a coincidence, it seems. The gene, which Mr Gellatly has described in his research, could be the reason why this happened.

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. And, in most countries, for as long as records have been kept, more boys than girls have been born. In the United States, for example, there are currently about 105 males born for every 100 females. It is also well-documented that more males die in childhood and before they are old enough to have children, with several studies offering possible reasons for higher male infant mortality.

But there's also a competition

Fairly new research led by The University of Western Australia has shown that the social conditions that a male experiences while growing up can influence the amount of X and Y chromosome sperm that he produces as an adult, meaning it's not left entirely up to genetics.

The findings, which were published in Evolution Letters, provide a look into how exposure to other males during sexual maturity may influence the sex of his offspring later in life. The senior author of the study Dr. Renée Firman, an ARC Research Fellow in UWA's School of Biological Sciences, said that the results had important implications for ways in which a father's developmental history may influence the gender of his children beyond just the genetics of gendered sperm production.

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"We knew from our previous research on mice that male exposure to different social conditions during development leads to changes in fertility," Dr. Firman said. "We experimentally controlled the exposure that males had to other individuals during development. In one environment we exposed males to other males to create the perception that there was strong male competition for a female mate. In a second environment, males were exposed only to females, so male competition was negligible. We found that males that had grown up in a competitive social environment produced more sperm and better quality sperm."

But beyond just producing more sperm, "We found that males exposed to a competitive environment during development produced larger numbers of Y sperm compared to males subjected to a non-competitive environment," she explained.

What does this mean for you and your offspring?

If you're a man coming from a family of several boys who also spent a great deal of adolescent-age time with other boys (think organized sports, armed forces, etc.), the odds are stacked in your favor to have more sons. If you're a man from a family with many girls who didn't experience a lot of male-to-male exposure and competition, you're more likely to carry many daughters.

Of course, there are several variables here, and you may truly have a nearly-even chance of conceiving either a son or daughter.

Love this article or have your own family genetics anecdote to share? Drop your comments below.