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Douglas_Knight comments on When does heritable low fitness need to be explained? - Less Wrong Discussion
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From the perspective of the parents of the gay couple, it's a 0% fitness cost, though.
Irrelevant.
We are discussing the dynamics of genes. If the gene has low fitness, it will be purged from the population and the rate of homosexuality will decline, even if the grandparents have full fitness. By using a donor, we are re-randomizing. Instead of a parent with the gene, we are replacing with a parent with a 50% chance of having the gene.
Similarly, what if two people with achondroplasia have children, repeatedly, until 2 survive infancy. Then the parents have fitness 1, but 1/4 of their children were homozygous and died in infancy. Their children are uniformly drawn from the remaining 3 children, consisting of 2 heterozygous, 1 homozygous normal. So the gene frequency has gone down from 1/2 in the parents to 1/3 in children. The parents have full fitness, but the gene has fitness only 2/3.