The recessive mutant allele is present on the X chromosome. Some of the genes present on the X chromosome functionally resemble the genes present on the autosomes. The recessive traits manifest only in a homozygous state or in a double dose. A mutation of a gene present on the X chromosome expresses the phenotype in males. These males show hemizygosity for the gene mutation since they have only one X chromosome. The females show homozygosity for the gene mutation. Since the females have two X chromosomes, they have a copy of the gene mutation on each X chromosome. Carriers have only one copy of the mutant gene. Hence, they do not express the phenotype. A carrier female transmits the gene to the next generation. An affected male transmits the mutant gene to all his daughters who become carriers in the future.

Consider a mating between a normal male and a carrier female. 25% of the females exhibit a normal phenotype, 25% of the males exhibit a normal phenotype, 25% of the females become carriers, and remaining 25% of the males exhibit a normal phenotype. Consider another mating example between the affected male and a normal female. In this case, an affected male will never transmit the disorder. 50% of the females will become carriers whereas 50% of the males will exhibit a normal phenotype.

Image 1: X-linked recessive inheritance (Affected father and unaffected mother)

Image 2: X-linked recessive inheritance (Unaffected father and carrier mother)

It severely affects the muscles. This type of dystrophinopathy is predominant in males. DMD involves severe muscular weakness and wasting. An affected person shows an awkward structure of the shoulders and arms while walking. They have very weak belly muscles. These weak muscles lead to the sticking of the belly, thereby affecting the sitting position of the person.

Individuals with DMD have weak thighs, weak muscles in the front leg, foot drop and bent knees. Children with DMD may walk on toes due to tight heel cord (contracture). The calf muscles get replaced by fat and the connective tissues. Most of them use a wheelchair since they show difficulty in walking.