Editing Most recent common ancestor (section)

==MRCA of a population identified by a single genetic marker==
{{Main|Coalescent theory}}
It is also possible to consider the ancestry of individual genes (or groups of genes, haplotypes) instead of an organism as a whole. [[Coalescent theory]] describes a stochastic model of how the ancestry of such [[genetic marker]]s maps to the history of a population.

Unlike organisms, a gene is passed down from a generation of organisms to the next generation either as perfect replicas of itself or as slightly mutated ''descendant genes''. While organisms have ancestry graphs and progeny graphs via [[sexual reproduction]], a gene has a single chain of ancestors and a tree of descendants. An organism produced by sexual cross-fertilization ([[allogamy]]) has at least two ancestors (its immediate parents), but a gene always has one ancestor per generation.