Recombination frequency
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In genetics, recombination frequency is when crossing-over will take place between two loci (or genes) during meiosis. Recombination frequency is a measure of genetic linkage and is used in the creation of a genetic linkage map. A centimorgan (cM) is a unit that describes a recombination frequency of 1%.
During meiosis, chromosomes assort randomly into gametes, such that the segregation of alleles of one gene is independent of alleles of another gene. This is stated in Mendel's Second Law and is known as the law of independent assortment. The law of independent assortment always holds true for genes that are located on different chromosomes, but for genes that are on the same chromosome, it does not always hold true.
As an example of independent assortment, consider the crossing of the pure-bred homozygote parental strain with genotype AABB with a different pure-bred strain with genotype aabb. A and a and B and b represent the alleles of genes A and B. Crossing these homozygous parental strains will result in F1 generation offspring with genotype AaBb. The F1 offspring AaBb produces gametes that are AB, Ab, aB, and ab with equal frequencies (25%) due to the law of independent assortment. Note that 2 of the 4 gametes (50 %)—Ab and aB—were not present in the parental generation. These gametes represent recombinant gametes. Recombinant gametes are those gametes that differ from both of the haploid gametes that made up the diploid cell. In this example, the recombination frequency is 50% since 2 of the 4 gametes were recombinant gametes.
The recombination frequency will be 50% when two genes are located on different chromosomes or when they are widely separated on the same chromosome. This is a consequence of independent assortment.
When two genes are close together on the same chromosome, they do not assort independently and are said to be linked. Whereas genes located on different chromosomes assort independently and have a recombination frequency of 50%, linked genes have a recombination frequency that is less than 50%.
As an example of linkage, consider the classic experiment by William Bateson and Reginald Punnett. They were interested in trait inheritance in the sweet pea and were studying two genes—the gene for flower color (P, purple, and p, red) and the gene affecting the shape of pollen grains (L, long, and l, round). They crossed the pure lines PPLL and ppll and then self-crossed the resulting PpLl lines. According to Mendelian genetics, the expected phenotypes would occur in a 9:3:3:1 ratio of PL:Pl:pL:pl. To their surprise, they observed an increased frequency of PL and pl and a decreased frequency of Pl and pL (see chart below).
| Phenotype and genotype | Observed | Expected from 9:3:3:1 ratio |
|---|---|---|
| Purple, long (P_L_) | 284 | 216 |
| Purple, round (P_ll) | 21 | 72 |
| Red, long (ppL_) | 21 | 72 |
| Red, round (ppll) | 55 | 24 |
Their experiment revealed linkage (or coupling) between the P and L alleles and the p and l alleles. The frequency of P occurring together with L and with p occurring together with l is greater than that of the recombinant Pl and pL. The recombinantion frequency cannot be computed directly from this experiment, but intuitively it is less than 50%.
When two genes are located on the same chromosome, the chance of a crossover producing recombination between the genes is directly related to the distance between the two genes. Thus, the use of recombinantion frequencies has been used to develop linkage maps or genetic maps.
[edit] Centimorgan, a unit of recombination frequency
In genetics, a centimorgan (abbreviated cM) is a unit of recombinant frequency for measuring genetic linkage. It is often used to imply distance along a chromosome. The centimorgan is equal to a 1% chance that a marker at one genetic locus on a chromosome will be separated from a marker at a second locus due to crossing over in a single generation. A 50 cM distance means that the genes will reassort when an odd number of crossings happen, which happens 31.8% of the time. Note that non-syntenic genes are inherently unlinked, and cM distances have no meaning.
Another unit of recombination frequency is the map unit (m.u.) with one m.u. equaling a recombination frequency of 1%. A map unit is synonymous with centimorgan.
The centimorgan was named in honor of geneticist Thomas Hunt Morgan by his student Alfred Henry Sturtevant. Note that the parent unit of the centimorgan, the morgan, is rarely used today.
[edit] See also
[edit] References
- Griffiths, Anthony J. F.; Miller, Jeffrey H.; Suzuki, David T; Lewontin, Richard C.; Gelbart, William M. (Eds.) (1993) An Introduction to Genetic Analysis (5th ed.) Chap. 5. New York: W.H. Freeman and Company. ISBN 0-7167-2285-2.
