genetics Genetics is the study of genes, genetic variation, and heredity in organisms.Hartl D, Jones E (2005) It is an important branch in biology because heredity is vital to organisms' evolution. Gregor Mendel, a Moravian Augustinian friar wor ...

, chromosomal polymorphism is a condition where one

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

contains members with varying

chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins are ...

counts or shapes. Polymorphism is a general concept in

biology Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of cells that process hereditary i ...

where more than one version of a trait is present in a population. In some cases of differing counts, the difference in chromosome counts is the result of a single chromosome undergoing fission, where it splits into two smaller chromosomes, or two undergoing fusion, where two chromosomes join to form one. This condition has been detected in many species. ''

Trichomycterus ''Trichomycterus'' is a genus of fish in the family Trichomycteridae, the largest genus of its family with over 170 species currently described. This genus is native to freshwater habitats in Central and South America. These fish are generally sm ...

davisi'', for example, is an extreme case where the polymorphism was present within a single chimeric individual. It has also been studied in alfalfa,

shrew Shrews (family Soricidae) are small mole-like mammals classified in the order Eulipotyphla. True shrews are not to be confused with treeshrews, otter shrews, elephant shrews, West Indies shrews, or marsupial shrews, which belong to differ ...

s, Brazilian rodents, and an enormous variety of other animals and plants. In one instance it has been found in a human. Another process resulting in differing chromosomal counts is

polyploidy Polyploidy is a condition in which the biological cell, cells of an organism have more than one pair of (Homologous chromosome, homologous) chromosomes. Most species whose cells have Cell nucleus, nuclei (eukaryotes) are diploid, meaning they ha ...

. This results in cells which contain multiple copies of complete chromosome sets. Possessing chromosomes of varying shapes is generally the result of a

chromosomal translocation In genetics, chromosome translocation is a phenomenon that results in unusual rearrangement of chromosomes. This includes balanced and unbalanced translocation, with two main types: reciprocal-, and Robertsonian translocation. Reciprocal translo ...

or chromosomal inversion. In a translocation, genetic material is transferred from one chromosome to another, either symmetrically or asymmetrically (a Robertsonian translocation). In an inversion, a segment of a chromosome is flipped end-for-end.

Implications for speciation

All forms of chromosomal polymorphism can be viewed as a step towards speciation. Polymorphisms will generally result in a level of reduced fertility, because some

gamete A gamete (; , ultimately ) is a haploid cell that fuses with another haploid cell during fertilization in organisms that reproduce sexually. Gametes are an organism's reproductive cells, also referred to as sex cells. In species that produce ...

s from one parent cannot successfully combine with all gametes of the other parent. However, when both parents contain matching chromosomal patterns, this obstacle does not occur. Further mutations in one group will not flow as rapidly into the other group as they do within the group in which it originally occurred. Further mutations can also cause absolute infertility. If an interbreeding population contains one group in which (for example) chromosomes A and B have fused, and another population in which chromosomes B and C have fused, both populations will be able to interbreed with the parent population. However, the two subpopulations will not be able to breed successfully with each other if the doubling of chromosome B is fatal. Similar difficulties will occur for incompatible translocations of material.

References

{{DEFAULTSORT:Chromosomal Polymorphism Classical genetics Chromosomes