Question

If all the sons of a couple are colour blind, then the

• A. Mother is colour blind.
• B. Mother is normal and father is colour blind.
• C. Mother is a carrier and father is colour blind.
• D. Mother is a carrier and father is normal.

Answer 1

Answer: (A)

Colour blindness is an X-linked recessive disorder. Since males are hemizygous for chromosome, One copy of the affected gene in males in each cell is sufficient to cause the disorder (X${}^c$Y ). Thus, the genotype of affected sons will be X${}^c$Y. Since father transmits its X chromosome to the daughters, not to son, so the disease condition of father would not affect the disease inheritance of his son. Son receives its X chromosome from the mother which is a carrier (X${}^c$X) which means mother should carry at least one copy of affected allele to have colour blind sons. Carrier mother (X${}^c$X) transmit a normal copy of X chromosome to half of her sons and another half will get the affected copy. Thus, there will be 50% probability of sons of carrier mother to have the disease. Only colour blind mother will deliver affected X-chromosome to all of her sons. Thus, the correct answer is option A.