A concerned parent presents with her four-year-old son whom you recently diagnosed with Duchenne muscular dystrophy. She states that she has always planned on having a large family; however now she worries that the disease will also affect all of her future children and grandchildren. What type of inheritance pattern does this condition follow, in addition to often having de novo mutations?

Question

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With an X-linked recessive condition, the chance of passing on the changed (non-working) copy of the gene to a child is different for males and females. Males only have one copy of the X chromosome from their mother and one copy of the Y chromosome from their father. Therefore, if their X chromosome has a mutation of their dystrophin gene, they will have Duchenne muscular dystrophy. Females, however, have two copies of X chromosomes. Since females have two copies of this gene, if one copy does not work, they have a second "back-up" copy to produce the dystrophin protein. A woman who has a genetic change in one of her two copies is said to be "a carrier" of Duchenne muscular dystrophy. Carriers do not have Duchenne muscular dystrophy and most are unaware that they even carry this change in their genetic material unless they have a family history. In addition to following an x-linked recessive pattern, "de novo" mutations are also possible, meaning the disorder could arise in any individual, with no previous family history of muscular dystrophy.

A Concerned Parent Presents with Her Four-Year-Old Son Whom You