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Modern Genetic Analysis Chapter 5. Recombination of Genes
Crossing-Over

The second major way of producing recombinants is crossing-over. When homologous pairs of sister chromatids are observed in their "stacked" configuration at meiosis, cross-like structures called chiasmata (singular, chiasma) are seen between nonsister chromatids of the homologous pair. Each homologous pair shows one to several chiasmata. Chiasmata can be anywhere along the length of a pair but are seen in different positions in different meiocytes. Indeed the chiasmata seem to occur more or less randomly. These chiasmata are the positions of crossovers, points at which a breakage-and-reunion event has taken place between homologous nonsister chromatids (see Genetics in Process 5-1 on the following page). It is a process rather like splicing movie film; a cut is made at an identical position in both homologs and then opposite homologs are spliced together. Figure 5-7 shows two chiasmata and their associated crossovers.

Genes on the same chromosome are said to be linked for the obvious reason that they are physically linked or joined together by the segment of chromosome between them. A gene is always found at the same place on a chromosome (called its locus), so any alleles of that gene also will be at the same locus. Hence in describing two genes on the same chromosome we could say that
The two genes are linked.
Alleles of the two genes are linked.
The two gene loci are linked.

it all means the same thing.

In a dihybrid in which the genes are linked, a crossover can lead to the production of recombinants. In order to illustrate this process, we need some new terms. There are two kinds of dihybrids involving linked genes. The difference lies in whether or not the mutant alleles of the two genes are on the same parental homolog (cis dihybrid) or on different ones (trans dihybrid). Assuming that the recessive alleles are the mutant alleles, the two arrangements are as follows:

Linked genes are typed with the alleles on one homolog to the left of the slash and those on the other homolog to the right. The genes are listed in the same order on both sides of the slash. The cis and trans dihybrids above would be represented as A B / a b and A b / a B, respectively. (Note there is no punctuation symbol between alleles on the same homolog.)

How can we tell which of these two arrangements (cis or trans) we are working withthey contain the same alleles and look exactly the same at the phenotypic level. There are two ways to tell them apart. First, often the two parents of the dihybrid are pure-breeding strains, and this automatically dictates the allelic arrangement in their offspring. For example, if the two parents of a dihybrid are A b / A b and a B / a B, their gametes must be A b and a B, and therefore, the dihybrid must be A b / a B (trans). Conversely, if the parents are A B / A B and a b / a b, the dihybrid offspring must be in cis, A B / a b. Another way to determine if a dihybrid for linked genes is in cis or trans is to work backward from the progeny ratios, as we shall see below.

Using a cis dihybrid, let's see how a crossover leads to recombinants. Because crossovers occur more or less randomly along the paired homologs, in the population of meiocytes undergoing meiosis, some will have a crossover between the two genes and some will not. The set of four meiotic products will be different in each case, as shown in Figure 5-8. In meioses with a crossover, two of the haploid meiotic products will be recombinant, of genotypes A b and a B. (If it were a trans dihybrid, the recombinants would be A B and a b.) A typical pedigree for a cis dihybrid is shown in Figure 5-9, which should be compared with Figure 5-4, the comparable situation for independent assortment.

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