what determines the sex of a baby

What Determines the Gender of the Child - A Plus Topper

A Newcastle University study involving thousands of families is helping prospective parents work out whether they are likely to have a boy or a girl.

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 brothers is more likely to have daughters.

This research involves the study of 927 family trees containing information about 556 387 people from North America and Europe going back to 1600.

"family tree study showed that whether you are likely to have sons -laki 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 brothers, however, in women, you just can not predict it, " Mr Gellatly explains.

Men determine the sex of the baby depends on whether their sperm carrying the X or Y. An X chromosome combines with the mother's X chromosome to make a baby girl (XX) and a Y chromosome combines with the mother to make boys (XY).

Newcastle University study showed that the US-yet undiscovered gene controls whether a man's sperm contains more X or more Y chromosomes, which influence the sex of their 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 show that men tend to carry two types of allele, which results in three possible combinations of genes that controls the ratio of X and Y sperm;

"The gene is inherited from both parents, which causes some people 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, women 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 researcher Newcastle University Mr Gellatly.

More boys born after the war

in many countries that fought in the World War, there was a sudden increase in the number of boys born afterwards. Years after World War I ended, an extra two boys were born for every 100 girls in the UK, compared to the year before the war began. Gene, which Mr Gellatly has described in his research, could explain why this happens.

As the opportunities that support a man with more children see the child back from the war, their children are more likely to father boys themselves because they inherited the tendency from their fathers. In contrast, men with more daughters may have lost their sons in the war and their only child will be more likely to father girls. This would explain why people who survived the war were more likely to have a boy, resulting in an explosion-infant child.

In most countries, 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.

It is well documented that more males die in childhood and before they were old enough to have children. So in the same way that the gene may cause more boys to be born after the war, may also cause more boys to be born each year.

How do genes work?

trees (above) illustrates how genes work. This is a simple example, where men either have only sons, only daughters, or the same amount of each, though in reality less clear cut. This shows that although the gene has no effect on women, they also carry the gene and pass it on to their children.

In the first family tree (A) 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, their children may also be just a tad (as shown). Grandchildren have mf combination of alleles, because they inherited m from their father and f from their mother. As a result, they have the same number of sons and daughters (great grandchildren).

In the second tree (B) 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 girls have children with a man who has the mm combination of alleles. male that determines the sex of the children, so the grandchildren are all male. Grandchildren have mf combination of alleles, because they inherited m from their father and f from their mother. As a result, they have the same number of sons and daughters (great-granddaughter)

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