I am working on a system that annotates variants in human DNA sequences. One of the pieces of information reported is the zygosity of the variant. I noticed that mitochondrial variants are also shown to be either homo- or heterozygous.
How does that make sense? Does it make sense? As far as I know, the mitochondrial DNA is just a single, circular DNA molecule. So I can understand considering all mitochondrial variants heterozygous, by definition. I could also accept that one could think of them as homozygous by definition. I just don't see how some variants can be homo- and others heterozygous. Am I missing something or is the annotation system I'm using not making sense?