《那些古怪又讓人憂心的問題》第71期:自體繁殖(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《龍與地下城》系統中，這就像一個2倍倍率。具有一個這樣的基因能讓你變得更強壯，但兩個配對染色體都是倍率染色體的話就會造成嚴重的缺陷。

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.

這兩種疾病能讓我們瞭解到基因多樣性有多重要。基因突變到處都在發生，但我們的冗餘染色體能夠幫助緩解突變帶來的影響。通過禁止近親繁殖，我們可以降低罕見卻有害的突變在配對染色體兩邊同時發生的機率。

Inbreeding coefficient

近交係數

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.

生物學家用一個叫作“近交係數”的數字來衡量一個人的配對染色體完全相同的可能性。一對沒有親緣關係的父母產下的孩子的近交係數爲0，而如果配對染色體完全相同，則近交係數爲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.

現在我們可以回答最初的那個問題了。一個自體繁殖的家長產下的孩子就像是家長的克隆體，但是帶有嚴重的基因缺陷。家長會具有孩子所擁有的所有基因，但孩子反過來不會擁有家長所有的基因。孩子體內有一半本應由另一位家長提供的染色體，會被這些染色體自己的副本所替代。

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.

這意味着孩子的近交係數爲0.5。這個數字非常高，連續三代近親結婚產下的孩子纔會有這麼高的近交係數。根據道格拉斯•斯科特•福爾科納的《數量遺傳學導論》，0.5的近交係數會導致孩子在10歲時的智商平均下降22點，身高會減少10釐米。有很大機率這個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.

這種近親結婚在努力保持血統“純正”的皇室家庭中最爲常見。公元第二個千年中期統治歐洲的哈布斯堡皇室就是近親結婚的典範，最後出了個西班牙國王卡洛斯二世。