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Random events in population genetics - What is random sampling?


In what sense is the sampling of gametes random?

We can see the exact meaning if we consider the first two offspring produced by an Aa parent. When it produces its first offspring, one gamete is sampled from its total gamete supply, and there is a 50% chance it will be an A and 50% that it will be an a .

Suppose it happens to be an A . The sense in which sampling is random is that it is no more likely that the next gamete to be sampled will be an a gene just because the last one sampled was an A : the chance that the next successful gamete will be an a is still 50%.

Coin flipping

Coin flipping is random in the same way: if you first flip a head, the chance that the next flip will be a head is still 1/2. The alternative would be some kind of 'balancing' system in which, after an A gamete had been successful in reproduction, the next successful gamete would be an a . If reproduction was like that, the gene frequency contributed by a heterozygote to its offspring would always be exactly 1/2A : 1/2a . Random drift would then be unimportant in evolution. In fact reproduction is not like that. The successful gametes are a random sample from the gamete pool.

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