Genetic Linkage

the joint transfer of two or more genes from parents to offspring. Genetic linkage occurs because such genes reside on the same chromosome, that is, they belong to the same linkage group and therefore cannot be accidentally recom-bined in meiosis, which occurs in the inheritance of genes residing on different chromosomes.

Genetic linkage was discovered in 1906 by the English geneticists W. Bateson and R. Punnett, who discovered in experiments on the crossing of plants the tendency of some genes to transfer together, thus violating the law of the independent combination of traits. This tendency was correctly explained by T. H. Morgan and his associates, who discovered a similar phenomenon in their study of inherited traits in the fruit fly (Drosophila).

Genetic linkage is measured by the frequency at which crossover gametes or spores are formed by a heterozygote on jointly transferring genes. In these gametes or spores, the genes occur in new combinations rather than in the original combinations, owing to the crossing-over of those parts of the homologous chromosomes bearing the genes. In some bacteria, another measure of genetic linkage is the frequency of joint transmission by inheritance of various genes in conjugation, genetic transformation, and transduction. The extent of genetic linkage may vary among the sexes: it is generally greater in the heterogametic sex. Genetic linkage may even be complete, without crossing-over, in one of the sexes, for example, in male Drosophila or in female Asiatic silkworms (Bombyx morí). The extent of genetic linkage may also vary with the age of the parents and with temperature. In addition, it may vary in the presence of chromosomal rearrangement or of mutant genes that influence the extent of genetic linkage.

S. M. GERSHENZON

genetic linkage

link·age

(lingk'ăj), 1. A chemical covalent bond. 2. The relationship between syntenic loci sufficiently close that the respective alleles are not inherited independently by the offspring; a characteristic of loci, not genes.

Fig. 170 Genetic linkage . The phenotypes produced by a testcross.

genetic linkage

the association between genes located (linked) on the same chromosome, thus producing proportions of gametes that are not those expected by INDEPENDENT ASSORTMENT although, unless there is very close linkage, the same TYPES of gamete will be produced. Instead, there tends to be association of particular alleles together in the gametes (the ‘parental’ types, in the same combination as the parent), with other combinations being less frequent (see RECOMBINATION and CROSSING OVER). By convention the amount of recombination between linked genes is a direct measure of their distance apart along the chromosome, 1% recombination being equivalent to the MAP UNIT. The amount of recombination occuring is best measured in a TESTCROSS calculated from the total number of recombinant types in the testcross progeny as a proportion of the total progeny. For example, a testcross with genes in COUPLING produced the PHENOTYPES in Fig. 170 where the amount of recombination would be:

and the genes A and B could be represented on a GENETIC MAP as:

Had the two genes been independently assorting (i.e., on different chromosomes) roughly equal numbers of each phenotype would have been expected in the progeny (about 70). Similarly, since the maximum expected amount of recombination between two genes is 50%, if two genes are located more than 50 map units apart on the same chromosome they will appear to be independent of each other.

单词	genetic linkage
释义	DictionarySeelinkage Genetic Linkage Genetic Linkage the joint transfer of two or more genes from parents to offspring. Genetic linkage occurs because such genes reside on the same chromosome, that is, they belong to the same linkage group and therefore cannot be accidentally recom-bined in meiosis, which occurs in the inheritance of genes residing on different chromosomes. Genetic linkage was discovered in 1906 by the English geneticists W. Bateson and R. Punnett, who discovered in experiments on the crossing of plants the tendency of some genes to transfer together, thus violating the law of the independent combination of traits. This tendency was correctly explained by T. H. Morgan and his associates, who discovered a similar phenomenon in their study of inherited traits in the fruit fly (Drosophila). Genetic linkage is measured by the frequency at which crossover gametes or spores are formed by a heterozygote on jointly transferring genes. In these gametes or spores, the genes occur in new combinations rather than in the original combinations, owing to the crossing-over of those parts of the homologous chromosomes bearing the genes. In some bacteria, another measure of genetic linkage is the frequency of joint transmission by inheritance of various genes in conjugation, genetic transformation, and transduction. The extent of genetic linkage may vary among the sexes: it is generally greater in the heterogametic sex. Genetic linkage may even be complete, without crossing-over, in one of the sexes, for example, in male Drosophila or in female Asiatic silkworms (Bombyx morí). The extent of genetic linkage may also vary with the age of the parents and with temperature. In addition, it may vary in the presence of chromosomal rearrangement or of mutant genes that influence the extent of genetic linkage. S. M. GERSHENZON genetic linkage link·age (lingk'ăj), 1. A chemical covalent bond. 2. The relationship between syntenic loci sufficiently close that the respective alleles are not inherited independently by the offspring; a characteristic of loci, not genes. Fig. 170 Genetic linkage . The phenotypes produced by a testcross. genetic linkage the association between genes located (linked) on the same chromosome, thus producing proportions of gametes that are not those expected by INDEPENDENT ASSORTMENT although, unless there is very close linkage, the same TYPES of gamete will be produced. Instead, there tends to be association of particular alleles together in the gametes (the ‘parental’ types, in the same combination as the parent), with other combinations being less frequent (see RECOMBINATION and CROSSING OVER). By convention the amount of recombination between linked genes is a direct measure of their distance apart along the chromosome, 1% recombination being equivalent to the MAP UNIT. The amount of recombination occuring is best measured in a TESTCROSS calculated from the total number of recombinant types in the testcross progeny as a proportion of the total progeny. For example, a testcross with genes in COUPLING produced the PHENOTYPES in Fig. 170 where the amount of recombination would be: and the genes A and B could be represented on a GENETIC MAP as: Had the two genes been independently assorting (i.e., on different chromosomes) roughly equal numbers of each phenotype would have been expected in the progeny (about 70). Similarly, since the maximum expected amount of recombination between two genes is 50%, if two genes are located more than 50 map units apart on the same chromosome they will appear to be independent of each other. FinancialSeeLinkage
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