英语听力文摘 近亲禁猎区
近亲之吻
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.
经典影片《激流四勇士》的粉丝可能还记得出名的五弦琴决斗场面吧!《激流四勇士》讲述的是四个都市朋友乘着独木舟沿着未开垦的乔治亚河冒险，一路上状况不断。在旅程之初，一个朋友挑战一个当地男孩，比赛谁弹五弦琴更快。曲终，镜头推向男孩，揭示男孩因近亲交配，身体不健全的事实。
表亲结婚这样的禁忌导致了这样的后果。遗传学者解释近亲更可能携带相同基因突变信息，他们的孩子先天缺陷的风险更大。
但是，这一风险似乎被夸大了。据一项研究显示，表亲婚后的孩子比非近亲婚后孩子先天有严重缺陷的几率仅高出1.7%至2.8%。尽管风险依然存在，但是没之前预想的严重。
长期来看，任何物种间的异血缘繁殖终会导致严重的先天缺陷。人口多的地方，个体间随机繁衍，恶性基因突变几率降低很多。反之，在人口小且封闭的地方，近亲交配很普遍，有血缘的夫妻携带相同基因突变的几率更大。
但是，恶性先天缺陷常见于家族多代异血缘交配。表亲生的孩子患先天缺陷的风险略高于无血缘夫妻生的孩子。虽然这样的风险不容忽视，但是表亲结婚并非注定基因灾难。
set out:出发
mutation：突变
contribute to：归因于
harbor：怀有
