Non-nuclear Inheritance

In eukaryotic cells, mitochondria and plastids, a category of plant cell organelles that includes chloroplasts, reproduce independently in the cytoplasm and they each have their own set of genes that directs their reproduction and function. Mitochondrial and plastid DNA takes the form of small, circular DNA molecules and is referred to as non-nuclear or cytoplasmic DNA. Each organelle carries many identical copies of its DNA, in contrast to the nucleus, where there are typically only two copies of each chromosome.

Further, during cell division, the organelles are randomly segregated into new daughter cells, so the non-nuclear DNA that is passed on does not follow a set pattern. During reproduction in animals and most plants, the cytoplasm in the zygote comes from the egg. Thus, while both parents contribute equally to the nuclear DNA, the non-nuclear DNA is passed down only from the mother. This means that certain traits, such as metabolic disorders resulting from mutations in the mitochondria, can be traced in a matrilineal fashion.

Additionally, some physical traits in plants, such as leaf color, are controlled by non-nuclear genes and are also passed down uniparentally.