Question

Which of the following is responsible for passing the gene of colour blindness to a boy born to a colour blind father and normal mother?

• A. His father
• B. His mother
• C. Either his father or his mother
• D. Both parents, this is an example of incomplete dominance
• E. It cannot be determined

Answer 1

The correct option is B His mother

Colour blindness is an X-linked recessive traits and always follow criss-cross inheritance. To have an affected son, the mother should carry atleast one copy of $X^c$ chromosome to pass it on to her sons. Here, the carrier mother ($X^c$X) has passed the disorder to the son. The affected father never passes the trait to son as the son obtains Y chromosome from father and X chromosome from mother. Thus, the correct answer is B.