Question

A boy with normal brother and colourblind sister has

• A. Father normal, mother colourblind
• B. Both normal
• C. Both colourblind
• D. Father colourblind, mother normal

Answer 1

The correct option is D Father colourblind, mother normal

Colour blindness is an X-linked recessive disorder. Father transmits its X chromosome to the daughters so the colour blind daughter has inherited her affected X chromosome from the father which means that father’s genotype is X${}^c$Y. Female has two copies of chromosomes, therefore, an X-linked recessive trait is expressed in homozygous recessive female only. This means that the daughter got one copy of affected X-chromosomes from each parent. Thus, genotype of father is X${}^c$Y and possible genotypes for her are X${}^c$X or XX. The brother is normal which means that the mother carries atleast one normal X-chromosome therefore genotypes of mother is X${}^c$X. A normal father cannot have a colour blind daughter (option A and B are incorrect) and a colour blind mother cannot have a normal son.

So, the correct answer is option D.