This phenomenon occurs when the genetic material duplicates but the cell fails to divide. Then the cell becomes tetraploid, containing twice the normal amount of genetic material (4n). If such an occurrence happens in germ cell formation, then when fertilized the zygote becomes pentaploid (5n). With their XX and XY mechanism of sex determination, animals are not able to successfully survive polyploidy conditions. However, fertilized eggs that contain even numbers of chromosomes survive better than those with odd chromosome complements. Why do you think this is the case? Think about it and then scroll down the window until the answer appears.
Odd numbers of chromosomes destroy the proper pairing on the metaphase plate either during meiosis or subsequent development of the zygote. Even number of chromosomes are not the greatest thing a cell experiences but at least with them, a homologue of every chromosome can be found in the mix and, crowded as it may be along the metaphase plate, some systems manage to survive. Plants are better at polyploid survival than animals but then they do not have an XX and Xy mechanism for sex determination.
