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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.
一项对于数百年家族宗系的研究表明，男人的遗传构造对于他所生育后代的性别可能起一定的作用。钮卡斯尓大学的研究人员发现，假如男人兄弟较多，他就可能生儿子，反之，如果姐妹多，就可能生女儿。
这种兄弟姐妹性别和孩子性别的联系，在女人方面却没有发现。
基因影响婴儿性别的确切方式至今尚未被证明出来。
但是进化生理学的研究能够揭示出一个长久的秘密——在一次大战之后，大量出生的是男孩。
女人总是将女性染色体X通过卵子遗传给她的孩子，而实际上是由父亲来决定孩子的性别：如果父亲的精子里带有另外一个X染色体，出生的就是女孩，如果带的是Y染色体，出生的就是男孩。
当男女出生率为50/50时，表明总体上男人精子中的X和Y染色体相当�？蒲Ъ也虏�，在具体的个别夫妻中，平衡会转换成有利于生男孩或是女孩。
对此有各种各样的解释，比如与性交发生于女人月经周期的时段有关，还比如与精液在睾丸中储存的时间有关等。
由柯瑞.杰拉特医生主持的钮卡斯尓大学的研究有力地表明生育的性别与遗传因素有关。
他发现在家庭中，有很多兄弟的男孩自己也可能生育较多的儿子，而姐妹较多的男孩会生育较多的女儿。
战争婴孩
杰拉特医生说，可能是遗传的差别影响男人产生的带X和Y染色体精子的相对数量。
这种仅在男人身上起作用的基因，男女都可能携带。
“家族宗系的研究显示，你是生男还是生女是由遗传决定的。”
他说，这种影响实际上起着平衡人口中男女比例的作用。
“假如在人口中男性过多，女性会比较容易找到伴侣，所以女儿较多的男人会有较多的机会遗传基因，导致下一代有较多的女性出生。”
在第一次世界大战后的若干年中，出生了大量的男孩。杰拉特医生说遗传的转换可以给出解释。
他说，几率会对男孩多的父亲有利，那些男孩各自都带着“生男孩基因”，然后他们从战争中幸存下来后生了儿子。而女儿较多的父亲可能遭遇到的是仅有的儿子在战争中死去。
他说，这意味着在下一代中男孩会比较多。
译后感言：可是我们都是独生子女，没有兄弟姐妹，怎么来判别呢？这又是中国特色，难解……，
