25.2: Recombination
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The term “recombination” is used in several different contexts in genetics. In reference to heredity, recombination is defined as any process that results in gametes with combinations of alleles that were not present in the gametes of a previous generation (see Figure \(\PageIndex{2}\)). In ter chromosomal recombination occurs either through independent assortment of alleles whose loci are on different chromosomes (Chapter 6). In tra chromosomal recombination occurs through crossovers between loci on the same chromosomes (as described below). It is important to remember that in both of these cases, recombination is a process that occurs during meiosis (mitotic recombination may also occur in some species, but it is relatively rare). If meiosis results in recombination, the products are said to have a recombinant genotype . On the other hand, if no recombination occurs during meiosis, the products have their original combinations and are said to have a non-recombinant, or parental genotype . Recombination is important because it contributes to the genetic variation that may be observed between individuals within a population and acted upon by selection to produce evolution.
As an example of interchromosomal recombination, consider loci on two different chromosomes as shown in Figure \(\PageIndex{2}\). We know that if these loci are on different chromosomes, there are no physical connections between them, so they are unlinked and will segregate independently as did Mendel’s traits. The segregation depends on the relative orientation of each pair of chromosomes at metaphase. Since the orientation is random and independent of other chromosomes, each of the arrangements (and their meiotic products) is equally possible for two unlinked loci as shown in Figure \(\PageIndex{2}\). More precisely, there is a 50% probability for recombinant genotypes, and a 50% probability for parental genotypes within the gametes produced by a meiocyte with unlinked loci. Indeed, if we examined all of the gametes that could be produced by this individual (which are the products of multiple independent meioses), we would note that approximately 50% of the gametes would be recombinant, and 50% would be parental. Recombination frequency (RF) is simply the number of recombinant gametes, divided by the total number of gametes. A frequency of approximately 50% recombination is therefore a defining characteristic of unlinked loci. Thus the greatest recombinant frequency expected is ~50%.