Recombination and genetic linkage.[]
Understanding of general Mendelian genetics and chromosome-based Punnett Square is an important tool for explaining genetic recombination, frequency of genetic recombination, crossing over during prophase of meiosis I, and genetic linkage. For a quick resource visit http://www.sciencecourseware.com/vcise/drosophila/.
Thomas Hunt Morgan in 1911, with the contribution of his Student (Alfred Sturtevant) used a fruit fly (Drosophila melanogaster), to explain the phenomenon of genetic recombination frequency and linkage. The nomenclature for genetic of fruit fly is a bit different, but simple; in other words the traits bi+ bl+ represents wild type (black color) fruit fly and vg+vg+ represent mutant type (vestigial wings). It is important to note that genes on the same exact chromosome tend to be transmitted together in genetic terms.
Using this link http://www.sciencecourseware.com/vcise/drosophila, make a cross between the two-parent, a wild type (bl+ bl+ vg+ vg+) and vestigial wing (bl bl vg vg ) fruit fly. During meiosis they can each produce the following gametes ( bl+ vg+) and (bl vg), a hybrid of both trait bl+ bl vg+ vg ( heterozygote) will be produced, a simple Mendelian genetics. Furthermore a cross of the FI hybrid (bl+bl vg+vg) with a mutant type bl bl vg vg (vestigial wing), tend to deviate from Mendelian genetics. The hybrid will produce four different gametes, while the mutant produce one gamete. These were a different predicted cross outcome from Mendelian genetics(Brooker,2001).
According to Mendelian genetics, a ratio of 1:1:1:1 was expected (i.e. a 4 by 1 Punnet square will produce 25% of each for type). These findings lead to the discovery of genetic linkage and recombination. After several crosses, instead of 1:1:1:1 ratio Thomas Morgan discovered a repetitive 83% of parental type ( bl+bl vg+ vg and bl bl vg vg) and 17% of recombinant (bi+bl vg vg, bl bl vg+ vg). With help from Alfred Sturtevant (Thomas Morgan’s student), came up with the explanation that the 17% genes are linked.
Genetic linkage means that when two or more genes are close together along the same chromosome and are usually transmitted together and therefor are linked.
During crossing over (a physical exchange of chromosomal pieces) in prophase of meiosis I, which is a random process. The closer the genes are on the chromosome the less likely they cut (recombine or blend together), but if the genes are far apart, the more likely they will cut and create a recombination. Genetic recombination is a phenomenon in which a chromosome is broken and rejoined to form a piece that is different from the original chromosome piece.
If the frequency of recombination was 17%, it means that the genes are relatively close together on the same chromosome and are linked. But 83% frequency of recombination means that the genes are on different chromosome and are not linked. This implies that frequency of recombination that is less than 50%, the genes are closer on the same chromosome and are linked; but if the frequency is greater than 50%, the genes are on different chromosomes and are not linked (Ott, J. 1999).
References
Brooker, R. (2001). Genetics linkage and Mapping in Eukaryotes. In Genetics: Analysis and principles (2d. ed., pp. 126-143). Boston.: Mcgraw Hill.
Ott, J. (1999). Recombination and genetic linkage. In Analysis of human genetic linkage (3rd ed., pp. 17-23). Baltimore: Johns Hopkins University Press.