Question

If a normal woman marries a colour blind man then

• A. All their children will be colourblind.
• B. All their sons will be colourblind but the daughters will be normal.
• C. All the children will be normal.
• D. All daughters will be colourblind and sons will be normal.
• E. All sons and 50% of daughters are normal but 50% daughters will be colour blind.

Answer 1

Answer: (C)

All the children will be normal.

Colourblind is a X-linked recessive disorder. Since, males are hemizygous for chromosome, one copy of the affected gene in each cell is sufficient to cause the disorder (X${}^c$Y). Females have two X chromosomes and hence need two copies of the affected gene to cause the disorder (X${}^c$X${}^c$). Females heterozygous (X${}^c$X) for this trait be normal but serve as a carrier of the disease. According to the question, the colour blind man marries a normal vision woman.
Parent generation : (XX) x X${}^c$Y

Gametes : (X${}^c$Y) --> XX	X${}^c$	Y
X	X${}^c$X carrier girl	XY normal boy
X	X${}^c$X carrier girl	XY normal boy

Thus, all children of a normal woman and colour blind man will be normal. Because father transmit its X-chromosome to the daughter and daughter need two copies of the affected alleles to express the disease. The wild type normal allele mask the effect of father's affected allele in daughter and they show normal phenotype. Sons receive their X-chromosome from mother who is normal here, hence no disease in them. Correct option is "C".