Questions: Maternal Inheritance and Extranuclear (Cytoplasmic) Genes

Question 1

A man carries a pathogenic mitochondrial DNA mutation. His wife has no mitochondrial mutations. Which prediction about their children is correct?

• A. Half of their children will carry the mutation, following a Mendelian dominant inheritance pattern
• B. All of their children will carry the mutation, since the father expresses it and it will be passed to all offspring
• C. None of their children will carry the mutation, because mitochondrial DNA is maternally inherited and fathers do not transmit it
• D. Only daughters will carry the mutation, because mitochondrial inheritance is X-linked

Answer: (C)

Mitochondria are inherited maternally: sperm contributes virtually no cytoplasm at fertilization, so only the egg's mitochondria are transmitted to offspring. A father's mitochondrial DNA mutations are not passed to any of his children, regardless of sex. Since the mother has no mutation, none of the children will inherit the paternal mitochondrial mutation. This is the defining feature of maternal inheritance — transmission is exclusively through the maternal line. X-linked inheritance (option D) involves nuclear chromosomal genes and follows completely different patterns.

Question 2

A heteroplasmic woman carries a mitochondrial disease mutation. She has four children with an unaffected father. One child is severely affected, two are mildly affected, and one is asymptomatic. Which explanation best accounts for this variation?

• A. Recombination shuffled the maternal and paternal mitochondrial alleles differently in each child
• B. Different children inherited different proportions of mutant and wild-type mitochondria due to stochastic vegetative segregation during oogenesis
• C. The asymptomatic child received a compensatory nuclear mutation that corrects for the mitochondrial defect
• D. Variable expressivity in X-linked conditions explains the range of severity across siblings

Answer: (B)

The mother is heteroplasmic — her cells contain a mixture of mutant and wild-type mitochondria. During oogenesis, these organelles are distributed to egg cells roughly at random. Some eggs receive a high proportion of mutant mitochondria, others a low proportion. Each child's disease severity tracks their proportion of mutant mitochondria: high mutant load → severe disease; low mutant load → mild or no symptoms. This stochastic vegetative segregation is the mechanistic explanation for variable expressivity among siblings born to the same heteroplasmic mother. Recombination (option A) does not occur between maternal and paternal mitochondria — there is no paternal mitochondrial contribution to recombine with.

Question 3

In a cross between a white-flowered plant and a green-flowered plant, if flower color follows maternal inheritance, then the offspring's color will match the maternal parent regardless of which parent provides the pollen.

• T. True
• F. False

Answer: True

Maternal inheritance means offspring phenotype is determined by the mother's cytoplasmic genotype, not the father's. Reciprocal crosses — white♀ × green♂ and green♀ × white♂ — give completely different results: offspring from white females are white regardless of pollen source; offspring from green females are green. This asymmetry between reciprocal crosses is the hallmark of maternal (cytoplasmic) inheritance, observed in Correns' original work with Mirabilis jalapa variegation. It contrasts sharply with nuclear Mendelian genes, where reciprocal crosses typically give the same F1 phenotype.

Question 4

A child born to a heteroplasmic mother with a mitochondrial disease mutation will generally express the disease because the mother passes most her mitochondria to most child.

• T. True
• F. False

Answer: False

Heteroplasmy means the mother's cells contain a MIXTURE of mutant and wild-type mitochondria — not all mutant. During oogenesis and subsequent cell divisions, organelles are distributed stochastically, so each egg receives a different random sample. If a child happens to receive a small proportion of mutant mitochondria, they may be asymptomatic or mildly affected. Many mitochondrial diseases only manifest when mutant load exceeds a tissue-specific threshold (often 70–90%). A child of a heteroplasmic mother can receive so few mutant mitochondria that they remain entirely unaffected — as the scenario above illustrates.

Question 5

Why does heteroplasmy make it difficult to predict mitochondrial disease severity in children of an affected mother, in ways that Mendelian nuclear gene inheritance would not?

Model answer: In Mendelian inheritance, each parent contributes exactly one allele per locus, so each child's allelic state at every nuclear locus is determinate. Mitochondrial inheritance involves hundreds to thousands of organelles per cell, each potentially carrying a different DNA variant. A heteroplasmic mother's egg cells receive a stochastic sample of these organelles, so each egg differs in its ratio of mutant to wild-type mitochondria. After fertilization, further stochastic segregation during cell division creates within-individual variation across tissues. Disease manifests only when mutant load exceeds a tissue-specific threshold, and the relevant tissues (brain, muscle, heart) may differ from sampled tissues. None of this continuous, probabilistic variation is possible with Mendelian nuclear genes.

This explains why prenatal diagnosis of mitochondrial disease is difficult: even measuring the mutant load in a biopsy sample may not reflect mutant loads in clinically affected tissues. The irreducible stochasticity of organellar inheritance — arising from the fact that hundreds of independent genetic units, rather than two alleles, are being sampled — creates uncertainty that is fundamentally different from the combinatorial predictability of Mendelian genetics.