Question

Duchenne muscular dystrophy (DMD) is a recessive X-linked disorder, which results in symptoms before the age of 6. A young girl, diagnosed with DMD has a 46 XX karyotype. The young girl’s brother is also living with DMD. Curiously, both parents are normal.

a. Given the information in the question, why does this young girl have DMD? (1 sentence)

b.How does this young girl have DMD? What mechanisms and how did this mechanism lead to this girl exhibiting DMD? (1 or 2 sentences)

Answer 1

# X-linked recessive disease mostly affect the male children. Even if the mother is carrier for the defective trait, she will transmit the disease to one of her sons. But female will be affected only when both the X alleles are defective. Mostly female children will act as a carrier for this disease.

Both the parents are normal. So, father is not having defective allele. But mother must be a carrier because, she transmitted the disease to her son. Here, according to the parental condition, young girl should not have the disease. But, she is having 46 XX Karyotype which means, there was a non-disjunction of X chromosome during gamete fromation. That caused an extra X chromosome in the gamete of the girl child.

She must be having two defective X trait in her choromosome.