A UK researcher has a new explanation for how the human race manages to keep a fairly even balance of males and females, despite massive deaths of young males in war and selective abortion¹ of female fetuses² in certain parts of the world.

Corry Gellatly, a research scientist at Newcastle University, proposes that there's a gene³ that determines whether a man will father more sons, more daughters, or equal numbers of each. When females are in short supply, they have a better chance of snagging a mate, and are thus more likely to pass the gene for fathering daughters on to their offspring. And when men are scarce, they have a better chance of mating and passing along the gene for having sons.

"It's kind of a counter-balancing mechanism," Gellatly explained. "You can't get a population that becomes too skewed toward males or too skewed toward females."

The ratio of male to female births jumped significantly at the end of each of the world wars in countries involved in the fighting. A number of hypotheses have been floated to explain why. One idea is that returning soldiers have extra-frequent sex with their partners, which could lead to fertilization earlier in the menstrual cycle, possibly making male births more likely.

After sorting through 927 family trees from North America and Europe, including 556,387 people in all, Gellatly proposes another explanation.

In an article published online in the journal Evolutionary⁴ Biology, the researcher suggests that men carry a gene that controls their ratio of X to Y sperm⁵, and thus the likelihood of their fathering sons or daughters.

Gellatly made a computer model simulating how the gene would act over 500 generations, and examined whether offspring sex ratios in the real-life family trees supported his hypothesis. Both experiments bore out his idea of a gene for gender⁶.

Almost all of our genes⁷ come in pairs, with one being inherited from each parent. Gellatly hypothesizes that the gender-controlling gene comes in a "male" and "female" version, with three possible combinations of the two.

A man could have a "male-male" gene, which would promote the formation of Y sperm; a "male-female" gene, which would cause him to produce about the same number of X and Y sperm; and a "female-female" gene, which would cause him to make more X sperm.

"The structure of the proposed gene is essentially⁸ very basic, and its function is simply to say 'produce more boys' or 'produce more girls,'" Gellatly explains.

 即便是很多年轻男性死于战场，女婴在某些地区被选择堕胎，人类仍能维持男女性别比例的基本平衡。这是为什么呢？英国一位研究人员对此做出了新的解释。

英国纽卡斯尔大学的研究员考利•盖里特利提出，是某种基因决定了男性后代的性别比例。当女性数量较少时，她们更容易找到配偶，因此生女孩的基因遗传给后代的可能性更大。而当男性数量较少时，他们在择偶时则处于优势，所以生男孩基因遗传给后代的几率更大。

盖里特利解释说：“这是一种平衡机制。就某一地区的人口而言，不会出现某种性别人口过多的现象。”

两次世界大战末期，参战国都曾出现“男婴潮”，当时关于这一现象的种种推理层出不穷。其中一种解释是回国士兵与伴侣的性生活更为频繁，使得女性的受孕期提前，而这有可能提高生男孩的几率。

盖里特利在对北美洲和欧洲927个族谱的556387人进行研究后做出了另一种解释。

他在发表于《进化生物学》网络版上的研究报告中指出，男性携带的一种基因控制着他们X和Y染色体的比例，从而影响他们后代的男女性别比例。

盖里特利制作了一个计算机模型来模拟这种基因历经500次遗传会如何发生作用，并调查现实生活中家族后代的性别比例是否符合他的假设。两项实验都验证了他有关基因决定后代性别的观点。

我们携带的几乎所有基因都是成对的，分别遗传自父亲和母亲。盖里特利假定，性别控制基因由“m”和“f”组成，它们有三种组合方式。

第一种方式是“mm”：携带“mm”基因的男性能产生更多的Y染色体精子；第二种方式是mf，能产生相同数量的X、Y染色体；第三种是ff，能产生更多的X染色体。

盖里特利解释说：“这种基因的结构从本质上来说很简单，它的功能也很好解释，那就是‘更容易生男还是女’。”