Deficiency of recombinant genotypes in a trans test cross

With respect to the arrangement of linked loci in dihybrid individuals (AaBb), a trans arrangement refers to the presence of the alternate dominant and recessive alleles at either locus on either chromosome, (Ab/) and (aB/). The genotype of the dihybrid individual is written as (Ab//aB). Because crossover between loci is more or less uncommon, the offspring of a cross between a trans dihybrid and an ab//ab tester will produce a larger fraction of the parental phases (Ab//ab and aB//ab) than of the recombinant types (AB//ab and ab//ab). Note that the recombinant types are in cis, with the dominant alleles at both loci on the same chromatid.

Figure ©2002 by Griffiths et al.; all text material ©2014 by Steven M. Carr