Coupling and Repulsion Theory

Once two genes are known to be linked, the next question is how their particular alleles are arranged on the two homologous chromosomes, because that arrangement, called the linkage phase, determines which combinations of traits appear together most often in the offspring. If the two dominant alleles of two different genes sit together on one chromosome (with the two recessive alleles together on its homolog), the arrangement is called coupling or the cis configuration; because the two dominant alleles are already riding on the same physical chromosome, they overwhelmingly travel together into the same gamete, so most gametes are either doubly dominant or doubly recessive. If instead a dominant allele of one gene sits on the same chromosome as the recessive allele of the other gene (with the opposite dominant-recessive pairing on the homolog), the arrangement is called repulsion or the trans configuration; here the two dominant alleles start out on different homologs, so most gametes carry one dominant and one recessive allele together. In Bateson and Punnett's sweet pea cross, the purple-flower/long-pollen parent supplied a chromosome carrying both dominant alleles together (cis), which is exactly why the parental-type combinations so heavily outnumbered the recombinant types in the testcr …