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Homologous chromosome

 

 
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Homologous chromosome
 
 
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Homologous chromosomes are chromosomes in a biological cell that pair (synapse
Synapsis

Synapsis is the pairing of two homologous chromosomes that occurs during meiosis. It is a form of chromosomal crossover. Synapsis takes place during prophase I....
) during meiosis
Meiosis

In biology or life science, meiosis is a process of reductional division in which the number of chromosomes per cell is halved. In animals, meiosis always results in the formation of gametes, while in other organisms it can give rise to spores....
, or alternatively, non-identical chromosomes that contain information for the same biological features and contain the same gene
Gene

A gene is the basic unit of heredity in a living organism. All living things depend on genes. Genes hold the information to build and maintain their cell and pass genetic trait to offspring....
s at the same loci
Locus (genetics)

In the fields of genetics and evolutionary computation, a locus is a fixed position on a chromosome such as the position of a genetic marker that may be occupied by one or more genes....
 but possibly different genetic information, called allele
Allele

An allele is one member of a pair or series of different forms of a gene. Usually alleles are coding region, but sometimes the term is used to refer to a junk DNA....
s, 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
Ploidy

Ploidy is the number of complete sets of non-homologous chromosomes in a biological cell. In humans, the somatic cells that comprise the body are diploid , but sex cells are haploid....
.






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Homologous chromosomes are chromosomes in a biological cell that pair (synapse
Synapsis

Synapsis is the pairing of two homologous chromosomes that occurs during meiosis. It is a form of chromosomal crossover. Synapsis takes place during prophase I....
) during meiosis
Meiosis

In biology or life science, meiosis is a process of reductional division in which the number of chromosomes per cell is halved. In animals, meiosis always results in the formation of gametes, while in other organisms it can give rise to spores....
, or alternatively, non-identical chromosomes that contain information for the same biological features and contain the same gene
Gene

A gene is the basic unit of heredity in a living organism. All living things depend on genes. Genes hold the information to build and maintain their cell and pass genetic trait to offspring....
s at the same loci
Locus (genetics)

In the fields of genetics and evolutionary computation, a locus is a fixed position on a chromosome such as the position of a genetic marker that may be occupied by one or more genes....
 but possibly different genetic information, called allele
Allele

An allele is one member of a pair or series of different forms of a gene. Usually alleles are coding region, but sometimes the term is used to refer to a junk DNA....
s, 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
Ploidy

Ploidy is the number of complete sets of non-homologous chromosomes in a biological cell. In humans, the somatic cells that comprise the body are diploid , but sex cells are haploid....
. In diploid organisms (most plants and animals), each member of a pair of homologous chromosomes is inherited from a different parent
Parent

A parent is a mother or father; one who sexual reproduction or gives birth to and/or nurtures and raises an offspring. The different roles of parents vary throughout the tree of life, and are especially complex in human culture....
. 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
X chromosome

The X chromosome is one of the two sex determination system chromosomes in many animal species, including mammals . It is a part of the XY sex-determination system and X0 sex-determination system....
 is considerably larger than the Y chromosome
Y chromosome

The Y chromosome is the Sex-determination system chromosome in most mammals, including humans. In mammals, it contains the gene SRY, which triggers testicle development, thus determining sex....
. These chromosomes share only small regions of homology
Homology (biology)

In evolutionary biology, homology refers to any similarity between characteristics that is due to their common descent. The word homologous derives from the ancient Greek ??????e??, 'to agree'....
.

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
X chromosome

The X chromosome is one of the two sex determination system chromosomes in many animal species, including mammals . It is a part of the XY sex-determination system and X0 sex-determination system....
); males have an X
X chromosome

The X chromosome is one of the two sex determination system chromosomes in many animal species, including mammals . It is a part of the XY sex-determination system and X0 sex-determination system....
 and a Y chromosome
Y chromosome

The Y chromosome is the Sex-determination system chromosome in most mammals, including humans. In mammals, it contains the gene SRY, which triggers testicle development, thus determining sex....
.

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
Mitosis

Mitosis is the process in which a eukaryotic cell separates the chromosomes in its cell nucleus, into two identical sets in two daughter nuclei....
 or meiosis II
Meiosis

In biology or life science, meiosis is a process of reductional division in which the number of chromosomes per cell is halved. In animals, meiosis always results in the formation of gametes, while in other organisms it can give rise to spores....
.

Homology of chromosomes is different from homology of genetic sequences
Homology (biology)

In evolutionary biology, homology refers to any similarity between characteristics that is due to their common descent. The word homologous derives from the ancient Greek ??????e??, 'to agree'....
, 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
Synteny

In classical genetics, synteny describes the physical co-localization of Locus on the same chromosome within an individual or species. The concept is related to genetic linkage: Linkage between two loci is established by the observation of lower-than-expected recombination frequencies between two or more loci....
 (or gene homology)—which refers to genes located on the same section of chromosome between individuals of the same or different species.

External links

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

In classical genetics, synteny describes the physical co-localization of Locus on the same chromosome within an individual or species. The concept is related to genetic linkage: Linkage between two loci is established by the observation of lower-than-expected recombination frequencies between two or more loci....
 definition:
  • Server for Synteny Identification and Analysis of Genome Rearrangement—the Identification of synteny and calculating reversal distances.