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Modern Genetic Analysis Chapter 5. Recombination of Genes
Introduction

Hereditary variation is a key element in genetics. Two processes are responsible for genetic variation, recombination (the subject of this chapter) and mutation. Mutation is the ultimate source of genetic change; new alleles arise in all organisms, some spontaneously, others as a result of exposure to mutagenic agents in the environment. These new alleles become the raw material for a second level of variation, effected by recombination. As its name suggests, recombination is the process whereby alleles of different genes become associated in new combinations. In eukaryotes, most recombination takes place at meiosis. Meiotic recombination is a gene-shuffling process that takes heterozygous allele pairs and deals them out in different combinations into the products of meiosis (eggs and sperm in plants and animals). If for the sake of illustration we assume a diploid organism is heterozygous at only 10 genes (a gross underestimate in most organisms), then the total possible number of sex cell genotypes is 210 = 1024. The cellular processes responsible for recombination also occur when alleles are homozygous, but then the recombination event cannot be detected at the genetic level. In general, it should always be remembered that the geneticist depends on heterozygositythat is, genetic differencesin order to carry out genetic analysis.

In nature mutation and recombination provide the hereditary variation that is the raw material for natural selection to act on. However, geneticists make use of mutation and recombination to provide the raw material for experimental genetic analysis. One important use of recombination is to create specific genotypes needed for experimental purposes, by combining alleles present in established genetic stocks. Sometimes these interesting alleles have been isolated from nature, but more often they have been induced experimentally using mutation analysis. Recombination is also used to determine on which specific chromosomes genes of interest are located and to map their specific chromosomal positions. Both these uses are central aspects of genetics.

How can we define recombination? We have seen that it is a process of bringing together alleles in new combinations. However, geneticists define it in a precise way that can be applied experimentally. Since recombination in eukaryotes is largely a meiotic process, for the present we shall define it in terms of meiosis, as shown in the following message.

MESSAGE
Recombination is any process in a meiocyte that generates a haploid product of meiosis with a genotype differing from both the haploid genotypes that originally combined to form the diploid meiocyte. The product of meiosis so generated is called a recombinant.

This definition makes the important point that we detect recombination by comparing the output genotypes of meiosis with the input genotypes (Figure 5-1). The input genotypes are the two haploid genotypes that combined to form the meiocyte, the diploid cell that undergoes meiosis. In a plant or animal the focus is on gametes: the allele combinations of the gametes produced by an organism (output) are compared with the allele combinations in the gametes that originally fused to create the organism in the first place (input). In the example in Figure 5-1 the input genotypes are A B and a b, and the output genotypes are A B and a b plus two new genotypes, A b and a B, which are by definition recombinants.

Recombination is most simply illustrated in the life cycles of haploid fungi. As an example let's cross cultures of two different mutants of the fungus Neurospora. One is frost, a mutant phenotype of an allele f that causes a highly branched growth pattern resembling frost on a windowpane. The wild-type allele f+ causes spreading, less branched growth. The other mutant is yellow, caused by an allele y. The wild-type allele y+ promotes orange pigment production. The cross can be designated

Cells of these cultures unite to form a diploid meiocytethey represent meiotic "input." Meiosis takes place and results in the production of haploid products of meiosis called ascospores, according to the standard haploid life cycle of this fungus. The phenotypes of the cultures growing from the ascospores will be

The definition of recombination tells us that the spreading, orange and frost, yellow progeny are clearly recombinantsthey represent new combinations of alleles when compared with the meiotic input genotypes contributed by the cross parents.

There are two meiotic processes that cause recombination; these are independent assortment of genes on different chromosome pairs and crossing-over between genes on the same chromosome pair, and they are discussed in the following two sections.

© 1999 by W. H. Freeman and Company. All rights reserved.