Final answer:
Polyploidy is the state where a cell or organism has extra sets of chromosomes beyond the normal diploid number. This condition is common in plants, making them larger and more robust, but in animals, polyploidy is rare and often results in sterility.
Step-by-step explanation:
The condition in which a normally diploid cell or organism acquires one or more additional sets of chromosomes is known as polyploidy. This can arise, for example, when there is fertilization of an abnormal diploid egg with a normal haploid sperm, resulting in a triploid zygote. Polyploid animals are quite rare and are mostly sterile because meiosis cannot proceed normally with an odd number of chromosome sets.
In contrast, polyploidy is very common in plants, and polyploid plants are often larger and more robust than their euploid counterparts. There are two main types of polyploidy, which include autopolyploidy, where an individual has multiple sets of chromosomes from its own species. This is different from aneuploidy, a condition where there is a gain or loss of chromosomes resulting in monosomy or trisomy, such as Down syndrome in humans.