Homologous chromosome
Encyclopedia
Homologous chromosomes are chromosome
pairs of approximately the same length, centromere
position, and staining pattern, with genes
for the same characteristics at corresponding loci
. One homologous
chromosome is inherited from the organism's mother; the other from the organism's father. They are usually not identical.
Homologous chromosomes pair (synapse
) during meiosis
- the cell division that occurs as part of the creation of gamete
s. Sections of the DNA can sometimes cross over
between homologous pairs.
Each chromosome in the pair contains genes for the same biological features, such as eye color, at the same locations (loci
) on the chromosome. However, each can contain either the same allele
(e.g., both alleles for blue eyes) or different alleles (e.g., one allele for blue eyes and one allele for brown eyes) for each feature.
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 sex chromosomes share only small regions of homology
.
Humans have 22 pairs of homologous non-sex chromosomes (called autosomes), and one pair of sex chromosomes, making a total of 46 chromosomes in a genetically normal human. Each member of a pair is inherited from one of the two parents. In addition to the 22 pairs of homologous autosomes, female humans have a homologous pair of sex chromosomes (two Xs
), while males have an X
and a Y chromosome
.
of an organism is the number of homologous versions it has of each chromosome. If the chromosomes of an organism were separated into sets of completely non-homolougous chromosomes, the ploidy would be the number of such sets (and the number of chromosomes that would be in each such set is termed the monoploid number).
Most plants and animals are diploid organisms. Their chromosomes come in pairs; each chromosome in a diploid organism has exactly one homologous partner. Each chromosome from these pairs is inherited from a different parent
(presuming the organism reproduces sexually from two parents). In fact the chromosomes physically arrange themselves into homologous pairs during meiosis
, and divide into monoploid (i.e., haploid) sets when forming gametes.
Polyploid organisms have more than two homologous chromosomes.
(the driver of the process of evolution
) depends.
(Sister chromatids in mitosis are each strand of the two identical conjoined chromosomes (the product of chromosome replication) prior to separation during the anaphase
.)
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...
pairs of approximately the same length, centromere
Centromere
A centromere is a region of DNA typically found near the middle of a chromosome where two identical sister chromatids come closest in contact. It is involved in cell division as the point of mitotic spindle attachment...
position, and staining pattern, with genes
Gênes
Gênes is the name of a département of the First French Empire in present Italy, named after the city of Genoa. It was formed in 1805, when Napoleon Bonaparte occupied the Republic of Genoa. Its capital was Genoa, and it was divided in the arrondissements of Genoa, Bobbio, Novi Ligure, Tortona and...
for the same characteristics at corresponding loci
Locus (genetics)
In the fields of genetics and genetic computation, a locus is the specific location of a gene or DNA sequence on a chromosome. A variant of the DNA sequence at a given locus is called an allele. The ordered list of loci known for a particular genome is called a genetic map...
. One homologous
Homology (biology)
Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...
chromosome is inherited from the organism's mother; the other from the organism's father. They are usually not identical.
Homologous chromosomes pair (synapse
Synapsis
Synapsis is the pairing of two homologous chromosomes that occurs during meiosis. It allows matching-up of homologous pairs prior to their segregation, and possible chromosomal crossover between them. Synapsis takes place during prophase I. When homologous chromosomes synapse, their ends are...
) during meiosis
Meiosis
Meiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....
- the cell division that occurs as part of the creation of gamete
Gamete
A gamete is a cell that fuses with another cell during fertilization in organisms that reproduce sexually...
s. Sections of the DNA can sometimes cross over
Chromosomal crossover
Chromosomal crossover is an exchange of genetic material between homologous chromosomes. It is one of the final phases of genetic recombination, which occurs during prophase I of meiosis in a process called synapsis. Synapsis begins before the synaptonemal complex develops, and is not completed...
between homologous pairs.
Each chromosome in the pair contains genes for the same biological features, such as eye color, at the same locations (loci
Locus (genetics)
In the fields of genetics and genetic computation, a locus is the specific location of a gene or DNA sequence on a chromosome. A variant of the DNA sequence at a given locus is called an allele. The ordered list of loci known for a particular genome is called a genetic map...
) on the chromosome. However, each can contain either the same allele
Allele
An allele is one of two or more forms of a gene or a genetic locus . "Allel" is an abbreviation of allelomorph. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation...
(e.g., both alleles for blue eyes) or different alleles (e.g., one allele for blue eyes and one allele for brown eyes) for each feature.
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-determining chromosomes in many animal species, including mammals and is common in both males and females. 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 one of the two sex-determining chromosomes in most mammals, including humans. In mammals, it contains the gene SRY, which triggers testis development if present. The human Y chromosome is composed of about 60 million base pairs...
. These sex chromosomes share only small regions of homology
Homology (biology)
Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...
.
Humans have 22 pairs of homologous non-sex chromosomes (called autosomes), and one pair of sex chromosomes, making a total of 46 chromosomes in a genetically normal human. Each member of a pair is inherited from one of the two parents. In addition to the 22 pairs of homologous autosomes, female humans have a homologous pair of sex chromosomes (two Xs
X chromosome
The X chromosome is one of the two sex-determining chromosomes in many animal species, including mammals and is common in both males and females. It is a part of the XY sex-determination system and X0 sex-determination system...
), while males have an X
X chromosome
The X chromosome is one of the two sex-determining chromosomes in many animal species, including mammals and is common in both males and females. 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 one of the two sex-determining chromosomes in most mammals, including humans. In mammals, it contains the gene SRY, which triggers testis development if present. The human Y chromosome is composed of about 60 million base pairs...
.
Ploidy
The ploidyPloidy
Ploidy is the number of sets of chromosomes in a biological cell.Human sex cells have one complete set of chromosomes from the male or female parent. Sex cells, also called gametes, combine to produce somatic cells. Somatic cells, therefore, have twice as many chromosomes. The haploid number is...
of an organism is the number of homologous versions it has of each chromosome. If the chromosomes of an organism were separated into sets of completely non-homolougous chromosomes, the ploidy would be the number of such sets (and the number of chromosomes that would be in each such set is termed the monoploid number).
Most plants and animals are diploid organisms. Their chromosomes come in pairs; each chromosome in a diploid organism has exactly one homologous partner. Each chromosome from these pairs is inherited from a different parent
Parent
A parent is a caretaker of the offspring in their own species. In humans, a parent is of a child . Children can have one or more parents, but they must have two biological parents. Biological parents consist of the male who sired the child and the female who gave birth to the child...
(presuming the organism reproduces sexually from two parents). In fact the chromosomes physically arrange themselves into homologous pairs during meiosis
Meiosis
Meiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....
, and divide into monoploid (i.e., haploid) sets when forming gametes.
Polyploid organisms have more than two homologous chromosomes.
Sister chromatids
In meiosis sister chromatids are each strand of the conjoined chromosomes which result from the replication and crossover of homologous chromosomes during that process, prior to separation during each of the two anaphases. This causes each of the four resultant gametes to have a unique genetic combination from both parents, providing the genetic variation upon which natural selectionNatural selection
Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....
(the driver of the process of evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
) depends.
(Sister chromatids in mitosis are each strand of the two identical conjoined chromosomes (the product of chromosome replication) prior to separation during the anaphase
Anaphase
Anaphase, from the ancient Greek ἀνά and φάσις , is the stage of mitosis or meiosis when chromosomes move to opposite poles of the cell....
.)
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) links to a number of resources.