Q.Define - "Atavism".

Atavism (Reversion)

Occasionally an individual is born showing a feature that its species lost long ago — a trait that has been absent for many generations suddenly reappears. This surprising reappearance of an ancestral character is called atavism, or reversion.

What atavism is

In atavism, a trait characteristic of a distant ancestor — one that normal members of the species no longer show — turns up again in an individual. Classic textbook examples include a human born with a short tail-like extension, or extra pairs of nipples, or a horse born with extra toes reflecting the multi-toed ancestors of modern single-hoofed horses. The trait is not new; it is a throwback to an ancestral form.

Why it happens — the genetic basis

The important idea is that atavistic genes are usually not lost during evolution — merely switched off. The genetic information for an ancestral feature can remain present but silenced (inactive) in the genome. Atavism occurs when such dormant genes become expressed again, which can happen through:

• Recombination bringing together a particular combination of genes,
• Gene interaction or the failure of a gene that normally suppresses the ancestral trait, or
• A rare change that reactivates the silenced developmental pathway.

Because the underlying genes were retained all along, the "old" trait can resurface.

Why atavism is significant …