Homologous chromosomes share characteristics and are inherited from different parents. Humans have 23 pairs of homologous chromosomes, with the same length and alleles, but may have different traits. Sex chromosomes are an exception, with the Y chromosome being shorter. Chromosome pairings can be disrupted in genetic disorders, identified through karyotyping. Homologous traits also reflect mutual genetic makeup in organisms.
A homolog is a chromosome that shares characteristics with another chromosome inherited from a different parent. During meiosis, when chromosomes pair to duplicate, 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 encode various traits located at the same positions along their lengths. However, the alleles may be different. A person may have one allele for blond hair and one for brown hair, for example. Whichever is dominant, only one trait will be expressed. Some people inherit two identical alleles, one from each parent, in which case the homologous pair will have identical copies.
Each homologue is able to find the chromosome it corresponds to and to mate during meiosis. The genetic information on both chromosomes is copied for the purpose of making new cells. People who study genetics number pairs for convenience, and when chromosomes are analyzed, one of the first steps is to pair all homologues to confirm that someone has the expected number of chromosomes and to look for any obvious signs of a problem.
Sex chromosomes are a special exception to the rule that homologs 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 homologous pair where the halves of the pair are of 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, the pairings of chromosomes can be disrupted in some way. Sometimes chromosomes have missing information, causing nonsex chromosomes to be of different sizes, some people are completely missing certain chromosomes, and some people have too many chromosomes. This can be identified in a karyotype, a picture of the chromosomes created with the use of staining and microscopy.
The term “homologous” is also used in another sense in biology to describe traits passed down to organisms from common ancestors. Many organisms have shared physical characteristics that reflect their mutual genetic makeup, such as the forelimbs seen on mammals from horses to domestic cats. When an evolutionary divergence occurs and physical characteristics evolve for new purposes and in different ways, they are said to be analogous.
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