A man's genetic make-up may play a role in whether he has sons or daughters, a study of hundreds of years of family trees suggests.

Newcastle University researchers found men were more likely to have sons if they had more brothers and vice versa if they had more sisters.

They looked at 927 family trees, with details on 556,387 people from North America and Europe, going back to 1600.

The same link between sibling sex and offspring sex was not found for women.

The precise way that genes can influence baby sex remains unproven.

But the Evolutionary Biology study could clear up a long-standing mystery - a flood of boy babies after World War I.

While a woman will always pass a female "X" chromosome via her egg to her child, 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.

While the birthrate is almost 50/50, suggesting that overall men will deliver equal amounts of "X" sperm and "Y" sperm, scientists have suspected that in some individual couples the balance is shifted in favour of either boys or girls.

Various explanations have been put forward for this, ranging from differences in the time in the woman's monthly cycle that sex happens, to the amount of time that

sperm spend waiting in the testicles.

The Newcastle study, by Dr Corry Gellatly, is strong evidence that there is a genetic component.

He found that within families, boys with lots of brothers were more likely to have a higher number of sons themselves and those with lots of sisters were more likely to have lots of daughters.

War babies

Dr Gellatly said it was likely that a genetic difference affected the relative numbers of "X" and "Y" sperm within those produced by the man.

This gene, while only active in the man, could be carried by men and women.

"The family tree study showed that whether you're likely to have a boy or a girl is inherited."

He said that the effect was to actually balance out the proportion of men and women in the population.

"If there 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."

In the years after World War I, there was an upsurge in boy births, and Dr Gellatly said that a genetic shift could explain this.

The odds, he said, would favour 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.

However, this would mean that more boys would be fathered in the following generation, he said.

一項對于數百年家族宗系的研究表明，男人的遺傳構造對于他所生育后代的性別可能起一定的作用。鈕卡斯尓大學的研究人員發現，假如男人兄弟較多，他就可能生兒子，反之，如果姐妹多，就可能生女兒。

這種兄弟姐妹性別和孩子性別的聯系，在女人方面卻沒有發現。

基因影響嬰兒性別的確切方式至今尚未被證明出來。

但是進化生理學的研究能夠揭示出一個長久的秘密——在一次大戰之后，大量出生的是男孩。

女人總是將女性染色體X通過卵子遺傳給她的孩子，而實際上是由父親來決定孩子的性別：如果父親的精子里帶有另外一個X染色體，出生的就是女孩，如果帶的是Y染色體，出生的就是男孩。

當男女出生率為50/50時，表明總體上男人精子中的X和Y染色體相當。科學家猜測，在具體的個別夫妻中，平衡會轉換成有利于生男孩或是女孩。

對此有各種各樣的解釋，比如與性交發生于女人月經周期的時段有關，還比如與精液在睪丸中儲存的時間有關等。

由柯瑞.杰拉特醫生主持的鈕卡斯尓大學的研究有力地表明生育的性別與遺傳因素有關。

他發現在家庭中，有很多兄弟的男孩自己也可能生育較多的兒子，而姐妹較多的男孩會生育較多的女兒。

戰爭嬰孩

杰拉特醫生說，可能是遺傳的差別影響男人產生的帶X和Y染色體精子的相對數量。

這種僅在男人身上起作用的基因，男女都可能攜帶。

“家族宗系的研究顯示，你是生男還是生女是由遺傳決定的。”

他說，這種影響實際上起著平衡人口中男女比例的作用。

“假如在人口中男性過多，女性會比較容易找到伴侶，所以女兒較多的男人會有較多的機會遺傳基因，導致下一代有較多的女性出生。”

在第一次世界大戰后的若干年中，出生了大量的男孩。杰拉特醫生說遺傳的轉換可以給出解釋。

他說，幾率會對男孩多的父親有利，那些男孩各自都帶著“生男孩基因”，然后他們從戰爭中幸存下來后生了兒子。而女兒較多的父親可能遭遇到的是僅有的兒子在戰爭中死去。

他說，這意味著在下一代中男孩會比較多。

譯后感言：可是我們都是獨生子女，沒有兄弟姐妹，怎么來判別呢？這又是中國特色，難解……，