Question

The gene for colorblindness is carried on the X chromosome and is recessive. A man, whose father was colorblind, has a colorblind daughter. Is this man colorblind? Why?

Answer 1

Let me hope this would be helpful to you...

let X = normal X chromosome
let x = colorblind trait on X chromosome
let Y = normal Y chromosome

woman whose father is colorblind is Xx (she must get the x from her father, he only has one to give)

her color-blind husband must be xY (no other option for him)

what genotypes are possible for the mother of the color blind man she has married?
she can only be Xx (she must have given her son the x, the father cannot give an x to a son), but rarely color-blind girls do occur, so xx is also possible

what are the chances that the first child from this marriage will be a color-blind boy?
this is a probability question:
take the chance that first child is a boy (50% or 1/2) and multiply by the chance that the boy gets x not X from his mom (also 50% or 1/2)
multiply the probabilities
the answer is 1/4 or 25%

of the daughters of these parents, what fraction are expected to be color-blind?
silly question in real life
but, theoretically the girls will always inherit the x from dad
they have a 50% chance of inheriting the x from mom (she can give either X or x)
so 50% of the girls may be colorblind (only xx girls will be colorblind, Xx girls will be carriers