pierceessentials3e

Coupling and Repulsion

In crosses for linked genes, the arrangement of alleles on the homologous chromosomes is critical in determining the outcome of the cross. For example, consider the inheritance of two genes in the Australian blowfly, Lucilia cuprina. In this species, one locus determines the color of the thorax: a purple thorax (p) is recessive to the normal green thorax (p⁺). A second locus determines the color of the puparium: a black puparium (b) is recessive to the normal brown puparium (b⁺). The loci for thorax color and puparium color are located close together on the same chromosome. Suppose that we test-cross a fly that is heterozygous at both loci with a fly that is homozygous recessive at both. Because these genes are linked, there are two possible arrangements on the chromosomes of the heterozygous parent. The dominant alleles for green thorax (p⁺) and brown puparium (b⁺) might reside on one chromosome of the homologous pair, and the recessive alleles for purple thorax (p) and black puparium (b) might reside on the other homologous chromosome:

This arrangement, in which wild-type alleles are found on one chromosome and mutant alleles are found on the other chromosome, is referred to as the coupling or cis configuration. Alternatively, one chromosome might carry the alleles for green thorax (p⁺) and black puparium (b), and the other chromosome might carry the alleles for purple thorax (p) and brown puparium (b⁺):

This arrangement, in which each chromosome contains one wild-type and one mutant allele, is called the repulsion or trans configuration. Whether the alleles in the heterozygous parent are in coupling or repulsion determines which phenotypes will be most common among the progeny of a testcross.

When the alleles are in the coupling configuration, the most numerous progeny types are those with a green thorax and brown puparium and those with a purple thorax and black puparium (Figure 5.7a). However, when the alleles of the heterozygous parent are in repulsion, the most numerous progeny types are those with a green thorax and black puparium and those with a purple thorax and brown puparium (Figure 5.7b). Notice that the genotypes of the parents in Figure 5.7a and 5.7b are the same (p⁺pb⁺b × pp bb) and that the dramatic difference in the phenotypic ratios of the progeny in the two crosses results entirely from the configuration—coupling or repulsion—of the chromosomes. Knowledge of the arrangement of the alleles on the chromosomes is essential to accurately predict the outcome of crosses in which genes are linked.

Figure 5.7: The arrangement (coupling or repulsion) of linked genes on a chromosome affects the results of a testcross. Linked loci in the Australian blowfly (Lucilia cuprina) determine the color of the thorax and that of the puparium.

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CONCEPTS

In a cross, the arrangement of linked alleles on the chromosomes is critical for determining the outcome. When two wild-type alleles are on one homologous chromosome and two mutant alleles are on the other, they are in the coupling configuration; when each chromosome contains one wild-type allele and one mutant allele, the alleles are in repulsion.

CONCEPT CHECK 2

The following testcross produces the progeny shown: Aa Bb × aa bb → 10 Aa Bb, 40 Aa bb, 40 aa Bb, 10 aa bb. Were the A and B alleles in the Aa Bb parent in coupling or in repulsion?

Repulsion

CONNECTING CONCEPTS

Relating Independent Assortment, Linkage, and Crossing Over

We have now considered three possible situations among genes at different loci. First, the genes may be located on different chromosomes; in this case, they exhibit independent assortment and combine randomly when gametes are formed. An individual heterozygous at two loci (Aa Bb) produces four types of gametes (A B, a b, A b, and a B) in equal proportions: two types of nonrecombinants and two types of recombinants. In a testcross, these gametes will result in four types of progeny in equal proportions (Table 5.1).

Second, the genes may be completely linked—meaning that they are on the same chromosome and lie so close together that crossing over between them is absent. In this case, the genes do not recombine. An individual heterozygous for two completely linked genes in the coupling configuration

produces only nonrecombinant gametes containing alleles A B or a b. The alleles do not assort into new combinations such as A b or a B. In a testcross, completely linked genes will produce only two types of progeny, both nonrecombinants, in equal proportions (see Table 5.1).

The third situation, incomplete linkage, is intermediate between the two extremes of independent assortment and complete linkage. Here, the genes are physically linked on the same chromosome, which prevents independent assortment. However, occasional crossovers break up the linkage and allow the genes to recombine. With incomplete linkage, an individual heterozygous at two loci produces four types of gametes—two types of recombinants and two types of nonrecombinants—but the nonrecombinants are produced more frequently than the recombinants because crossing over does not take place in every meiosis. In the testcross, these gametes result in four types of progeny, with the nonrecombinants more frequent than the recombinants (see Table 5.1).

TABLE 5.1 Results of a testcross (Aa Bb × aa bb) with complete linkage, independent assortment, and linkage with some crossing over

Situation	Progeny of Testcross
Independent assortment	Aa Bb (nonrecombinant)	25%
	aa bb (nonrecombinant)	25%
	Aa bb (recombinant)	25%
	aa Bb (recombinant)	25%
Complete linkage (genes in coupling)	Aa Bb (nonrecombinant)	50%
Complete linkage (genes in coupling)	aa bb (nonrecombinant)	50%
Linkage with some crossing over (genes in coupling)		more than 50%
Linkage with some crossing over (genes in coupling)		less than 50%

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Earlier in the chapter, the term recombination was defined as the sorting of alleles into new combinations. We’ve now considered two types of recombination that differ in their mechanisms. Interchromosomal recombination takes place between genes located on different chromosomes. It arises from independent assortment—the random segregation of chromosomes in anaphase I of meiosis—and is the kind of recombination that Mendel discovered while studying dihybrid crosses. A second type of recombination, intrachromosomal recombination, takes place between genes located on the same chromosome. This recombination arises from crossing over—the exchange of genetic material in prophase I of meiosis. Both types of recombination produce new allele combinations in the gametes, so they cannot be distinguished by examining the types of gametes produced. Nevertheless, they can often be distinguished by the frequencies of types of gametes: interchromosomal recombination produces 50% nonrecombinant gametes and 50% recombinant gametes, whereas intrachromosomal recombination frequently produces more than 50% nonrecombinant gametes and less than 50% recombinant gametes. However, when the genes are very far apart on the same chromosome, crossing over takes place in every meiotic division, leading to 50% recombinant gametes and 50% nonrecombinant gametes. This result is the same as that for the independent assortment of genes located on different chromosomes (interchromosomal recombination). Thus, the intrachromosomal recombination of genes that lie far apart on the same chromosome and interchromosomal recombination are phenotypically indistinguishable.