When a couple is expecting a baby, there are many speculations about the sex of the baby that is on the way. There are those who believe that the size and shape of the belly can predict it – this is not true – and even that the cravings you have in pregnancy can give you a clue if it will be a boy or a girl – this is not true either.
But then what determines the sex of the baby? Chance, mother, father, God, food, stress …? The truth is that it is not known for sure. It can be a combination of many factors, but according to research that looked at hundreds of years of family trees, the key is in the father’s genes .
Will it be a boy or a girl? The key is in the father’s genes
A study at the University of Newcastle in England, conducted by researcher Corry Gellatly, suggests that a man’s genes play an essential role in whether he has sons or daughters . Men inherit the tendency to have more sons or more daughters from their fathers. This means that a man with many brothers is more likely to have children, 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 dating back to 1600.
How to determine the sex of the baby
Men determine the sex of a baby depending on whether their sperm has an X or Y chromosome. An X chromosome combines with the mother’s X chromosome to form a girl (XX) and a Y chromosome will combine with the mother’s. to form a child (XY).
The Newcastle University study suggests that an undiscovered gene controls whether a man’s sperm contains more X or more Y chromosomes , which affects the sex of his children.
Sons or daughters?
A gene consists of two parts, known as alleles, one inherited from each parent. In her study, Gellatly shows that men are likely to have two different types of alleles , resulting in three possible combinations in a gene that controls the ratio of X and Y sperm in a man.
- Men with the first allele combination, known as mm, produce more Y sperm, and have more male children .
- Men with the second allele combination, known as mf, produce roughly equal numbers of X and Y sperm, and therefore have roughly equal numbers of sons and daughters .
- Men with the third allele combination, known as ff, produce more X sperm and will likely have more daughters than sons .
The unknown gene passed down from both parents, which is what causes some men to have more sons and others more daughters, may explain why we see roughly balanced numbers of men and women in a population.
And the mother?
The mother must have an influence in determining the sex of the baby, but it cannot be predicted based on whether they have more brothers or sisters, but they do carry the gene that they pass on to their offspring.
“The family tree study showed that whether you are likely to have a boy or a girl is inherited. We now know that men are more likely to have children if they have more brothers, but are more likely to have daughters if they have more sisters. However, in women, it just can’t be predicted, “explains Mr. Gellatly.
How does the gene work?
The trees below illustrate how the gene works. It is a simplified example, in which men have only sons, only daughters or the same number of each, although in reality it is less clear. It shows that although the gene has no effect on women, they also carry the gene and pass it on to their children.
Examples:
In the first family tree (A) the grandfather is mm , so all his children are boys. She only transmits the m allele, so her children are more likely to have the mm allele combination. As a result, those children may also have only children. Grandchildren have the mf allele combination, because they inherited an m from their father and an f from their mother. As a result, they have the same number of sons and daughters (the great-grandchildren).
In the second tree (B) the grandfather is ff , so all his daughters are women who have the allele combination ff because their father and mother are both ff. One of the girls has children of her own with a man who has the mm allele combination. That man determines the sex of the children, so the grandchildren are all men. Grandchildren have the mf allele combination, because they inherited an m from their father and an f from their mother. As a result, they have the same number of sons and daughters (the great-grandchildren).
Balance in the population
His theory explains why we see the number of men and women roughly balanced in a population.
“The gene that is passed from both parents can explain why we see that the number of men and women is more or less balanced in a population. If there are too many men in the population, for example, women will find a partner more easily, for example what men who have more daughters will pass on more of their genes, which will cause more women to be born in later generations ”, explains researcher Gellatly.
For example, the year after the end of World War I, two more boys were born for every 100 girls in the UK, compared to the year before the start of the war.
He argues that since men with more children were more likely to see a child return from war, those children were more likely to father children themselves because they inherited that tendency from their parents. In contrast, men with more daughters may have lost their only sons in war and those sons would have been more likely to father girls. This would explain why men who survived the war were more likely to have male children, resulting in the baby boy boom.
Photos | Amina Filkins on Pexels and iStockphoto
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