Question

Both husband and wife have normal vision though their fathers were colourblind and mothers did not have any gene for colourblindness. The probability of their daughters becoming colourblind is

• A. 50%
• B. 75%
• C. 0%
• D. 25%

Answer 1

The correct option is C 0%

Colourblindness 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 fathers will be X${}^c$Y. Since father transmits its X chromosome to the daughters, not to son, and mothers of both husband and wife are normal; the wife is carrier for the disease (X${}^c$X) and the husband is normal (XY). As daughters receive their one copy of X chromosome from father and other from mother; 50% daughters of carrier wife (X${}^c$X) and normal husband (XY) will get one copy of affected X chromosome from mother and second copy of normal X chromosome from normal father while rest 50% will get both normal copies from both parents. Since females with two copies of the affected gene show the disorder (X${}^c$X${}^c$ ) while females heterozygous for this trait will be normal but serve as a carrier of the disease; all the daughters will be normal. The correct answer is C.