First determine the genotype of the boy. He is color-blind and we know from the question that this is a recessive condition. What must his genotype be?

X^BX^B	X^BY
X^BX^b	X^bY
X^bX^b

OK, The boy had to get his Y chromosome from his father. If the X^b came from his mother and we know that she has normal vision, what must her genotype be?

X^BX^B

X^BX^b

X^bX^b

Now that we know what the mother's genotype is, we need to find out what the genotypes of her parents were. As a female, she must have two X chromosomes. Where do they come from?

We know that the grandfather had normal sight, which means his genotype must be what?

X^BY

X^bY

So the boy's mother got her normal X^B chromosome from her father. This means that the recessive X^b chromosome came from her mother who has normal sight. What does this make the grandmother's genotype?

X^BX^B

X^BX^b

X^bX^b

Now return to the question page and select the correct answers.