Question

You have a female friend and her father has muscular dystrophy. Her mother does not. She is concerned that she may both contract the disease and/or possibly pass the disease to her children. Explain to her why this is not possible. What would have to be different for her to be concerned about contracting the disease?

Answer 1

Muscular dystrophy is a X linked recessive disorder. This means the disease will only be expressed if both the X chromosomes has the mutant recessive allele responsible for muscular dystrophy. Males are hemizygous for X chromosome, i.e, they have only one X chromosome, and have a Y chromosome in place of the second X chromosome. Thus, in males, having the recessive allele on the single X chromosome is enough to contract the disease.

Reason why the female friend will not contract muscular dystrophy:

Now, coming to the example of the said female friend, it is extremely unlikely that she will contract muscular dystrophy. As she is female, she would have two X chromosomes, inheriting one from her father and the other from her mother. Now,as her father is affected with muscular dystrophy he has the X chromosome bearing the diseased allele. Hence she would definitely inherit the diseased allele from her father. However since her mother is unaffected, and she would receive unaffected dominant allele from her mother. Therefore she would have one unaffected and one affected allele for muscular dystrophy. Since it is a recessive disease, the dominant allele will save her from contracting muscular dystrophy. This is why it is highly unlikely that she would have muscular dystrophy.

Reason why the female friend does have a chance of passing the disease onto her children:

However, as seen above, she still carries the diseased allele in one of her X chromosomes. This makes her a carrier for muscular dystrophy. Now, considering she marries a man without muscular dystrophy, her children would have a 25% chance of contracting the disease. None of her daughters would contract muscular dystrophy as they would inherit the normal unaffected allele from their father. Hence even if they inherit the diseased allele from their mother, they would not contract the disease.

On the other hand, her sons would have a 50% chance of contracting muscular dystrophy. They would inherit the Y chromosome from their father. This leaves them with a 50% chance of inheriting either the diseased allele or the normal allele from their mother. If they do inherit the disease allele from her, they would definitely contract muscular dystrophy. As the chances of having a son is 50% in the first place, the overall risk of her children contracting muscular dystrophy is 25%.

But, if she marries a man with muscular dystrophy, that risk goes upto 50%. The risk for the sons remains the same as the previous case. However, the daughters now stands a 50% chance of inheriting diseased allele from both parents and a 100% chance of inheriting the diseased allele from their father.

Factors that would increase the risk for her contracting the disease:

The said friend would however have a 50% chance of contracting muscular dystrophy if her mother was a carrier for the disease. Carriers would be heterozygotes for the trait, having one normal and one diseased allele. Hence they would not show the symptoms of muscular dystrophy, but are capable of passing on the disease.

If her mother was a carrier for the disease, she would have a 50% chance of inheriting the X chromosome containing the diseased allele from her mother. Given that she would have definitely inherited the diseased allele from her father, she would have had a 50% chance of contracting the disease.

She would have a 100% chance of contracting the disease if both her parents had muscular dystrophy. As it is a recessive disorder, it would mean all the X chromosomes in both of her parents contains the diseased allele, and there would have been no way for her to inherit a dominant unaffected allele and be safe from contracting the disease.

Coming to the conclusion, it is impossible for the said female friend to both contract the disease and pass it on to her children. However, in the given situation, she does have the chance of passing on the disease to her sons without contracting the disease herself.

[ Note: If the said friend was male however, he would have inherited Y chromosome from his father and unaffected X chromosome from his mother, and would have neither contracted the disease nor passed it onto his children.]