In Biology, What Is Homology?

In the theory of biological phylogeny, if two or more structures have the same ancestor, they are called homology.

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Here the same ancestor can refer to both ancestors in the evolutionary sense, that is, two structures evolved from a common ancestor (in this sense, the wings of a bat are homologous to the arms of a human), or it can mean development Ancestors, that is, two structures developed from the same tissue during the embryonic period (in this sense, the human female ovary is homologous to the male testis).

The concept of homology needs to be distinguished from similarity. For example, insect wings, bat wings, and bird wings are similar, but have different origins. This phenomenon is called non-homologous similarity (or homomorphism, Homoplasy). These similar

In genetics, the concept of homology mainly refers to sequence homology, indicating that two or more proteins or DNA sequences have the same ancestor. Homologous sequences are also likely to have similar functions. The two sequences are either homologous or from different sources, and there is no such concept as "homology". The homologous part of a sequence is also called reserved.

The homology of proteins and DNA is often determined by the similarity of their sequences, especially in bioinformatics. For example, if two genes have almost the same DNA sequence, they are likely to be homologous, but there are exceptions: they may not have a common ancestor, but evolved to meet the need to bind to a particular protein The same form (such as a transcription factor). Such sequences are similar, but from different sources.

Non-professionals occasionally use terms such as "percent homology", which is not true. The term "percent similarity" does exist, but it can only be used to measure the similarity of biological molecular sequences, not their homology. However, if several sequences are only partially homologous (are presumed to share descent), we can call them partially homologous.

Many algorithms are able to cluster protein sequences into several families, and the sequences in each family are homologous.

Orthologs and paralogs

Homologous sequences can be divided into two types: orthology and paralogy. Orthologous sequences are separated due to speciation: if a gene originally exists in a species and the species differentiates into two species, then the genes in the new species are orthologous; Paralogous sequences are separated by gene duplication: if a gene in an organism is duplicated, the two copies are paralogous. A pair of orthologous sequences is called orthologs, and a pair of paralogous sequences is called paralogs.

Orthologs usually have the same or similar functions, but not necessarily to paralogs: due to the lack of the power of original natural selection, reproduced gene copies can freely mutate and acquire new functions.

Myoglobin and hemoglobin are considered ancient paralogs. Similarly, the four known hemoglobins (hemoglobin A, hemoglobin A2, hemoglobin S, and fetal hemoglobin) are all paralogs. They are all capable of transporting oxygen

However, there is a slight differentiation in function: fetal hemoglobin (hemoglobin F) has a higher affinity for oxygen than adult hemoglobin.

Another example is the insulin gene in rodents, such as mice. Rodents have a pair of paralogous insulin genes, and it is unclear whether their functions are differentiated.

Paralogs are common in the same species, but not absolutely: human hemoglobin and African chimpanzee myoglobin are paralogs. This is a big difficulty in predicting gene function using bioinformatics methods: even if genes of different species are homologous, we cannot immediately infer that they have the same or similar functions, because they may be paralogs with different functions.

Homologous chromosome pair

A pair of homologous chromosomes in a diploid cell is a matched pair of chromosomes. They are derived from biological parents. Except for sex chromosomes, the two chromosomes in a homologous chromosome pair are quite similar in length and usually have the same gene sequence. The two sex chromosomes have fewer similar parts than the other chromosomes. We speculate that the chromosomes are paralogs.