Genetics Problem Help?
A question from Mark J: genetics problem help?
mabel’s sister died of cystic fibrosis as a child. mabel herself does not have the disease and neither do her parents. mabel is pregnant with her first child. if you were a genetic counselor what would you tell her about the probability that her child will have cystic fibrosis.
The best answer:
Answer by Asst Prof
If there is no history of CF in her husband’s family, I would tell her the chance of having a child with CF is 0.
There is a 2/3 chance of Mabel’s being a carrier, since her parents are both heterozygous. But since CF is an autosomal recessive, if her husband does not have the CF allele, it won’t matter.
Agree or disagree? Leave your own thoughts below.
Tagged with: GENETICS • help • problem
Filed under: Cystic Fibrosis
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it depends.
from your scenario it seems that it is a recessive trait.
unless mabel and the father of the baby both carry the recessive gene the baby should not get it.
or
it could be a trait carried on the X chromosome and mable’s sister got it from her mother.
Cystic Fibrosis is inherited recessively. This means that both parents must be carriers for the gene in order for the child to get it. Since Mabel’s sister had it, both her parents must at least be a carrier. Since neither parent has the disease, then both parents must have the heterozygous genotype. By making a Punnett square with to heterozygous genotypes, you see that Mabel had 1/4 chance of homozygous dominant, 1/4 chance of homozygous recessive, and 1/2 chance of heterozygous. She does not have the disease, so homozygous recessive can be eliminated as an option. Then, she has a two thirds chance of carrying the gene for the disease, which means there is a 2/3 chance that she will pass the gene to her child. Whether or not the child has the disease, though, depends on the father of the child and his family history with the disease.