- Homologous chromosome
Homologous chromosomes are
chromosomesin a biological cellthat 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 diploidorganisms (most plants and animals), each member of a pair of homologous chromosomes is inherited from a different parent. But polyploidorganisms have more than two homologous chromosomes.
Homologous chromosomes are similar in length, except for
sex chromosomes in several taxa, where the X chromosomeis 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.
Homologous chromosomes are not to be confused with
sister chromatids, the identical chromosomes that separate during mitosisor meiosis II.
Homology of chromosomes is different than "homology of genetic sequences", and predates that use of the term "homology". An exception arises with
allopolyploidywhere 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.
* [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
* [http://cinteny.cchmc.org/ Synteny server] Server for Synteny Identification and Analysis of Genome Rearrangement—the Identification of synteny and calculating reversal distances.
Wikimedia Foundation. 2010.
Look at other dictionaries:
Chromosome segregation — is a step in cell reproduction or division, where chromosomes pair off with their similar homologous chromosome. In mitosis, a complete copy of each one is made. In meiosis, one chromosome from each pair migrates to opposite ends of the cell and… … Wikipedia
Homologous recombination — Figure 1. During meiosis, homologous recombination can produce new combinations of genes as shown here between similar but not identical copies of human chromosome 1. Homologous recombination is a type of genetic recombination in which nucleotide … Wikipedia
Chromosome — For a non technical introduction to the topic, see Introduction to genetics. Diagram of a replicated and condensed metaphase eukaryotic chromosome. (1) Chromatid – one of the two identical parts of the chromosome after S phase. (2)… … Wikipedia
chromosome — chromosomal, adj. chromosomally, adv. /kroh meuh sohm /, n. Genetics. any of several threadlike bodies, consisting of chromatin, that carry the genes in a linear order: the human species has 23 pairs, designated 1 to 22 in order of decreasing… … Universalium
Chromosome — A visible carrier of the genetic information. The 3 billion bp (base pairs) in the human genome are organized into 24 distinct, physically separate microscopic units called chromosomes. All genes are arranged linearly along the chromosomes. The… … Medical dictionary
chromosome — Synonyms and related words: Altmann theory, DNA, De Vries theory, Galtonian theory, Mendelianism, Mendelism, RNA, Verworn theory, W chromosome, Weismann theory, Weismannism, Wiesner theory, X chromosome, Y chromosome, Z chromosome, accessory… … Moby Thesaurus
Chromosome regions — Diagram of a duplicated and condensed (metaphase) eukaryotic chromosome. (1) Chromatid one of the two identical parts of the chromosome after S phase. (2) Centromere the point where the two chromatids touch, and where the microtubules attach. (3) … Wikipedia
homologous — adjective Showing a degree of correspondence or similarity. : Corresponding to a similar structure in another life form with a common evolutionary origin. :, : Lobules, homologous in structure, recur again only in the Gondwanalandic families… … Wiktionary
Homologous — The relationship between two chromosomes that are paired and so are homologs of each other. * * * Corresponding or alike in certain critical attributes. 1. In biology or zoology, denoting organs or parts corresponding in evolutionary origin and… … Medical dictionary
Chromosome duplication — Part of a chromosome in duplicate. A particular kind of mutation involving the production of one or more copies of any piece of DNA, including sometimes a gene or even an entire chromosome. A duplication is the opposite of a deletion.… … Medical dictionary