Thus, 12% of the progeny exhibit new combinations of traits resulting from crossing over.

Coupling and Repulsion

In crosses for linked genes, the arrangement of alleles on the homologous chromosomes is critically important in determining the outcome of the cross. For example, consider the inheritance of two linked genes in the Australian blowfly, Lu-cilia cuprina. In this species, one locus determines the color of the thorax: 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+). These loci are located close together on the second chromosome. Suppose 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 fly. The dominant alleles for green thorax (p+) and brown puparium (b+) might reside on the same chromosome, 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

I 7.7 Crossing over between linked genes produces nonrecombinant and recombinant offspring. In this testcross, genes are linked and there is some crossing over. For comparison, this cross is the same as that illustrated in Figure 7.5.

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