Introduction to Genetic Analysis 629

Introduction to Genetic Analysis 629 - 44200_19_p611-642...

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Most mutation rates that have been determined are the sum of all mutations of A to any mutant form with a detectable effect. The mutation process is even slower if we consider the increase of a particular new allelic type. Any specifc base substitution is likely to be at least two orders of magnitude lower in frequency than the sum of all changes. Variation from recombination When a new mutation of a gene arises in a population, it occurs as a single event on a particular copy of a chromosome carried by some individual. But that chro- mosome copy has a particular allelic composition for all the other polymorphic genes on the chromosome. So, if the mutant allele a arose at the A locus on a chromo- some copy that already had the allele b at the B locus, then without recombination all gametes carrying the a allele would also carry the b allele in future generations. The population would then contain only the original A B haplotype and the new a b haplotype that arose from the mutation. Recombination between the A gene and the B gene in the double heterozygote A B/a b , how- ever, would produce two new haplotypes A b and a B . The consequence of repeated recombination be- tween genes is to randomize combinations of alleles of different genes. If the allele frequency of a at locus A is, say, .2, and the frequency of allele b at locus B is, say, .4, then the frequency of a b
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