genetic linkage

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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.
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Genetic linkageclick for a larger image
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.

Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005
References in periodicals archive ?
Once students understand coupling and repulsion crosses, they can predict the outcomes of the F2 generations in the coupling and repulsion crosses for linked genes, if no crossing over has occurred.
Using Table 1, students determine the approximate map distance between their two linked genes.
Studies on the genetic fine structure of these linked genes will help enhance our understanding of wilt resistant gene clusters in chickpea.
[T.sub.2] progeny from Family 20 were tested against genetic ratios of 3:l for linked genes from a hemizygous parent, and 15:1 for two linked insertions of RNase L and 2-5A synthetase, and for linked or unlinked genes from a homozygous parent (1:0).
[T.sub.2] progeny from Family 20 were tested against genetic ratios of 3:1 for linked genes from a hemizygous parent, and 15:1 for two linked insertions of RNase L and 2-5A synthetase, and for linked or unlinked genes from a homozygous parent (1:0).
The [Pl.sub.2] gene is closely linked to the complex resistance locus [Pl.sub.6], which consists of at least two very tightly linked genes (Vear et al., 1997).
Washington, Feb 2 (ANI): A new research has linked genes to our ability to orient ourselves to the world around us and then navigate through it.
It has not been determined if smut resistance associated with the tr allele is conferred by that allele or by closely linked genes. Secondly, although forage yield is not adversely affected by the tr allele (Burton and Werner, 1991), it is not known if grain yield in the absence of disease is affected by this gene.
Three crown rust resistance (CRR) genes, Pc-38, Pc-62, and Pc-63, are inherited as allelic or tightly linked genes (Harder et al., 1980).
Mozhui and colleagues (2008) have, for example, dissected a trans eQTL cluster on distal mouse chromosome 1 and identified a candidate gene (Fmn2) that they propose has a major influence on the expression of linked gene networks.