Mitochondrial inheritance and heteroplasmy
Mitochondrial DNA (the small ring of DNA inside mitochondria) is inherited only from the mother, and the mix of normal and mutant copies in a cell is called heteroplasmy.
Remember
A mother can pass a mitochondrial DNA mutation to all her children, but only her daughters can pass it on, and severity differs between children because normal and mutant copies are mixed.
Memory aid
mitochondria are heirlooms handed down on the mother's side only
Mitochondria are handed down like heirlooms, on the mother's side only.
Mitochondria are small structures inside cells that turn fuel into usable energy. Each one has a little DNA of its own, known as mitochondrial DNA (mtDNA), which is separate from the DNA in the nucleus.
At fertilisation the egg supplies the embryo's mitochondria, while sperm do not contribute any. This is called maternal inheritance: the mitochondrial DNA comes from the mother alone. A woman with an mtDNA mutation can pass it to her sons and her daughters. An affected man cannot pass it to any of his children, so only daughters can carry it into the next generation.
Each cell holds many copies of mtDNA, so a person can carry a mixture of normal and mutant copies. This mixture is called heteroplasmy. When an egg forms, it receives only a small sample of the mother's copies, a narrowing known as the mitochondrial bottleneck. Chance therefore sets the proportion of mutant copies in each egg, so one child may stay well with a low proportion while another child, with a high proportion, is severely affected.
Tissues that need a lot of energy, such as muscle and brain, show problems first once the proportion of mutant mitochondrial DNA passes a threshold. The outcome for one child is hard to predict.
Key facts
- Source
- mtDNA comes through the egg, because sperm contribute no mitochondria to the embryo.
- Mother
- she can pass the mutation to her sons and her daughters alike.
- Sons
- they can be affected but cannot pass it on.
- Daughters
- they can be affected and can pass it on to their children.
- Heteroplasmy
- normal and mutant mtDNA are mixed within a cell, so severity can differ between children.
- Bottleneck
- only a small sample of mitochondrial DNA copies enters each egg, so the proportion of mutant copies varies.
Exam traps
- Sperm do contain mitochondria, but a man with a mitochondrial DNA mutation does not pass it to his children, because the embryo's mitochondria come only from the egg.
- Mitochondrial disease is not limited to one sex, because both sexes inherit mitochondrial DNA from the egg and can be affected, so daughters can develop it too.
- A child can inherit a mitochondrial mutation even if the father does not carry it, because only the egg contributes mitochondria to the embryo.
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