1. A color-blind man marries a woman with normal vision. The woman's father was also color-blind. (a) What is the chance that their first child will be a color-blind son? A color-blind daughter? (b) If they have four children, what is the chance that two will be color-blind sons? (Be careful on this one)
Color vision is an X-linked recessive trait. In order for a woman (XX) to be color blind, both her X chromosomes must have the mutated gene. If she only gets one mutated gene, her other X chromosome can make up for it and she won't be colorblind, but she will be able to pass down the bad gene. If a man (XY) receives a defective gene on his X chromosome, he will express the trait since the Y chromosome can't compensate for the defect.
In this case, the woman's father was color blind, so his X chromosome contains the mutation. She is XX- one X from her mother and one from her father. Therefore, she must have inherited her father's X chromosome, and she is a carrier of the color blind trait. If her husband is normal, he definately doesn't have the defect.
So if the two were to have children, the girls will get the father's good X chromosome, and either the wildtype or the mutated gene from their mother. Either way, they'll be phenotypically normal since they are guaranteed at least one good gene from their dad. They will, though, have a 50% chance of being a carrier like their mom.
The sons are at a higher risk. The get a Y from their father, and one of their mother's X chromosomes. If they get the mutated gene they'll be colorblind, so her boys have a 50% chance of being affected.
Quick Reply
