Mothers will pass on a mitochondrial defect to their offspring. In a type of gene therapy, one approach to circumvent this problem is to have two different maternal contributions, with the nucleus of the female with the defective mitochondria being placed in an enucleated egg derived from a female with normal mitochondria. After fertilization, the resulting offspring would have three parental sources of DNA—with nuclear DNA derived from a mother and a father, and mitochondrial DNA derived from another 'mother.' Recently, children with this genetic makeup have been born, but the elimination of defective mitochondria is not complete, with the amount of defective mitochondria derived from the defective mother ranging from 0 to 9%. Discuss potential complications resulting from such a mixture of genomes.

Payne, B. A. et al. (2013) present evidence that a low level of heteroplasmic mtDNA exists in all tested healthy individuals.

What genetic conditions within a given mitochondrion are likely to contribute to such a variable pool of mitochondria?

385

The dodo bird (Raphus cucullatus) lived on the Mauritius Islands until the arrival of European sailors, who quickly hunted the large, placid, flightless bird to extinction. Rapid morphological evolution such as often accompanies island isolation had caused the bird's huge size and obscured its physical resemblance to any near relatives. However, sequencing of mitochondrial DNA from dodo bones reveals that they were pigeons, closely related to the Nicobar pigeon from other islands in the Indian Ocean. Why was mitochondrial DNA suited to the study of this extinct species?

735

The mtDNA accumulates mutations at a rate approximately ten times faster than nuclear DNA. Thus geneticists can use mtDNA variations as a 'molecular clock' to study genetic variation and the movement of ancestral human populations from Africa to different areas of the world more than 125,000 years ago. Propose an explanation for how an analysis of mtDNA can be used to construct family trees of human evolution.

386

Because offspring inherit the mitochondrial genome only from the mother, evolutionarily the mitochondrial genome in males encounters a dead end. The mitochondrial genome in males has no significant impact on the genetic information of future generations. Scientists have proposed that this can result in an accumulation of mutations that have a negative impact on genetic fitness of males but not females. Experiments with Drosophila support this possibility. What experimental data or evidence would you want to evaluate or consider to determine if an accumulation of mtDNA mutations negatively impacts the fitness of males of any species?

678

Which best describes mitochondrial DNA (mtDNA)?

Eukaryotic chromatin is composed of which of the following macromolecules?

In bacteria, what typically happens to linear DNA that is not integrated into the bacterial chromosome?