Homologous chromosome

Homologous chromosome

Homologous chromosomes are chromosomes in a biological cell that pair (synapse) during meiosis, or alternatively, non-identical chromosomes that contain information for the same biological features and contain the same genes at the same loci but possibly different genetic information, called alleles, at those genes. For example, two chromosomes may have genes encoding eye color, but one may code for brown eyes, the other for blue.

Non-homologous chromosomes representing all the biological features of an organism form a set, and the number of sets in a cell is called ploidy. In diploid organisms (most plants and animals), each member of a pair of homologous chromosomes is inherited from a different parent. But polyploid organisms have more than two homologous chromosomes.

Homologous chromosomes are similar in length, except for sex chromosomes in several taxa, where the X chromosome is considerably larger than the Y chromosome. These chromosomes share only small regions of homology.

Humans have 22 pairs of homologous non-sex chromosomes (called autosomes). Each member of a pair is inherited from one of their two parents. In addition, female humans have a homologous pair of sex chromosomes (2 X's); males have an X and a Y chromosome.

Homologous chromosomes are two pairs of sister chromatids that have gone through the process of crossing over and meiosis. In this process the homologous chromosomes cross over (not the sister chromatids)each other and exchange genetic information. This causes each final cell of meiosis to have genetic information from both parents, a mechanism for genetic variation. The homologous chromosomes are similar in length.

Other terms

Homologous chromosomes are not to be confused with sister chromatids, the identical chromosomes that separate during mitosis or meiosis II.

Homology of chromosomes is different than "homology of genetic sequences", and predates that use of the term "homology". An exception arises with allopolyploidy where chromosomes pair up by age, so sequence similarity is used to determine the original chromosomal sets.

Homologous chromosomes are also very similar to, and often confused with the term synteny (or gene homology)—which refers to genes located on the same section of chromosome between individuals of the same or different species.

External links

* [http://www.ncbi.nlm.nih.gov/Homology/ Comparative Maps] NIH's National Library of Medicine NCBI link to Gene Homology resources, and Comparative Chromosome Maps of the Human, Mouse, and Rat.
* [http://www.ncbi.nlm.nih.gov/ NCBI Home Page] NIH's National Library of Medicine NCBI (National Center for Biotechnology Information) link to a tremendous number of resources.

See also a link from the synteny definition:
* [http://cinteny.cchmc.org/ Synteny server] Server for Synteny Identification and Analysis of Genome Rearrangement—the Identification of synteny and calculating reversal distances.


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