Chromosomes Explained

A study involving hundreds of years of family trees suggests that a man's genetic makeup may play a role in whether he has sons or daughters.
Men were more likely to have sons if they had more brothers and vice versa if they had more sisters, according to Newcastle University researchers.
The research, published online in the journal Evolutionary Biology, involved a study of 927 family trees containing information on 556,387 people from North America and Europe going back to 1600.
But scientists say the precise way that genes can influence baby sex is still unclear. But the Evolutionary Biology study could clear up a long-standing mystery - a flood of boy babies after World War I.
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 favor 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.
However, Dr. Corry Gellatly’s study shows strong evidence that there is a genetic component.
"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," Gellatly explains.
It was likely that a genetic difference affected the relative numbers of "X" and "Y" sperm within those produced by the man, Dr. Gellatly said. He said men and women could carry this gene, but it is only active in the man.
“The family tree study showed that whether you're likely to have a boy or a girl is inherited. The effect was to actually balance out the proportion of men and women in the population.”
He said 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.
Dr. Gellatly said that in the years after World War I, there was an upsurge in boy births and that a genetic shift could explain this.
“The odds would favor fathers with more sons - each carrying the "boy" gene - having a son return from war alive, compared with fathers who had more daughters, who might see their only son killed in action,” Gellatly said.
He said this would mean that more boys would be fathered in the following generation.