Boy or Girl

Science tells us that dad’s sperm determines whether a baby will be a boy or a girl. About half of his sperm will make a boy and half a girl. The sex of the baby depends on which sperm gets to the egg first. In a perfect world, it would all be up to chance. Both types of sperm would have an equal chance of reaching the egg first. And once fertilized, each type of egg would have an equal chance of developing fully into a baby.

But as you know, the world is not perfect. There is always genetics and environmental factors at play. Science now tells us that certain environmental factors can influence the sex of the child, including the egg's receptivity to the male's chromosomes. A number of studies have suggested that factors like nutrition and how the parents live can affect the odds of having a boy or a girl.  

As many people know (or have heard), the sperm that carry male chromosomes (which will make a boy) can swim faster because they carry less genetic material and are lighter. So though male and female sperm have equal muscular power in their flippers, the lighter ones will travel faster (just like a smaller cyclist will go faster than a larger one, if all other things are equal, because there's less mass to move).

But even if the male sperm outrace the female sperm, the male sperm will die off if the egg isn't in proper position for fertilization, leaving the female sperm clear for landing. Although male sperm are faster, female sperm are like tortoises; they keep pushing ahead slowly, hanging out, and waiting a few days. To have a boy, having sex around ovulation is critical, or the female sperm will win.

A recent Newcastle University study involving thousands of families is also helping prospective parents work out whether they are likely to have sons or daughters.

The work by Corry Gellatly, a research scientist at the university, has 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.

"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 explains.

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. On a larger scale, the number of men with more X sperm compared to the number of men with more Y sperm affects the sex ratio of children born each year.

Sons or daughters?

A gene consists of two parts, known as alleles, one inherited from each parent. In his paper, Mr. Gellatly demonstrates that it is 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.

“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,” says Newcastle University researcher Mr. Gellatly.

Journal Reference: Note: Content may be edited for style and length.

Gellatly et al. Trends in Population Sex Ratios May be Explained by Changes in the Frequencies of Polymorphic Alleles of a Sex Ratio GeneEvolutionary Biology, Dec 11, 2008; DOI: 10.1007/s11692-008-9046-3 

Newcastle University

Stanford University

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