Deficiency of recombinant genotypes in a Cis test cross

With respect to the arrangement of linked loci in dihybrid individuals (AaBb), a cis arrangement refers to the presence of both dominant alleles on one chromosome (AB/), and both recessive alleles on the other (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 cis 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 trans, with the dominant and recessive alleles on the same chromatid.

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