The ploidy of an organism or type of organism is a measure of how many copies of its basic chromosome set it has. This only makes sense for EukaryoticOrganisms, because ProkaryoticCells don't have Chromosomes. Many organisms, such as mammals and many trees, are diploid, meaning that their somatic (body) cells have two copies of each chromosome from their basic set. The Gametes that are produced by diploid organisms are haploid, meaning that they have only one copy of each chromosome -- half the amount of genetic material as the parent organism has.