Homologous chromosome

Homologous chromosomes are chromosomes in a biological cell that pair (synapse) during cell division during the creation of gametes (meiosis).

Each chromosome pair contains genes for the same biological features, such as eye color, at the same locations (loci) on the chromosome.

Each chromosome pair can contain the same gene (both genes for blue eyes) or different genes (one gene for blue eyes and one gene for brown eyes) for each feature.

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 are separated during mitosis or meiosis II.

Homology of chromosomes is different from 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

• Comparative Maps NIH's National Library of Medicine NCBI link to Gene Homology resources, and Comparative Chromosome Maps of the Human, Mouse, and Rat.
• 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:

• Synteny server Server for Synteny Identification and Analysis of Genome Rearrangement—the Identification of synteny and calculating reversal distances.