User blog comment:B214/Question and Answers/@comment-166.216.194.86-20131015003019/@comment-9339850-20131015013550

i found an actual explination on Wikipedia

"Atavism  is the tendency to revert to ancestral type. In biology, an atavism is an  evolutionary  throwback, such as traits reappearing which had disappeared generations before. [2]  Atavisms can occur in several ways. One way is when  genes  for previously existing  phenotypical  features are preserved in  DNA, and these become expressed through a  mutation  that either knock out the overriding genes for the new traits or make the old traits override the new one. A number of traits can vary as a result of shortening of the  fetal development  of a trait ( neoteny ) or by prolongation of the same. In such a case, a shift in the time a trait is allowed to develop before it is fixed can bring forth an ancestral phenotype. "