《那些古怪又让人忧心的问题》第71期:自体繁殖(5)

In the D&D system, this is like a “2x”multiplier. One copyof the gene can make you stronger, but two copies-double multipliers-lead to aserious disorder.

在《龙与地下城》系统中，这就像一个2倍倍率。具有一个这样的基因能让你变得更强壮，但两个配对染色体都是倍率染色体的话就会造成严重的缺陷。

These two diseases illustrate one reasonthat genetic diversity is important. Mutations pop up all over the place, butour redundant chromosomes help blunt this effect. By avoiding inbreeding, apopulation reduces the odds that rare and harmful mutations will pop up at thesame place on both sides of the chromosome.

这两种疾病能让我们了解到基因多样性有多重要。基因突变到处都在发生，但我们的冗余染色体能够帮助缓解突变带来的影响。通过禁止近亲繁殖，我们可以降低罕见却有害的突变在配对染色体两边同时发生的几率。

Inbreeding coefficient

近交系数

Biologists use a number called the“inbreeding coefficient” to quantify the percentage of someone's chromosomesthat are likely to be identical. A child from unrelated parents has aninbreeding coefficient of 0, while one who has a completely duplicated set ofchromosomes has an inbreeding coefficient of 1.

生物学家用一个叫作“近交系数”的数字来衡量一个人的配对染色体完全相同的可能性。一对没有亲缘关系的父母产下的孩子的近交系数为0，而如果配对染色体完全相同，则近交系数为1。

This brings us to the answer to theoriginal question. A child from a parent who self-fertilized would be like aclone of the parent with severe genetic damage. The parent would have all thegenes the child would, but the child wouldn't have all the genes of the the child's chromosomes would have their “partner”chromosomesreplaced by a copy of themselves.

现在我们可以回答最初的那个问题了。一个自体繁殖的家长产下的孩子就像是家长的克隆体，但是带有严重的基因缺陷。家长会具有孩子所拥有的所有基因，但孩子反过来不会拥有家长所有的基因。孩子体内有一半本应由另一位家长提供的染色体，会被这些染色体自己的副本所替代。

This means the child would have aninbreeding coefficient of 0.50. This is very high; it's what you would expectin a child of three generations of consecutive sibling marriages. According toD. S. Falconer's Introduction to Quantitative Genetics, an inbreedingcoefficient of 0.50 would result in an average of a 22-point reduction in IQand a 4-inch reduction in height at age ten. There would be a very good chancethat the resulting fetus would not survive to birth.

这意味着孩子的近交系数为0.5。这个数字非常高，连续三代近亲结婚产下的孩子才会有这么高的近交系数。根据道格拉斯•斯科特•福尔科纳的《数量遗传学导论》，0.5的近交系数会导致孩子在10岁时的智商平均下降22点，身高会减少10厘米。有很大几率这个孩子都不能活到出世。

This kind of inbreeding was famouslyexhibited by royal families attempting to keep their bloodlines “pure.” TheEuropean House of Hapsburg, a family of European rulers from the mid-secondmillennium, was marked by frequent cousin marriages, culminating in the birthof King Charles II of Spain.

这种近亲结婚在努力保持血统“纯正”的皇室家庭中最为常见。公元第二个千年中期统治欧洲的哈布斯堡皇室就是近亲结婚的典范，最后出了个西班牙国王卡洛斯二世。