Heteroplasmy in mitochondrial disease
A woman with a disorder caused by a mitochondrial DNA mutation has three children with an unaffected man. One child is severely affected, one is mildly affected, and one has no symptoms. What best explains the difference among the children?
Answer D: explanation for every choice
Sperm mitochondria are destroyed after fertilization, so all of a child's mitochondria come from the egg.
Mitochondrial DNA is separate from the nuclear chromosomes, including the X. The severity differences come from the mother's mitochondria, not the child's sex.
Dominance describes two alleles of a nuclear gene in a diploid cell. A cell holds hundreds of mtDNA copies, and what matters is the fraction that carries the mutation.
Correct. The mother's cells carry a mix of normal and mutant mtDNA (heteroplasmy). Only a small sample of mitochondria passes into each egg, so the mutant fraction varies from child to child, and so does severity.
Reviewed 2026-11-05 by a medical student. Spot an error? Report it with question ID bb61.