陈丽

A Newcastle University study involving thousands of families is helping prospective parents work out whether they are likely to have sons or daughters.英國纽卡斯尔大学一项有千万家庭参与的研究能够帮助准父母预测孩子性别。

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 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 youre 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 cant 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 mothers X chromosome to make a baby girl （XX） and a Y chromosome will combine with the mothers to make a boy （XY）.

The Newcastle University study suggests that an as-yet undiscovered gene controls whether a mans 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;

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.

“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 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.

科里·格拉特利是英国纽卡斯尔大学的一位研究员，他的研究表明，男性生男生女的倾向遗传自父母。这就意味着，兄弟多的男性更可能生儿子，而姐妹多的男性则更可能生女儿。

这项研究调查了927个家庭的族谱，包含北美和欧洲自1600年以来共556，387人的信息。

“这项族谱研究表明，生男生女的倾向是遗传而来的。我们现在明白了，男性如果兄弟多则容易生儿子，姐妹多则容易生女儿。然而，在女性身上却无法预测这一点。”格拉特利先生解释道。

男性的精子里携带的是X还是Y染色体，这一点决定了婴儿的性别。X染色体与母亲的X染色体结合，就会生出女孩（XX）;而Y染色体和母亲的X染色体结合则会生出男孩（XY）。

纽卡斯尔大学的这项研究表明，一个至今尚未发现的基因决定了男性的精子里含有更多的X还是Y染色体，而精子所含的染色体会影响孩子的性别。从更大范围来看，产生更多含X染色体精子的男性与产生更多含Y染色体精子的男性数量之比，影响着每年新生儿的性别比例。

生男还是生女？

一个基因包含两个部分，它们被称为等位基因，从父母双方各遗传一个。格拉特利先生在其论文中指出，很可能男性携带两个不同类型的等位基因，这就导致了基因在控制包含X和含Y染色体的精子比率时，会出现三种可能的组合。

第一种基因组合称为mm，携带此类组合的男性会产生更多含Y染色体的精子，从而更可能生儿子。……