A homologue is a chromosome that shares characteristics with another chromosome inherited from a different parent. During meiosis, when chromosomes pair up to duplicate, the homologues join together. Humans have 23 pairs of homologous chromosomes in their body, consisting of 23 chromosomes inherited from each parent.
The chromosomes in a pair are the same length, and have the same alleles to code for various traits located in the same positions along their lengths. However, the alleles may be different. A person may have one allele for blonde hair and one for brown hair, for example. Depending on which is dominant, only one trait will be expressed. Some people inherit two identical alleles, one from each parent, in which case the homologue pair will have identical copies.
Each homologue is capable of finding the chromosome it matches and pairing up during meiosis. The genetic information on both chromosomes is copied for the purpose of making new cells. People who study genetics number the pairs for convenience and when the chromosomes are analyzed, one of the first steps is to pair up all of the homologues to confirm that someone has the number of chromosomes expected and to look for any obvious signs of a problem.
The sex chromosomes are a special exception to the rule that homologues are usually the same length. While the X chromosomes found in genetic females are the same size, the Y chromosome in males is actually shorter than the X chromosome. These two chromosomes create a homologue pair where the halves of the pair are different sizes. The variation in the structure of the X and Y chromosomes is a reflection of human evolution.
In people with certain genetic disorders, chromosome pairs may be disrupted in some way. Sometimes chromosomes have missing information, causing non-sex chromosomes to be different sizes, some people are missing certain chromosomes entirely, and other people have too many chromosomes. This can be identified in a karyotype, an image of the chromosomes created with the use of staining and microscopy.
The term "homologue" is also used in another sense in biology to describe traits passed down to organisms by common ancestors. Many organisms have shared physical characteristics that reflect their mutual genetic heritage, such as the forelimbs seen on mammals from horses to house cats. When evolutionary divergence occurs and physical characteristics evolve for new purposes and in different ways, they are said to be analogous.