<P> Ploidy is the number of complete sets of chromosomes in a cell . </P> <Ul> <Li> Polyploidy, where there are more than two sets of homologous chromosomes in the cells, occurs mainly in plants . It has been of major significance in plant evolution according to Stebbins . The proportion of flowering plants which are polyploid was estimated by Stebbins to be 30--35%, but in grasses the average is much higher, about 70% . Polyploidy in lower plants (ferns, horsetails and psilotales) is also common, and some species of ferns have reached levels of polyploidy far in excess of the highest levels known in flowering plants . <P> Polyploidy in animals is much less common, but it has been significant in some groups . </P> <P> Polyploid series in related species which consist entirely of multiples of a single basic number are known as euploid . </P> </Li> <Li> Haplo - diploidy, where one sex is diploid, and the other haploid . It is a common arrangement in the Hymenoptera, and in some other groups . </Li> <Li> Endopolyploidy occurs when in adult differentiated tissues the cells have ceased to divide by mitosis, but the nuclei contain more than the original somatic number of chromosomes . In the endocycle (endomitosis or endoreduplication) chromosomes in a' resting' nucleus undergo reduplication, the daughter chromosomes separating from each other inside an intact nuclear membrane . In many instances, endopolyploid nuclei contain tens of thousands of chromosomes (which cannot be exactly counted). The cells do not always contain exact multiples (powers of two), which is why the simple definition' an increase in the number of chromosome sets caused by replication without cell division' is not quite accurate . This process (especially studied in insects and some higher plants such as maize) may be a developmental strategy for increasing the productivity of tissues which are highly active in biosynthesis . The phenomenon occurs sporadically throughout the eukaryote kingdom from protozoa to humans; it is diverse and complex, and serves differentiation and morphogenesis in many ways . </Li> <Li> See palaeopolyploidy for the investigation of ancient karyotype duplications . </Li> </Ul> <Li> Polyploidy, where there are more than two sets of homologous chromosomes in the cells, occurs mainly in plants . It has been of major significance in plant evolution according to Stebbins . The proportion of flowering plants which are polyploid was estimated by Stebbins to be 30--35%, but in grasses the average is much higher, about 70% . Polyploidy in lower plants (ferns, horsetails and psilotales) is also common, and some species of ferns have reached levels of polyploidy far in excess of the highest levels known in flowering plants . <P> Polyploidy in animals is much less common, but it has been significant in some groups . </P> <P> Polyploid series in related species which consist entirely of multiples of a single basic number are known as euploid . </P> </Li> <P> Polyploidy in animals is much less common, but it has been significant in some groups . </P>

A karyotype with an extra set of chromosomes indicates