Mixing together of the genetic material coming from different sources.
Recombination comes in a number of types, at least some of which are examples of what can be described as genetic recombination. These types are molecular recombination where polynucleic acids – typically but not always DNA – line up and exchange parts and independent assortment where separate polynucleic acids are randomly mixed together. The difference is that one involves making and breaking of chemical bonds whereas the other entails a random mixing of genetic material that individually remain fully, physically intact.
Not necessarily included among genetic recombination but definitely a form of recombination is what is known as site-specific recombination as practiced by temperate bacteriophages. There is also illegitimate recombination – involving very low levels of homology between sequences – which also arguably is not what is being considered on the heading of genetic recombination but nonetheless is a form of recombination (and an important one as well, particularly evolutionarily, though somewhat rare in its occurrence).
Recombination is seen during meiosis in sexual eukaryotes but also is seen in a large number of situations in which polynucleic acids from one organism, again, usually DNA, find their way into the genomes of another organism.
Alternatively, otherwise separate genetic parts within the same organism can become assembled in different ways that involves a combining of sequences. The result, ultimately, is an alternative to mutation as a means of modifying an organism's genetic complement (and, again, is not exactly equivalent to the concept of genetic recombination).
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