Question

use the information below to answer questions 1–6.
the protein dystrophin holds muscle fibers together and stabilizes the cell membrane of muscle cells. with the condition duchenne muscular dystrophy (dmd), individuals have a version of dystrophin that is less effective at holding muscle fibers together which leads to damage and progressive muscle weakness. dmd is a heritable genetic condition that follows an x - linked recessive pattern of inheritance.
which of the two following conclusions can be drawn based on the knowledge that duchenne muscular dystrophy follows an x - linked recessive pattern of inheritance?

• dmd presents in all individuals where one copy of the gene codes for dmd.
• dmd presents in individuals where one copy of the gene codes for dmd as long as the individual has female sex chromosomes.
• dmd presents in individuals where both copies of the gene code for dmd.
• dmd presents in individuals where one copy of the gene codes for dmd as long as the individual has male sex chromosomes.

Explanation:

For X - linked recessive inheritance: Males (XY) have one X chromosome. If the X chromosome carries the DMD - causing allele, they will express the disorder because they don't have a second X to mask it. Females (XX) need two copies of the recessive allele (one on each X) to express the disorder.

• Option 1: In X - linked recessive, males with one copy and females with one copy (carriers) don't all present DMD (females with one copy are carriers). So this is wrong.
• Option 2: Females with one copy are carriers, not affected. So this is wrong.
• Option 3: Females would need two copies, but males only need one. So saying both copies is incorrect (applies more to autosomal recessive in general, not X - linked recessive for males).
• Option 4: Males have XY. If their X has the DMD allele (one copy), they will present DMD since they have no second X to provide a normal allele. This matches X - linked recessive inheritance for males.

Answer:

D. DMD presents in individuals where one copy of the gene codes for DMD as long as the individual has male sex chromosomes.