Chromosomal polymorphism
Encyclopedia
In genetics
Genetics
Genetics , a discipline of biology, is the science of genes, heredity, and variation in living organisms....

, chromosomal polymorphism is a condition where one species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 contains members with varying chromosome
Chromosome
A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...

 counts or shapes. Polymorphism
Polymorphism (biology)
Polymorphism in biology occurs when two or more clearly different phenotypes exist in the same population of a species — in other words, the occurrence of more than one form or morph...

 is a general concept in biology
Biology
Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...

 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
Trichomycterus is a genus of catfishes, the largest genus in the family Trichomycteridae with over 100 species currently described.-Taxonomy:This genus is defined by the lack of specializations found in other trichomycterids and is certainly polyphyletic...

 davisi
, for example, is an extreme case where the polymorphism was present within a single chimeric
Chimera (genetics)
A chimera or chimaera is a single organism that is composed of two or more different populations of genetically distinct cells that originated from different zygotes involved in sexual reproduction. If the different cells have emerged from the same zygote, the organism is called a mosaic...

 individual.

It has also been studied in alfalfa
Alfalfa
Alfalfa is a flowering plant in the pea family Fabaceae cultivated as an important forage crop in the US, Canada, Argentina, France, Australia, the Middle East, South Africa, and many other countries. It is known as lucerne in the UK, France, Australia, South Africa and New Zealand, and known as...

, shrew
Shrew
A shrew or shrew mouse is a small molelike mammal classified in the order Soricomorpha. True shrews are also not to be confused with West Indies shrews, treeshrews, otter shrews, or elephant shrews, which belong to different families or orders.Although its external appearance is generally that of...

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
Polyploid is a term used to describe cells and organisms containing more than two paired sets of chromosomes. Most eukaryotic species are diploid, meaning they have two sets of chromosomes — one set inherited from each parent. However polyploidy is found in some organisms and is especially common...

. 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
Chromosomal translocation
In genetics, a chromosome translocation is a chromosome abnormality caused by rearrangement of parts between nonhomologous chromosomes. A gene fusion may be created when the translocation joins two otherwise separated genes, the occurrence of which is common in cancer. It is detected on...

 or chromosomal inversion
Chromosomal inversion
An inversion is a chromosome rearrangement in which a segment of a chromosome is reversed end to end. An inversion occurs when a single chromosome undergoes breakage and rearrangement within itself. Inversions are of two types: paracentric and pericentric.Paracentric inversions do not include the...

.

In a translocation, genetic material is transferred from one chromosome to another, either symmetrically or asymmetrically (a Robertsonian translocation
Robertsonian translocation
Robertsonian translocation is a common form of chromosomal rearrangement that in humans occurs in the five acrocentric chromosome pairs, namely 13, 14, 15, 21, and 22. Other translocations occur but do not lead to a viable fetus. They are named after the American biologist William Rees Brebner...

).

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
Speciation
Speciation is the evolutionary process by which new biological species arise. The biologist Orator F. Cook seems to have been the first to coin the term 'speciation' for the splitting of lineages or 'cladogenesis,' as opposed to 'anagenesis' or 'phyletic evolution' occurring within lineages...

. Polymorphisms will generally result in a level of reduced fertility, because some gametes 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.
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