Ploidy
Ploidy describes chromosome set number in cells.
Ploidy is the number of complete sets of chromosomes in a cell, determining the number of possible alleles for autosomal and pseudoautosomal genes. Somatic cells, tissues, and organisms are described by ploidy level, ranging from monoploid (1 set) to polyploid (3 or more sets). Virtually all sexually reproducing organisms have diploid or greater somatic cells, but ploidy varies widely among organisms, tissues, and life stages. Polyploidy is common in plants, with half of all known plant genera containing polyploid species and about two-thirds of grasses being polyploid. In animals, polyploidy is frequent in invertebrates, reptiles, and amphibians, while mammals and birds typically experience fatal ploidy changes. Humans are normally diploid, with 23 pairs of chromosomes in somatic cells.
- definition
- Number of complete sets of chromosomes in a cell
- key_terms
- haploid, diploid, polyploid, monoploid, aneuploid
- human_ploidy
- diploid (46 chromosomes, 23 pairs)
- common_in
- plants, fungi, invertebrates, reptiles, amphibians
- speciation_role
- major driver in plants and fungi; evidence in animals
- etymology
- back-formation from haploidy and diploidy; coined by Eduard Strasburger in 1905
Lore & Background
The term ploidy is a back-formation from haploidy and diploidy. 'Ploid' combines Ancient Greek -πλόος ('-fold') and -ειδής ('form, likeness'). Polish-German botanist Eduard Strasburger coined haploid and diploid in 1905, possibly based on August Weismann's conception of the id. The terms entered English via William Henry Lang's 1908 translation of a Strasburger textbook. Haploid has two definitions: the number of sets in a gamete (n) or a single set of chromosomes (x). Monoploid is used for a single set to avoid ambiguity. Gametes are haploid by definition, even if numerically diploid in polyploid organisms.
Reader's Guide
Ploidy is fundamental to genetics and evolutionary biology. It defines chromosome number in cells and influences reproduction, development, and speciation. In plants and fungi, ploidy changes between generations drive speciation, with polyploidy common in plant genera and grasses. In animals, polyploidy occurs in invertebrates, reptiles, and amphibians, while mammals and birds typically cannot survive ploidy changes. Humans are diploid, with 23 chromosome pairs; aneuploidy, such as Turner syndrome (45,X), involves an abnormal chromosome number. The distinction between haploid (n) and monoploid (x) is important in polyploid species like common wheat, where x and n differ. Ploidy concepts underpin understanding of heredity, evolution, and chromosomal disorders.
Did You Know?
- Half of all known plant genera contain polyploid species, and about two-thirds of all grasses are polyploid.
- In mammals and birds, ploidy changes are typically fatal, but polyploidy is common in invertebrates, reptiles, and amphibians.
- The terms haploid and diploid were coined by Polish-German botanist Eduard Strasburger in 1905.
- A person with Turner syndrome has a (45,X) karyotype, an example of aneuploidy.
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