Question

In myoclonic epilepsy with ragged red fibers syndrome (MERRF), expression of the disease is highly variable, with some family members being severely affected while others are not affected at all. Which of the following explains the variable expression of this disease?

(A) variable expressivity
(B) incomplete penetrance
(C) heteroplasmy
(D) allelic heterogeneity

Answer 1

The variable expression of MERRF syndrome is best explained by c) heteroplasmy, which refers to the varying mix of normal and mutated mitochondrial DNA in cells, leading to different disease presentations among individuals.

Step-by-step explanation:

The highly variable expression of Myoclonic Epilepsy with Ragged Red Fibers (MERRF) syndrome within a family can best be explained by heteroplasmy. Heteroplasmy refers to the presence of a mix of both normal and mutated mitochondrial DNA within a cell. Since the mitochondria are responsible for energy production and have their own DNA separate from the nuclear DNA, mutations in mitochondrial DNA can cause diseases like MERRF.

The proportion of mutated to normal mitochondrial DNA can vary among different tissues and even between family members, leading to differences in the severity and presence of symptoms. This is in contrast with other inheritance patterns, such as autosomal dominant, autosomal recessive, or allelic heterogeneity, which involve nuclear DNA.

Hence, the correct answer is Option C.