Q.What is Atavism ?

Atavism is a rare 'throwback' in which a trait typical of an organism's distant ancestors briefly reappears in a present-day individual, because the underlying genes for it, though normally silent, are still present and occasionally get reactivated.

Over evolutionary time, structures or characters that were useful in ancestral forms may become reduced, vestigial or entirely suppressed in descendant species as they are no longer needed, even though the underlying genetic instructions for building them are often not completely lost -- they remain present but normally switched off. Atavism refers to the occasional, sporadic reappearance of such an ancestral character in an individual member of the descendant species, due to an unusual reactivation of these dormant genes (through mutation or unusual patterns of gene expression during development), even though the character is absent in the rest of the population. Classic examples in humans include the rare birth of an individual with a small tail-like caudal appendage (reflecting our tailed ancestors), individuals with unusually excessive body/facial hair (hypertrichosis, echoing …