Gene coupling

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In genetics, gene linkage is understood as the phenomenon that some features encoded by genes are inherited together. The genes do not behave according to the third Mendelian rule ( rule of independence). This phenomenon is explained by the fact that the number of genes far exceeds the number of homologous chromosome pairs. In humans, for example, around 20,000 - 25,000 genes have to be distributed over 23 chromosomes . A chromosome can also be referred to as a coupling group because the genes can be inherited together.

During meiosis , genetic material can be exchanged between chromosomes. While the four chromatids of two homologous chromosomes are arranged side by side to form a tetrad ( synaptonemaler complex ), the chromatids are superimposed and broken, which are linked again crosswise ( chiasmata ) and thus recombine . This process is also called crossing-over . The genes of a chromosome can also be separated from one another (they are "decoupled"). Genes on different chromosomes are always decoupled, which can be seen from the fact that they are inherited according to Mendel's third rule.

The closer two genes are to each other, the less often they are separated, so the probability of recombination is proportional to the distance between the genes. This connection allows mapping on a chromosome with the help of at least three genes . This is done with a so-called coupling analysis . The relative distance between genes can also be determined ( gene map ). If two genes are separated per 100 meioses in one case, it has been defined that they are separated by 1 centiMorgan (cM). In humans, 1 cM is an average of about 1 million base pairs , but this can vary widely.

There are also accumulation points ( hot spots ) for the formation of chiasmata, so that certain genes are more often inherited together than would be statistically the case. An example of gene coupling are the genes that are responsible for self-incompatibility in plants .

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