gene dominance

[dmanganye]

the brother to a woman's father's was affected with haemophilia but her father was unaffected, she worries that she may have an affected son, should she worry? explain.

[kotoreru]

You could have at least rephrased your homework instead of just copying out the question exactly as you saw it.

[blcr11]

OK, I’m not a geneticist, so if I get it wrong here, I hope someone who is a geneticist will correct me.

Let’s back up for a second and think it through. Most (all?) forms of hemophillia are X-linked abnormalities in clotting factors. Heterozygous women are carriers of the trait, but are usually not affected—the normal X-chromosome apparently supplies enough normal clotting factor to compensate for whatever defect there is in the mutant X’ chromosome. An X’X’ homozygous woman is affected with the disease—there is no source of normal clotting factor in this case. An X’y male is also affected for the same reason.

The girl is concerned that she might be a carrier of an X’ chromosome. But can she be? We aren’t told anything about her mother. If her mother is X’X then that will change the story, but assuming her mother is XX, can she have inherited an X’ chromosome from her father? No. Why? Because her father is unaffected. Think about that: if he were affected, then he must have been X’y, but he’s not, so he could only have been Xy—this is the crucial piece of information in this problem. Her father must have been Xy. She must inherit her father’s X chromosome, otherwise she would be a he. Assuming that her mother is XX, there is no way she can inherit an X’ chromosome—from her father at least. I think you should be able to see how the story would change if the girl’s mother was X’X—she would have a 50% chance of being a carrier in that case. Right? And if her mother was X'X'? What then? I'll let you figure that one out.

[dmanganye]

THANXX A LOT FOR YOUR EFFORT, I REALLY APPRECIATE IT

[david23]

blcr you are too modest.