Discussion
Assertion (A): Two siblings can inherit the same mitochondrial mutation and yet differ greatly in how severely they are affected.
Reason (R): The proportion of mutant mitochondria passed on varies from one egg to another.
Answer: B
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason names heteroplasmy. A cell carries many mitochondrial genomes, and a mutation is usually present in only a fraction of them; that fraction is partitioned unevenly among the eggs a mother produces. Since disease appears only above a threshold proportion, siblings inheriting different loads can differ from unaffected to severely affected - which is why mitochondrial disorders are so difficult to counsel about.
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