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Modern Genetic Analysis Chapter 5. Recombination of Genes
Recombination within a Gene

Meiosis in a diploid that is heterozygous for two different alleles of the same gene (say, a1/a2) will result in products half of which are a1 and half a2, according to the law of segregation. However, it is likely that these two mutant alleles will have their altered mutant site in different places within the gene, and if this is the case, there is a low probability that recombination will occur between the two sites. The products will be a wild-type chromatid and a mutant with two mutant sites:

These rare recombinants that occur at frequencies of 103 to 106 can be detected in some systems. For example, in fungi if the mutant alleles determine auxotrophy (for example, arginine requirement), then the wild types can be detected by simply spreading the meiotic products on medium containing no arginineonly the wild-type recombinants (which are prototrophic) will be able to grow to form colonies. Indeed mutant sites within a gene can be mapped by this method in an approach essentially identical with that used in chromosome mapping.

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