Polyploidy refers to the phenomenon observed in some species of having more than two sets of chromosomes in their cells. A simple (haploid) set of chromosomes contains each chromosome once, while a double (diploid) set contains each chromosome twice. An odd-numbered chromosome set is referred to as an aneuploidy, while an even-numbered set is called an orthoploidy. Polyploidy occurs during meiosis when chromosomes are duplicated. If spindle fibers are not formed or if the homologous chromosome pairs are not separated during the reduction division for other reasons, diploid gametes are produced. The causes of such failures in separation can be due to metabolic disorders, environmental influences, or human intervention.