Question

3.   Why is carrier status for Duchenne muscular dystrophy (DMD) usually not transmitted from affected fathers to daughters, even though this disorder has an X-linked transmission pattern
4.   Why is there such a high rate of spontaneous mutation among
the genes responsible for classic hemophilia, Duchenne muscular dystrophy, and achondroplasia?

Answer 1

3 . DMD is inherited in an X-linked pattern because the gene that can carry a DMD-causing mutation is on the X chromosome. Every boy inherits an X chromosome from his mother and a Y chromosome from his father, which is what makes him male. Girls get two X chromosomes, one from each parent.

Each son born to a woman with a dystrophin mutation on one of her two X chromosomes has a 50 percent chance of inheriting the flawed gene and having DMD. Each of her daughters has a 50 percent chance of inheriting the mutation and being a carrier. Carriers may not have any disease symptoms but can have a child with the mutation or the disease. DMD carriers are at risk for cardiomyopathy.

When a girl inherits a flawed dystrophin gene from one parent, she usually also gets a healthy dystrophin gene from her other parent, giving her enough of the protein to protect her from the disease. Males who inherit the mutation get the disease because they have no second dystrophin gene to make up for the faulty one.

Usually, girls do not experience the full effects of DMD the way boys do, although they still have symptoms of muscle weakness. A minority of females with the mutation, called manifesting carriers, have some signs and symptoms of DMD.

For these women, the dystrophin deficiency may result in weaker muscles in the back, legs, and arms that fatigue easily. Manifesting carriers may have heart problems, which can show up as shortness of breath or an inability to do moderate exercise. The heart problems, if untreated, can be quite serious, even life-threatening.

In very rare instances, a girl may lack a second X chromosome entirely, or her second X may have sustained serious damage. In these cases, she makes little or no dystrophin (depending on the type of dystrophin mutation), and she develops a dystrophinopathy just as a boy would.