Homologous chromosomes (also called homologs or homologues) 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.[1] They are usually not identical.
Homologous chromosomes pair (synapse) during meiosis - the cell division that occurs as part of the creation of gametes. 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.
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The ploidy 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.
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 selection (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.)