If a woman is a carrier for the color-blind recessive allele and her husband is normal, what are their chances that a son will be color-blind?
a. none since the father is normal.
b. 50% since the mother is the only carrier.
c. 100% because the mother has the gene.
d. 25% because the mother is a hybrid.
e. none since he will also be just a carrier.

b. 50% since the mother is the only carrier.

Explanation:

Color blindness is a X linked recessive trait characterized by difficulty in differentiating between some colors. Let us assume

C = dominant allele

c  = recessive allele

Given,

mother is a carrier = XCXc

father is normal = XCY

Hence their children:   XC          Xc

                        XC       XCXC     XCXc

                          Y        XCY        XcY

The sons can either be XCY (normal) or XcY (color blind). Hence they have half or 50% chance to be colorblind since half of the sons will get one recessive allele from the carrier mother.