Boy or girl? It's in the father's genes

December 11, 2008

A Newcastle University study involving thousands of families is 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, published online today by 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.

"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;"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.

More boys born after the wars

In many of the countries that fought in the World Wars, there was a sudden increase in the number of boys born afterwards. The year 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 started. The gene, which Mr Gellatly has described in his research, could explain why this happened.

As the odds were in favour of men with more sons seeing a son return from the war, those sons were more likely to father boys themselves because they inherited that tendency from their fathers. In contrast, men with more daughters may have lost their only sons in the war and those sons would have been more likely to father girls. This would explain why the men that survived the war were more likely to have male children, which resulted in the boy-baby boom.

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.

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

How does the gene work?

The trees (below) illustrate how the gene works. 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 (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, 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 (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 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).

http://www.ncl.ac.uk/press.office/press.release/photos/516diagram_06.1.jpg
-end-
Academic paper: Trends in Population Sex Ratios May be Explained by Changes in the Frequencies of Polymorphic Alleles of a Sex Ratio Gene. Corry Gellatly.

Published in: Evolutionary Biology, DOI 10.1007/s11692-008-9046-3

Newcastle University

Related Sperm Articles from Brightsurf:

Nut consumption causes changes in sperm DNA function
Researchers have evaluated for the first time the effect of a short/middle-term consumption of a mixture of tree nuts (almonds, hazelnuts and walnuts) on sperm DNA methylation patterns in healthy individuals reporting eating a Western-style diet.

Collecting sperm from Covid-19 patients
How does Covid-19 affect sperm and thus the next generation┬┤s immune system?

Paleontology -- The oldest known sperm cells
An international team of paleontologists has discovered giant sperm cells in a 100-million year-old female ostracod preserved in a sample of amber.

Research captures how human sperm swim in 3D
Using state-of-the-art 3D microscopy and mathematics, Dr Hermes Gadêlha from the University of Bristol, Dr Gabriel Corkidi and Dr Alberto Darszon from the Universidad Nacional Autonoma de Mexico, have reconstructed the movement of the sperm tail in 3D with high-precision.

Overweight and obesity are associated with a low sperm quality
Researchers from the Rovira i Virgili University in collaboration with researchers from the University of Utah have carried out a systematic review and meta-analysis evaluating the association between adiposity (normal weight, overweight, obesity, and low weight) and the sperm quality.

Diet has rapid effects on sperm quality
Sperm are influenced by diet, and the effects arise rapidly.

Sperm may offer the uterus a 'secret handshake'
Why does it take 200 million sperm to fertilize a single egg?

Long duration of sperm freezing makes no difference to live birth rates in large sperm bank study
Despite a time limit imposed in many countries on the freeze-storage of sperm, a new study from China has found that the long-term cryopreservation of semen in a sperm bank does not affect future clinical outcomes.

An important function of non-nucleated sperm
Some animals form characteristic infertile spermatozoa called parasperm, which differ in size and shape compared to fertile sperm produced by single males.

DNA of sperm taken from testicles of infertile men 'as good as sperm from fertile men'
Scientists have found that sperm DNA from the testicles of many infertile men is as good as that of ejaculated sperm of fertile men.

Read More: Sperm News and Sperm Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.