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Men are responsible for the gender of their children, not women according to science ... read on to find out how ...

Q: A lady in distress sent me this message: My mother-in-law and husband say that I am responsible for the birth of our two daughters. I am saddened by these allegations. My friend told me to contact you and find out the truth about it. Please let me know whether women are really responsible for the birth of girls? 

Krishna: I feel extremely sad when I read stories about making women solely responsible for the birth of a girl. 

It is actually the X chromosome contributed by a man that is responsible for the gender determination of a child! According to several studies, a man's genetic makeup may play a role in whether he has sons or daughters. A research paper, published online in the journal Evolutionary Biology in 2008 (the Newcastle study, 1), involved a study of 927 family trees containing information on 556,387 people from North America and Europe going back to 1600.

Scientists say the precise way that genes can influence baby sex is still unclear. A woman will always pass a female "X" chromosome via her egg to her child, but the father effectively "decides" the sex of the child by passing on either another "X" in his sperm, making a girl, or a "Y" chromosome, making a boy. The birthrate suggests that overall men will deliver equal amounts of "X" sperm and "Y" sperm, but scientists have suspected that in some individual couples the balance is shifted in favour of either boys or girls.

Explanations in the past range from differences in the time in the woman's monthly cycle when sex happens, to the amount of time that sperm spend waiting in the testicles. In most countries, for as long as records have been kept, more boys than girls have been born. In the UK and US, for example, there are currently about 105 males born for every 100 females.

Now the family tree study showed that whether you're likely to have a boy or a girl is inherited. Men inherit a tendency to have more sons or more daughters from their parents. 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.

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.

That was old research.

However, a recent study (2) shows offspring sex ratio is not a heritable trait. Century-old theories that having girls or boys 'runs in families' have been upended by a University of Queensland study, proving parents' genes do not determine their child's gender and concluded the sex of offspring is essentially random. 

The researchers found individuals don't have an innate tendency to have offspring of one or the other gender. The chances are more like 51 to 49 of having a boy, but the genes of the mother and father don't play any role. These findings have crucial implications for biological and evolutionary theories of offspring sex ratios.

A gene consists of two parts, known as alleles, one inherited from each parent. In this paper, the scientists demonstrate 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.

 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.

How does the gene work? It is a simplified example, in which men either have only sons, only daughters, or equal numbers of each, though in reality it is less clear cut. It shows that although the gene has no effect in females, they also carry the gene and pass it to their children.

In the first family tree  the grandfather is mm, so all his children are male. He only passes on the m allele, so his children are more likely to have the mm combination of alleles themselves. As a result, those sons may also have only sons (as shown). The grandsons have the mf combination of alleles, because they inherited an m from their father and an f from their mother. As a result, they have an equal number of sons and daughters (the great grandchildren).

In the second tree the grandfather is ff, so all his children are female, they have the ff combination of alleles because their father and mother were both ff. One of the female children has her own children with a male who has the mm combination of alleles. That male determines the sex of the children, so the grandchildren are all male. The grandsons have the mf combination of alleles, because they inherited an m from their father and f from their mother. As a result, they have an equal number of sons and daughters (the great-grandchildren).

Science, again, disproves these false allegations made by some people. Lady, if you still can't convince your husband and in-laws, please meet me in person along with your tormentors.  We need to talk to them and explain to them in detail to remove their misconceptions.


1.  Evolutionary Biology, DOI 10.1007/s11692-008-9046-3


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