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.