Mitochondrial DNA inheritance through the maternal line
Mitochondrial DNA (mtDNA) is the small separate genome inside mitochondria, and it is passed down the maternal line (from the mother, through the egg) to children of either sex.
Remember
The egg supplies the mitochondria of the embryo, so mitochondrial DNA comes from the mother. An affected woman can pass a mutation to all of her children, sons and daughters alike, but an affected man cannot pass it on.
Mitochondria make the energy a cell uses. Each one contains a small circle of DNA, called mitochondrial DNA (mtDNA), which is separate from the DNA in the nucleus.
Mitochondrial DNA is passed on only down the maternal line, so it comes from the mother and not the father. At fertilisation, the egg supplies the mitochondria of the embryo. In a sperm, the mitochondria are used to provide movement, so they are discarded when the sperm fuses with the egg.
All of an affected woman's children can inherit the mutation, whether sons or daughters. An affected man does not pass it on. Sons and daughters can both be affected, yet only the daughters of an affected woman can pass the mutation to the next generation. On a family tree, the condition appears along the maternal line, in both sexes, with no affected people related only through a father.
A cell holds many mitochondria, and in a person with a mutation some copies of mtDNA can be normal and some mutant (they carry the mutation). This mixture is called heteroplasmy. A mother passes a random selection of her mitochondria to each child, so each child can receive a different proportion of mutant mtDNA. The severity of the disease follows that proportion, together with the tissues involved, so children of one family can show different features.
Tissues with high energy needs tend to be hit hardest, for example muscle, brain and heart. Symptoms that are frequently observed include muscle weakness and hearing loss.
Key facts
- Source of mtDNA
- the egg supplies the mitochondria, so the DNA inside them comes from the mother.
- Affected mother
- can pass the mutation to children of either sex.
- Affected father
- does not pass the mutation on, because a sperm's mitochondria are discarded.
- Family tree
- affected people of both sexes along the maternal line, and no affected people related only through a father.
- Heteroplasmy
- a mixture of normal and mutant mtDNA in the cells, with severity depending on the proportion of mutant mtDNA and the tissues involved.
- Tissues hit hardest
- those with high energy needs, such as muscle, brain and heart, tend to be affected most.
Exam traps
- Autosomal recessive inheritance, which needs two altered copies of a gene in the nucleus, one from each parent, does not explain mitochondrial DNA mutations, because mitochondrial DNA is separate from the DNA in the nucleus and is passed on by the mother only.
- However active the mitochondria in sperm are, mitochondrial DNA still does not come from the father, because the embryo's mitochondria come from the egg.
- A father's mitochondria do not replace the mother's after fertilisation, because the sperm's mitochondria are discarded after fertilisation and the egg supplies the embryo's mitochondria.
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