If a heterozygous parent (ab/ab) is testcrossed to a homozygous recessive (ab/ab), what will the progeny phenotypic ratio be if genes a and b are completely linked?

Phenotypic ratio 1:1:1:1

Explanation:

A test cross is a cross between an individual with heterozygous genotype and a homozygous recessive individual, in order to ascertain the actual genotype of the dominant parent i.e whether homozygous or heterozygous.

In this case, which is a dihybrid cross involving two genes A and B. The allele A is dominant over allele a, since it masks the expression in a heterozygous state. The same applies for allele B and b.

Therefore, allele A and B are dominant alleles while allele a and b are recessive alleles.

In a cross between an heterozygous parent (AaBb) and a homozygous recessive (aabb), each individual will undergo meiosis and produce gametes. According to Mendel's law of independent assortment, the alleles of each individual get sorted into gametes independent of one another.

The four possible gametes produced by each parent are:

AaBb: AB, Ab, aB, ab

aabb: ab, ab, ab, ab

Thus, using a punnet square (see attached image), the possible 16 progenies will possess four different genotypes: AaBb (4), Aabb (4), aaBb(4), aabb(4)

Considering the four distinct genotypes, the phenotypes will also differ. Hence, the phenotypic ratio: 4:4:4:4 ~ 1:1:1:1

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