Recombination
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[edit] Introduction
Recombination is a means by which a sexually reproducing organism passes on to the next generation not one or the other of each of its pairs of homologous chromosomes, but rather a mixture of the two.
[edit] Chromosomes and reproduction: a recap
Organisms that reproduce sexually are diploid: that is, they have two of each chromosome, with each of these homologous pairs of chromosomes having the same genes, but possibly different alleles of these genes.
When a diploid organism produces gametes (sperm or eggs) these cells have to contain only one of each chromosome, so that when the sperm and the egg combine their genes in fertilization to produce a zygote, the resulting organism is diploid again.
It would seem from this bare description that what must happen is that each organism passes on one of each of its pairs of homologous chromosomes to each gamete. Instead, something rather more complicated happens. Before the production of the gametes, the two homologous chromosomes get together and swap around homologous chunks of their DNA. This process is called recombination or crossing over. The chromosome that each organism transmits via the gamete is therefore neither of its own chromosomes, but rather a mixture of the two.
The whole process of reproduction is described in our article on chromosomes, particularly the section on meiosis. This article focuses on the mechanics of recombination and its consequences.
[edit] Recombination
When the process begins, the chromosomes have already undergone DNA synthesis, so that each chromosome consists of two identical sister chromatids. Each homologous pair of chromosomes in the cell pair up and overlap. Then they literally cross over one another. The chromosomes then break at the crossing points and then rejoin in such a way as to swap around their DNA. (It is also possible for the chromosomes to break at one of the crossing-points and then rejoin exactly the same way as they were, which of course has no net effect on the chromosomes. We haven't shown such an event on the diagram.)
When this process is finished, the two chromosomes part: their genes have been shuffled so that none of the chromatids of the chromosomes need be the same as it started, but each is some mixture of the original pair of sister chromosomes.
After two more cell divisions, and the parting of the sister chromatids, each of the four will end up in one of four different haploid gametes: for more information, see the section on meiosis in our article on chromosomes.
[edit] Recombination and evolution
To see what recombination is good for, consider a world without recombination. Suppose a beneficial mutation occurs. We must here stress the point that a mutation is something which initially is present in one chromosome of one individual.
Suppose this beneficial mutation occurs on a chromosome which is otherwise of poor quality compared to homologous chromosomes in other members of the species (there is, after all, no reason at all, why beneficial mutations should only happen to the fittest members of a species).
Now, if there was no recombination, then one of two things might happen, neither of them entirely desirable. If the poor quality of the chromosome outweighs the benefits of the mutation, then natural selection will tend to remove that variant of the chromosome from the gene pool; so natural selection will eliminate this new mutation even though in and of itself it is beneficial. If, on the other hand, the mutation is so beneficial that it outweighs the poor quality of the rest of the chromosome, then natural selection would favor this chromosome, despite its other defects, which will spread through the gene pool along with the new mutation. In short, if there was no recombination, natural selection would operate on chromosomes, rather than genes, which would be less efficient.
Some organisms, such as bacteria, do reproduce asexually and so do labor under these handicaps: however, in the evolutionary stakes, they make up for this by their large numbers and rapid rate of reproduction.
[edit] Linkage
Because each chromosome pair undergoes only a few crossings-over, or even none, there is a tendency for genes that lie close together on the chromosome to still be together after recombination. Whether two genes will stay together depends on chance: if there happens to be a crossing-over between them, then they will be separated. It follows from this that the closer the genes are on the chromosome, the less likely is that a crossing-over between them, and the more likely it is that they stick together. In the diagram above, for example, the red and orange genes have assorted together in every chromatid, sticking with the partners they came with; as have green and turquoise; and blue and purple. This tendency is known as linkage.
This principle allowed biologists to make maps of the genome of the fruit fly by a series of breeding experiments. The following explanation will only make sense to the reader familiar with the terminology of Mendelian genetics; for those who are not, we recommend our article on the subject.
Suppose we want to find out whether two genes are on the same chromosome, and, if so, how close they are together. Suppose further that each of these genes comes in at least two possible alleles, one of which is dominant to the other. Let's call the two alleles of the first gene A and a, with A dominant to a; and similarly call the alleles of the second gene B and b.
Take two true-breeding lines of the species in question, one with genotype AABB and one with genotype aabb, and cross them together. The generation produced by this first cross (the F1 generation) will have genotype AaBb.
If we then cross the F1 generation back with the aabb line, then if the two genes are on different chromosomes, and assort independently, then the result of this cross will be divided equally between AaBb, Aabb, aaBb and aabb, in accordance with Mendel's law of independent assortment. Because A is dominant to a, and B to b, it follows that AaBb, Aabb, aaBb and aabb correspond to four different phenotypes, so it is easy to check if this is the case.
However, if the genes lie on the same chromosome, then we should see a different pattern. For in that case each member of the F1 generation received an AB chromosome from one parent and an ab chromosome from the other. When the F1 generation produce their gametes, recombination may produce some Ab and aB type gametes, but if the two genes display linkage, then most of the gametes produced will either be AB or ab.
This means that when we cross the F1 generation back with aabb, we should not get equal proportions of AaBb, Aabb, aaBb and aabb as a result: rather, we should see more of AaBb and aabb and less of Aabb and aaBb. And the closer the genes lie together on the chromosome, the less likely recombination is to mix them up, so the greater the divergence from equal proportions, the closer the two genes lie together on the chromosomes.
