atavism

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The reappearance of a characteristic in an organism after several generations of absence.
• noun An individual or a part that exhibits atavism.
• noun The return of a trait or recurrence of previous behavior after a period of absence.

from The Century Dictionary.

• noun In biology, reversion, through the influence of heredity, to ancestral characters; resemblance exhibited by a given organism to some remote ancestor; the return to an early or original type by its modified descendants; restoration of structural characters which have been lost or obscured.
• noun In pathology, the recurrence of any peculiarity or disease of an ancestor in remote generations.

from the GNU version of the Collaborative International Dictionary of English.

• noun The recurrence, or a tendency to a recurrence, of the original type of a species in the progeny of its varieties; resemblance to remote rather than to near ancestors; reversion to the original form.
• noun (Biol.) The recurrence of any peculiarity or disease of an ancestor in a subsequent generation, after an intermission for a generation or two.
• noun recurrence of or reversion to a past style, outlook, approach, or manner.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The reappearance of an ancestral characteristic in an organism after several generations of absence.
• noun The recurrence or reversion to a past behaviour, method, characteristic or style after a long period of absence.
• noun sociology Reversion to past primitive behavior, especially violence.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a reappearance of an earlier characteristic