mitochondrial inheritance

Not all of your DNA sits in the cell nucleus. A small, separate genome lives inside the mitochondria, the tiny structures that power the cell. Mitochondrial inheritance is how the traits encoded by that genome pass from one generation to the next — and the route is almost entirely through the mother.

The reason is simple to picture. An egg cell is large and brings thousands of mitochondria into the new embryo; a sperm contributes essentially only its nuclear DNA, and the few paternal mitochondria it carries are normally destroyed. So mitochondrial DNA is inherited maternally: a mother passes it to all of her children, but her sons cannot pass it on.

This gives a pedigree a distinctive look — affected mothers transmit the trait to every child, while affected fathers transmit it to none. A further wrinkle is heteroplasmy: a cell can hold a mixture of normal and mutant mitochondrial DNA, and the proportion that lands in each egg varies, so the severity in children can differ widely and is hard to predict.