endoreduplication

[ĕn′dō-rĭ-dōō′plĭ-kāshən]
n.
  1. A form of polyploidy or polysomy characterized by redoubling of chromosomes, giving rise to four-stranded chromosomes at prophase and metaphase.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.