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4 7.8 The arrangement of linked genes on a chromosome (coupling or repulsion) 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.

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.

Connecting Concepts 9

Relating Independent Assortment, Linkage, and Crossing Over

We have now considered three situations concerning 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 (AaBb) produces four types of gametes (AB, ab, Ab, and aB) in equal proportions: two types of nonrecombinants and two types of recombinants.

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

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