Answer:
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
Knowing these genes are completely linked, after crossing an AB/ab individual with another ab/ab parent, the phenotypic ratio is 1:1 ⇒ 1/4 AB and 1/4 ab.
Many genes are placed far away from each other in the chromosome, so they segregate independently after crossing-over events.
However, some other genes are too close to each other and they do not segregate independently. These are the linked genes.
Linked genes do not exhibit an independent distribution.
Instead, they are inherited together more frequently because there are fewer probabilities of crossing over to occur between them.
In the exposed example, a heter0zyg0us parent (AB/ab) is crossed to a h0m0zyg0us recessive (ab/ab).
These genes are completely linked, meaning that they will not separate during gamete formation.
Cross: AB/ab x ab/ab
Gametes: AB ab ab ab
Punnett square) AB ab
ab AB/ab ab/ab
ab AB/ab ab/ab
F1) Genotype
2/4 = 1/2 = 50% of the progeny will express heter0zyg0us genotype, AB/ab
2/4 = 1/2 = 50% of the progeny will express h0m0zyg0us recessive genotype, ab/ab
Phenotype
2/4 = 1/2 = 50% of the progeny will express the phenotype coded by AB genes.
2/4 = 1/2 = 50% of the progeny will express the phenotype coded by ab genes.
The phenotypic ratio is 1:1 ⇒ 1/4 AB and 1/4 ab
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