Question

For full credit, you must show all your work. If you just write down the answer, you will not receive credit. You need to show me that you understand how to work the problem.

1. As a genetic counselor, you routinely advise couples about the possibilities of genetic disease in their offspring based on family histories. This morning, you met with an engaged couple, both of whom are phenotypically normal. The man (age 30), however, has a brother who died of Duchenne muscular dystrophy, an X-linked condition that results in death before age 20. The allele responsible for the diseases is recessive. His prospective bride has no history of the disease in her family.

a. Produce a Punnett square that you would use to explain to this couple what their children might look like genotypically and phenotypically.

b. The sister (age 24) of the man discussed above also is engaged. Her future groom has no history of Duchenne muscular dystrophy. Produce a Punnett square that you would use to explain to this couple what their children might look like genotypically and phenotypically.

Answer 1

Duchenne muscular dystrophy (DMD) is an X-linked recessive genetic disorder that affects boys (can affect girls, but rarely) which is characterized by progressive muscle wasting and weakness. Females are carriers and boys get sick if they inherit the mutant copy of the allele that codes for the protein dystrophin.

a) It is given to us that both the parents are phenotypically normal, but that the brother of the man was afflicted by DMD. From this, we can reasonably assume that their mother was a carrier of the disease. It is also mentioned that there is no family history of the disease on the bride's side of the family.

From this information, we can reasonably assume that the bride is not a carrier for the disease and that the groom is genetically fit (DMD is recessive, and since boys carry only one copy of the X chromosome, they get sick if they carry the mutant allele. Here the man is older than the median survival age of 20 and is said to be phenotypically normal). In this scenario, let us construct a punnet square to represent the expected phenotypes and genotypes of their future children. If we represent the dominant allele for Dystrophin by D and the recessive deleterious allele for it by d, then

Genotype of father - D _ (since it is X-linked, males only cary one copy of the allele)

Genotype of mother - DD

Father Mother D DD DD All healthy offspring regardless of gender # father carries only one copy of the Dystrophin gene as it is X linked.

As you can see, all the children will be healthy, regardless of their gender. Hence they have no reason to be worried.

b) In this scenario, the sister of the groom is engaged to a man with no family history of DMD. Since the mother of the groom was a carrier of the disease, and since she had two male children with different phenotypes (one normal and one with DMD), she was likely a heterozygous carrier of the disease causing allele (Dd genotype). Now this means that the sister could have either inherited the deleterious allele (d) or the normal allele (D) from the mother and the normal allele (D) from her father, since her father is not mentioned to be sick from DMD. This means that the genotype of the sister could be either Dd or DD, and the genotype of the groom is D _

If we consider both the scenarios and draw Punnett squares to represent them, this is how it will look:

Scenario 1 - Mother is of DD genotype Scenario 2 -Mother is of Dd genotype Father Father Mother Mother DD DD D DD D. dD dD-girl carrier boy with DMD 1 in 4 children can be affected by DMD All healthy offspring regardless of gender #father carries only one copy of the Dystrophin gene as it is X linked.

So in total there is a 1 in 8 probability ( 12.5 %) that a male child born out of this union would be affected by DMD, and a 1 in 8 chance that a girl child born would be a carrier for the disease.

I hope this helps :)