英語聽力文摘 近親禁獵區(qū)
近親之吻
Fans of classic movies will remember the famous dueling banjos scene in Deliverance, the story of four city friends who take an ill-fated boating trip down a backwoods Georgia river. As the friends set out, one of the friends challenges a local boy with a banjo to see who can play faster. At the end of the song, the camera zooms in to reveal the boy’s diseased, inbred features.
Such images have contributed to the taboo against marriage between first cousins. Geneticists explain that since closely related persons are more likely to harbor identical genetic mutations, their children have a greater chance of being born with defects.
But such risks might be overstated. According to one study, children from first cousin marriages are only 1.7 to 2.8 percent more likely to be born with serious birth defects than children of conventional marriages. Although the risk is still significant, it is lower than previously thought.
Prolonged interbreeding in any species can eventually lead to serious birth defects. Harmful genetic mutations are less likely to appear in large populations where individuals reproduce with random partners due to the wider variety of genetic mutations. But in small, isolated populations where inbreeding is common, there’s a greater chance that related partners carry the same mutations.
However, harmful effects are more common in groups with a history of interbreeding over several generations. First cousins that are not from such groups run only a slightly greater risk of producing children with birth defects than non-related couples. Although such risks should not be ignored, first cousin marriage is not necessarily a recipe for genetic disaster.
經典影片《激流四勇士》的粉絲可能還記得出名的五弦琴決斗場面吧!《激流四勇士》講述的是四個都市朋友乘著獨木舟沿著未開墾的喬治亞河冒險，一路上狀況不斷。在旅程之初，一個朋友挑戰(zhàn)一個當?shù)啬泻ⅲ荣愓l彈五弦琴更快。曲終，鏡頭推向男孩，揭示男孩因近親交配，身體不健全的事實。
表親結婚這樣的禁忌導致了這樣的后果。遺傳學者解釋近親更可能攜帶相同基因突變信息，他們的孩子先天缺陷的風險更大。
但是，這一風險似乎被夸大了。據(jù)一項研究顯示，表親婚后的孩子比非近親婚后孩子先天有嚴重缺陷的幾率僅高出1.7%至2.8%。盡管風險依然存在，但是沒之前預想的嚴重。
長期來看，任何物種間的異血緣繁殖終會導致嚴重的先天缺陷。人口多的地方，個體間隨機繁衍，惡性基因突變幾率降低很多。反之，在人口小且封閉的地方，近親交配很普遍，有血緣的夫妻攜帶相同基因突變的幾率更大。
但是，惡性先天缺陷常見于家族多代異血緣交配。表親生的孩子患先天缺陷的風險略高于無血緣夫妻生的孩子。雖然這樣的風險不容忽視，但是表親結婚并非注定基因災難。
set out:出發(fā)
mutation：突變
contribute to：歸因于
harbor：懷有
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