Gene Linkage
Gene linkage describes the physical relationship of genes. Specifically, linkage means that the genes are on the same chromosome and therefore do not assort independently into gametes (in humans, ovum and spermatozoa) during meiosis.
Gene linkage is the phenomenon where genes located on the same chromosome in an eukaryote tend to be transmitted together. Because of this co-transmittance, the traits associated with the genes sometimes do not segregate between two daughter cells, following crosses between the parental cells, as predicted by Mendelian genetics
The genes of most organisms can exist in different forms, called alleles, in a population. If the organism has identical alleles of a gene on each of its homologous chromosomes, it is called homozygous. If the alleles are different it is called heterozygous. During the cell division process, a separation of nuclear material into gametes occurs via meiosis. If an organism is heterozygous, two kinds of gametes are produced; if homozygous, it produces only one kind of gamete. At fertilization the male and female gametes combine and the random process that creates different unites the gametes into various combination. The ratio of the appearance of the observed traits, or phenotypes, produced by the pattern of separation of the dominant and recessive genes for that trait was predicted by Gregor Mendel following painstaking work and observation of the crosses between pea plants.
However, early in the twentieth century, William Bateson and Reginald Crundall Punnett, two British geneticists, observed that sometimes the expected Mendelian ratio of phenotypes did not occur. Their best explanation was that in some manner the phenotypic classes, the alleles, were coupled, and so did not sort independently into gametes. Proof of their explanation was provided by Thomas Hunt Morgan, using Drosophila eye color as the examined trait.
Morgan observed that test crosses between mutants in eye color and wing development deviated from the expected Mendelian 1:1:1:1 ratio for independent assortment. The observed ratio was, rather, consistent with the non-independent segregation of two genes that were close to each other on the same chromosome.
Linked genes do not observe the genotypic or phenotypic relationships predicted by Mendelian crosses that assume independent assortment of chromosomes and genes. In a cross the parental generation is designated P1 and the first generation of offspring are designated F1(first filial generation), and the offspring resulting from the fertilization between individuals of the F1 generation are called the F2 (second filial generation). When the F1 and F2 ratios deviate form the predicted Mendelian rations this is evidence of gene linkage.
The linkage of genes is used to generate so-called linkage maps, which give a measure of the distance between genes on a chromosome. The linkage map technique, which is based on the use of the percentage of recombinants, in which crossing over of DNA and expression of traits due to gene linkage has occurred, was devised in 1911 by Alfred Henry Sturtevant, an undergraduate student of Morgan's. The technique remains in use today as a means of producing an index of the distance between two genes.
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