Mitochondrial Inheritance

The Circular Chromosome of the Mitochondrion

A nuclear chromosome is huge and linear. The mitochondrial chromosome is circular and relatively small.

Mitochondria are Passed from Mother to Offspring

• In most animals father and mother each contribute the same number of chromosomes to the zygote.

• But there are thousands of mitochondria per egg cell, and only a few in the sperm. And once the sperm enters the egg, those sperm mitochondria are usually destroyed.

• The zygote ends up filled with mitochondria from the egg, therefore the zygote inherits the maternal mitochondrial DNA.

Because mitochondrial chromosomes almost never recombine and because they are inherited so differently, population bottlenecks lead to a very rapid loss of mitochondrial diversity.

In this example we begin with four mothers, each with a different mitochondrial chromosome.

If each mother has two offspring survive to maturity, the population is held constant.

Since only daughters pass on the mitochondria, mothers who only have sons end the transmission of that mitochondrial type.

After only a few generations (in this example) only one mitochondrial type remains:

…the mitochondria from Mother #2

Mitochondrial DNA Relationships

Mitochondrial Migrations

Because mitochondrial chromosomes don’t recombine like nuclear chromosomes, and because they are maternally inherited, mutations don’t spread laterally through a population because of mating.

Instead, they are passed, almost clonally, down through subsequent generations, often becoming “fixed” when small populations or individuals move to new areas.

Mitochondrial genetic markers are therefore the best tools to follow human migrations that have taken place over the millennia.

Mitochondrial Migrations

Source: McDonald Institute for Archaeological Research, University of Cambridge

Mitochondrial DNA Haplotypes

Mitochondrial DNA

• Eric Lander – Access Excellence• Argentina - Human Rights• National Geographic Genographic Project• Dolan DNA Center - Background on Mitochondrial

DNA

But, mussels are unique among animals!!!

Mytilus has two genetically different types of mitochondria and in addition to maternal inheritance there is also paternal inheritance!!!

More about that in the lab!