noun, plural: atavisms
(1) A reversion, or an individual reverting, to an earlier type; an evolutionary throwback.
(2) A reappearance of an ancestral characteristic in an organism after several generations of absence.
Atavisms occur due to chance recombination of genes. Genes coding for such traits are preserved in DNA although the genes may not be expressed in some organisms.
Word origin: Latin atavus, a great-grandfather's grandfather and, thus, more generally, an ancestor.
Related forms: atavist (noun), atavistic (adjective).
See also: pangenesis.