Question

A woman with normal vision but with colourblind father marries a colourblind man. The fourth child of the couple is a boy. This boy

• A. May or may not be colourblind
• B. Must be colourblind
• C. Must have normal vision
• D. Will be partially colourblind due to being heterozygous

Answer 1

The correct option is A May or may not be colourblind

Colourblindness is an X-linked recessive disorder. Father transmits its X chromosome to the daughters so the colourblind father will inherit its affected X chromosome to the daughter. Hence, the genotype of carrier female will be ($X^c$X) and that of colourblind man will be (($X^c$Y). As son gets its X-chromosome from the mother, the mother should be colourblind to have all affected sons. The carrier female transmits its one copy of affected X-chromosome to 50% sons. As we know that birth of a child is the independent event of the birth of a next/previous child which means that each time, there are 50% chances of their son to be colourblind. Option A is correct and C is incorrect. The normal copy of X-chromosome of the mother is transmitted to rest 50% sons who are normal; option B is incorrect. Males are hemizygous for X-chromosome which makes option D incorrect. The correct answer is option A.