Yes. There’s actually another small set of genes that we all possess, inside our cells but outside the cell nucleus. The cell nucleus is where most of our genes reside on the 23 pairs of chromosomes already discussed.

The additional genes, which make up less than 1 percent of a cell’s DNA, are the mitochondrial genes, and they exist as circular strands of DNA inside mitochondria, the “energy factories” of cells. (The singular for mitochondria is mitochondrion.)

What are genes doing inside the mitochondria?

There are 37 genes, mostly involved in energy production, inside the mitochondria. Scientists believe that mitochondria were once independent organisms resembling today’s bacteria, and that when they became part of human and animal cells, they kept their own genes.

These genes, arranged on structures that are like the nuclear chromosomes but are ring-like in shape, carry the recipes for 13 proteins needed for mitochondrial functions. They also carry codes for 24 specialized RNA molecules that are needed to assist in the production of other mitochondrial proteins.

For reasons that will become clear, it’s important to know that mitochondria also use proteins made by genes in the cell’s nucleus. These proteins are “imported” into the mitochondria.

Can disease-causing mutations occur in mitochondrial genes?

Yes. Disease-causing mutations can occur in the mitochondrial genes.

The disorders are often, as one would predict, associated with energy deficits in cells with high energy requirements, such as nerve and muscle cells. The disorders as a whole are called mitochondrial disorders. Mitochondrial disorders affecting muscle are known as mitochondrial myopathies.

How are mitochondrial mutations inherited?

Mitochondrial DNA inheritance comes only through the mother and is therefore completely different from nuclear (from the nucleus) DNA inheritance. The rules for recessive, dominant and X-linked inheritance don’t apply at all.

An embryo receives its mitochondria from the mother’s egg cell, not the father’s sperm cell, at conception. (Research suggests that sperm mitochondria are eliminated by the egg cell.)

Mutations can exist in some of the mitochondria in a person’s cells and never cause much, if any, trouble. (In fact, one theory of aging says that it’s caused by an accumulation of mutations in mitochondrial DNA.) The normal mitochondria are usually enough to produce the needed energy for the body. But once a person has a certain percentage of mutated mitochondria (perhaps 30 percent or so), the energy deficits become crucial and a mitochondrial disorder can result.

Mothers can pass on mitochondrial mutations to their children, but fathers can’t, so mitochondrial DNA inheritance follows a pattern called maternal inheritance. The severity of the child’s disorder depends on how many normal versus abnormal mitochondria the child receives from the mother.

Mitochondrial DNA mutations can also occur during development of an embryo. Not all mitochondrial mutations are inherited. Some occur as an embryo is developing in the womb. Researchers have found that embryonic mitochondrial mutations generally occur after sperm or egg cells have formed in the affected embryo, so, as far as has been observed, these mutations are not passed on to the next generation.

Does DNA from the cell’s nucleus affect the mitochondria?

Yes. DNA from the nucleus also affects mitochondrial function, so some mitochondrial disorders are inherited according to the same rules as are other genetic disorders.

Most mitochondrial proteins aren’t made in the mitochondria but come from genes in the cell’s nucleus. These nuclear proteins are later imported into the mitochondria, where they too help with energy production.

As you may have guessed, mutations also can occur in these nuclear genes that affect mitochondria. So, that’s another way to get a “mitochondrial disorder” — but one that’s not caused by mutated mitochondrial DNA.

Nuclear DNA that affects mitochondrial function is inherited according to autosomal and X-linked patterns.

For family planning, it’s important to know exactly what kind of DNA mutation exists in a family with a mitochondrial disorder — whether it’s a mitochondrial DNA mutation or a nuclear DNA mutation. As you can see, these have very different patterns of inheritance and implications for the family.