8.1 Mitochondria #
8.1.1 The human mitochondrial genome and maternal transmission #
Human mitochondrial DNA is a circle of about 16.6 kb, 16,569 base pairs in the reference, with 37 genes: 13 proteins, 22 tRNAs, and 2 rRNAs. The proteins are oxidative-phosphorylation subunits. All other mitochondrial proteins are nuclear-encoded and imported, which Cell Biology describes, along with the endosymbiotic origin. The genetic point of that origin is a genome inherited off the nuclear chromosomes. A few codons differ from the nuclear code: UGA is tryptophan, AUA is methionine, and AGA and AGG are stops. Reading them as nuclear codons miscalls the gene.
Mitochondrial
The egg supplies the cytoplasm. An affected mother can transmit to children of both sexes; an affected father does not. Rare paternal leakage does not change ordinary pedigree reading. Copy number is high, hundreds or thousands of molecules per cell, so genotype is a population of molecules, not a pair of alleles.
8.1.2 Heteroplasmy and threshold #
Heteroplasmy is a mixture of mitochondrial genotypes in one person. At mitosis the molecules are partitioned without pairing, so tissues drift apart by vegetative segregation. A bottleneck in the female germline reduces the number of molecules that found each oocyte lineage and then amplifies them, so children of one heteroplasmic mother can differ sharply, one nearly mutant and another nearly wild type.
Phenotype appears when the mutant fraction in a tissue passes a threshold that the tissue’s energy demand will not tolerate. The fraction is commonly high and is not a single number. It depends on the mutation and the tissue. These notes do not turn that into diagnosis or advice. Leber hereditary optic neuropathy is often a complex I mutation close to homoplasmy; MELAS and MERRF are often heteroplasmic tRNA mutations, which disturb all 13 proteins rather than one subunit.
8.2 Chloroplasts #
8.2.1 Correns and four o’clock variegation #
Correns followed branch colour in Mirabilis jalapa. Progeny match the ovule parent and ignore the pollen parent.
| Seed parent | Progeny |
|---|---|
| Green | Green |
| White | White, and the seedlings die for lack of photosynthesis |
| Variegated | Green, white, and variegated |
Plastids in the egg are the transmitted factor. Mitosis sorts a mixed cell into pure green, pure white, or still mixed sectors. This is not the nuclear incomplete dominance of Mirabilis flower colour, which pollen does transmit. Chloroplast genomes are often about 120–160 kb and encode only some photosynthetic proteins. Some species transmit plastids from both parents; this cross does not. The reciprocal cross is the test. Poky in Neurospora is the mitochondrial version of the same maternal transmission.
8.3 Maternal effect is not cytoplasmic #
8.3.1 Snail shell coiling #
In Lymnaea peregra, dextral coiling is dominant to sinistral, and the gene is nuclear, but the coil reports the mother’s genotype. She loads the egg with products that set the direction of spiral cleavage; Developmental Biology owns that mechanism.
A D− mother has all dextral young, even dd young. A dd mother has all sinistral young, even DD or Dd young. Those dextral dd animals, once they are mothers, produce sinistral progeny. The phenotype lags the genotype by one generation. It does not keep travelling in the cytoplasm after the nuclear genotype has changed. Sturtevant read this from coils and genotypes.
8.3.2 How to tell the mechanisms apart #
A reciprocal-cross difference is not, by itself, cytoplasmic inheritance. The next generation separates the mechanisms.
| Organelle | Maternal effect | X-linked | |
|---|---|---|---|
| Affected father to his children | Does not transmit the phenotype | Does not set their phenotype; his alleles can set the grandchildren through daughters | Transmits to daughters, not to sons |
| Child’s own nuclear genotype | Does not decide the organelle phenotype | Does not; the mother’s genotype does | Does, together with sex |
| A generation later | Maternal line continues | Resets to the new mother’s genotype | Ordinary X transmission |
| Maps to a nuclear chromosome | No | Yes | Yes, the X |
Sex-limited autosomal traits pass through the non-expressing sex. Y-linkage never does. Organelle transmission can reach sons and then stop, because those sons do not pass cytoplasm on.
8.4 Key terms, summary, and self-check #
Key terms. Maternal inheritance; heteroplasmy; threshold; bottleneck; vegetative segregation; Mirabilis; maternal effect; Lymnaea; dextral; sinistral.
Summary. Human mitochondrial DNA is about 16.6 kb and 37 genes, passed by the mother, with heteroplasmy, a germline bottleneck, and a tissue threshold. Correns’s four o’clock transmits plastid colour through the ovule, and sectors sort by mitosis. Snail coiling is a nuclear genotype expressed one generation late. Organelles persist down the maternal line; a maternal effect is reset by each new mother.
Self-check.
1. Why do children not inherit a mitochondrial phenotype from an affected father?
2. Why can two children of one heteroplasmic mother have very different mutant fractions?
3. Green Mirabilis seed parent × variegated pollen parent gives green progeny. What does that rule out?
4. A dd snail coils right. What was its mother, and what will its own progeny do?
5. What do the grandchildren show if the trait is a maternal effect rather than an organelle?
