Question

What shall be the ratio of heterozygous, homozygous and colour-blind hemizygous in offspring of a colour-blind husband and a carrier wife?
a. 1:1:2
b. 1:1:1
c. 2:1:1
d. 1:2:1

Answer 1

A test cross can be defined as a cross that happens between an individual with an unknown genotype with a homozygous recessive genotype. This is a method through which the genotype of an organism can be determined. The set of heritable genes of an individual is called the genotype.

Complete answer:
A test cross is a process of genetics through which the genotype of an organism can be determined. This system was first introduced by Gregor Mendel. The test cross involves the breeding of an individual with a phenotypically recessive individual. The obtained offspring phenotypes proportions are further analysed as well as the zygosity of the offspring is determined. Zygosity is the degree of similarity of the alleles in an organism. There are three types of zygosity which are heterozygous, homozygous, and hemizygous.

When identical alleles of the gene are found on both the chromosomes or homologous chromosomes then the cell will be called homozygous for a particular gene. A cell will be called heterozygous at a gene locus when it bears two different alleles of a gene which are one wild-type allele and one mutant allele. A cell will be homozygous when there is only one copy allele of the gene present.

Colour blindness is a type of an X-linked recessive disorder. The genotype of the carrier wife will be X’X and on the other hand, the genotype of colour-blind man will be X’Y. When these two genotypes cross the parent generation will be X’X and X’Y. After the cross, the four gametes will be as follows:
X’X, which is a homozygous colour blind girl
X’X, which is a heterozygous carrier girl
X’Y, which is a hemizygous colour blind boy
XY, which is a normal boy

So, the ratio of heterozygous, homozygous and colour-blind hemizygous in offspring will be 1:1:1.
Hence, the correct answer is option (b).

Note: A test cross is a system that takes between an individual with an unknown genotype with a homozygous recessive genotype. Through this process the genotype of an organism can be determined. In the given problem, the genotype of the female is X’X and the genotype of the male is X’Y and the produced gametes will be X’X (a homozygous colour blind girl), X’X (a heterozygous carrier girl) X’Y (a hemizygous colour blind boy), and XY (a normal boy). Then the ratio of heterozygous, homozygous and colorblind hemizygous in offspring will be 1:1:1.