Color blindness is inherited in an X-linked recessive manner. A woman who carries the colorblind gene and has normal vision and a man who is colorblind just got married and are wondering about their children.

What is the probability of a colorblind child? Draw a Punnett square

a. What proportion of the sons are colorblind?

b. What proportion of the daughters are colorblind?

c. Now draw a Punnett square of a mating between a colorblind woman and a man with normal vision.

d. What is the probability of a colorblind child?

e. What proportion of the sons are colorblind?

f. What proportion of the daughters are colorblind?

g. Why do sex-linked recessive traits appear more often in males than in females?

Since colour-blindness is inherited in an X-linked recessive manner, for a female to be affected, she has to carry the recessive trait on both X chromosome. But for a male to be affected, only one copy of the recessive trait is needed. let c represent the recessive trait

normal female with colorblind gene x colorblind male

XC Xc

Xc XCXc XcXc

Y XCY XcY

probability of a colour blind child is 1/2

proportion of sons being colorblind is 1/2

proportion of daughters being colorblind is 1/2

color blind female x normal male

Xc Xc

XC XCXc XCXc

Y XcY XcY

probability of a colorblind child is 1/2

proportion of sons being colorblind is 1

proportion of daughters being colorblind is zero.

Sex linked recessive traits appear more often in males because, the males get affected immediately by just carrying the recessive trait on the one X chromosomes as this is not linked to the Y chromosomes.