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chromosome, is referred to as coupling, or the cis configuration. Alternatively, one chromosome might bear the alleles for green thorax (p+) and black puparium (b), and the other chromosome would 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 green thorax and brown puparium and those with purple thorax and black puparium ( FIGURE 7.8a); but, when the alleles of the heterozygous parent are in repulsion, the most numerous progeny types are those with green thorax and black pupar-ium and those with purple thorax and brown puparium ( FIGURE 7.8b). Notice that the genotypes of the parents in Figure 7.8a and b are the same (p+p b+b X 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. It is essential to know the arrangement of the alleles on the chromosomes to accurately predict the outcome of crosses in which genes are linked.

là] AJJéteidh üWipJlnv -tüBfidii rdtiun hurí» hruwi pL purium

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