生男生女取決於父親基因
Fathering sons, daughters may be in man's genes
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 abortion of female fetuses in certain parts of the world.
Corry Gellatly, a research scientist at Newcastle University, proposes that there's a gene 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 Evolutionary Biology, the researcher suggests that men carry a gene that controls their ratio of X to Y sperm, 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 gender.
Almost all of our genes 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 essentially very basic, and its function is simply to say 'produce more boys' or 'produce more girls,'" Gellatly explains.
即便是很多年輕男性死於戰場，女嬰在某些地區被選擇墮胎，人類仍能維持男女性別比例的基本平衡。這是爲什麼呢？英國一位研究人員對此做出了新的解釋。
英國紐卡斯爾大學的研究員考利•蓋裏特利提出，是某種基因決定了男性後代的性別比例。當女性數量較少時，她們更容易找到配偶，因此生女孩的基因遺傳給後代的可能性更大。而當男性數量較少時，他們在擇偶時則處於優勢，所以生男孩基因遺傳給後代的機率更大。
蓋裏特利解釋說：“這是一種平衡機制。就某一地區的人口而言，不會出現某種性別人口過多的現象。”
兩次世界大戰末期，參戰國都曾出現“男嬰潮”，當時關於這一現象的種種推理層出不窮。其中一種解釋是回國士兵與伴侶的性生活更爲頻繁，使得女性的受孕期提前，而這有可能提高生男孩的機率。
蓋裏特利在對北美洲和歐洲927個族譜的556387人進行研究後做出了另一種解釋。
他在發表於《進化生物學》網絡版上的研究報告中指出，男性攜帶的一種基因控制着他們X和Y染色體的比例，從而影響他們後代的男女性別比例。
蓋裏特利製作了一個計算機模型來模擬這種基因歷經500次遺傳會如何發生作用，並調查現實生活中家族後代的性別比例是否符合他的假設。兩項實驗都驗證了他有關基因決定後代性別的觀點。
我們攜帶的幾乎所有基因都是成對的，分別遺傳自父親和母親。蓋裏特利假定，性別控制基因由“m”和“f”組成，它們有三種組合方式。
第一種方式是“mm”：攜帶“mm”基因的男性能產生更多的Y染色體精子；第二種方式是mf，能產生相同數量的X、Y染色體；第三種是ff，能產生更多的X染色體。
蓋裏特利解釋說：“這種基因的結構從本質上來說很簡單，它的功能也很好解釋，那就是‘更容易生男還是女’。”
Vocabulary:
selective abortion:選擇性墮胎
counter-balance:使平衡
bear out:證實
