Question

A colour-blind man marries a woman with normal vision. The offspring will be
A. All sons colour blind
B. All daughters colour blind
C. Both A and B
D. All sons and daughters normal but daughters are carriers

Answer 1

Colour blindness: Colour blindness is a recessive sex-linked trait in which the eye fails to distinguish red and green colours. The gene for normal vision is dominant The normal gene and its recessive allele are named by $X -$chromosome.

Complete answer: In female, colour blindness appears only when both the sex chromosomes carry the recessive gene $\left( {XcXc.{\text{ }}} \right)$The females have normal vision but function as carrier if a single recessive gene for colour blindness is present $\left( {XXc} \right).$However, in human males the defect appears in the presence of a single recessive gene $\left( {Xc} \right)$because Y-chromosomes of males do not carry any gene in colour Vision. Male transmits his trait to his grandson through daughter, while a female transmits the traits to her grand daughter through her son or it is transfer of trait from one sex to the offspring of the opposite sex.
Now if colour-blind man $\left( {XcY} \right)$marries a girl with normal vision$\left( {XX} \right)$, the daughter would have normal vision but would be a carrier , while sons would be normal.
Cross between colour blind man and normal woman :-
Now let us match this with given options:-

All sons colour blind :- this will only happen if both male and female are colour blind . Thus this option is not true .
All daughters colour blind :- this will only happen if both male and female are colour blind . Thus this option is not true .
Both A and B :- as both the options are wrong this option is also wrong.
All sons and daughters are normal but daughters are carriers :- the daughter will carry X gene from mother and $\left( {Xc} \right)$from father so they will be normal but carriers. Now the boy will get Y from male and X from female so it will be normal. Hence it is the correct option.

Note: It should be very much clear, colour blindness does not mean not seeing any colour at all, it means that those who are colour-blind have trouble in seeing the differences between certain colours. They can not tell the difference between red and green.