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Old 04-06-2009, 12:25 AM
Derleth Derleth is online now
Join Date: Apr 2000
Location: Missoula, Montana, USA
Posts: 20,294
A simple way to understand it is by analogy with cards: Every family has a deck of cards, representing their gene pool, and there are complex rules determining what comes out of any particular hand. A good rule to remember is that mostly pairs or other combinations actually have an observable outcome (expression in the phenotype).

Every child gets a hand created out of shuffling their parents' cards. Outbreeding, the usual way to select mates in our culture, ensures that the number of pairs is kept fairly low: Mom's hand isn't especially similar to dad's hand, so junior won't end up with a lot of repeats. Additionally, cards combinations that result in hands that never get reshuffled (people who never breed) will be destroyed and their component cards will be so lost in the shuffle they're essentially nonexistent. (This is why so few babies are born without lungs, for example.)

Inbreeding, however, keeps all the same cards circulating from one generation to the next. This results in pairs, flushes, and even inside straights, or, in genetic terms, a larger amount of phenotypes that are rare in the general population.

This doesn't always mean diseases. However, it results in diseases often enough that inbreeding is mostly a bad idea. Mainly, this is because we don't (and, to some extent, can't) do detailed genetic testing of our prospective mates. Hemophilia in European royalty is a famous example of what can result from a particularly intensive long-term human inbreeding project. One of the most dramatic examples was Charles II of Spain, also known as Charles the Hexed, who was descended from Joanna the Mad 14 different ways.
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Last edited by Derleth; 04-06-2009 at 12:28 AM.