. It's very rare for someone with two X chromosomes to have DMD. Explain how a person with two X chromosomes could have the symptoms of DMD using what you know about inheritance and random mutation.

Duchenne muscular dystrophy (DMD) is inherited through X-linked recessive inheritance mode. It is characterized by muscle weakness which mostly affect boys. It can only affect girls when a female carrier got married to affected male.

Explanation: In the union between a carrier female and an affected male, four types of gametes will be produced as shown in the attached image. After fertilization, the resulting offsprings will consist of female carrier, normal male, female affected and male affected each with 25% probability. See attached image for better illustration.

X-linked recessive traits have the following characteristics;

1. More males are affected than females.

2. Approximately ½ of a carrier mother's sons are affected.

3. It is never passed from father to son.

4. All daughters of affected fathers are carrier.

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