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.
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.

References in periodicals archive ?
We have been able to estimate further the recombination frequency in the Tsn1 region between fraction breakpoints 0.
infinity]], and ratio between the two, in two-locus additive versus epistatic genetic models as a function of the recombination frequency between loci, r, and the populations size N.
The selection response for this second selection step depends on the recombination frequency between the target gene and the flanking markers, and on the densities of the markers on the carrier and noncarrier chromosomes.
sl] is defined, one should determine the desirable recombination frequency between the flanking markers and the target gene and the number of genotypes selected at each generation, based on the objective and the constraints of the experiment.
Despite numerous questions raised by this experiment as to the identity of the chromosomes involved and the best approach to introgress the desired loci into wheat, it is clear that with low recombination frequency and structural differences between the donor and recipient chromosomes, transfers of small segments of alien chromatin by induced homoeologous recombination may be impractical.
A small difference in chromosome structure dramatically increased recombination frequency of Te relative to MA.
There is then a clear advantage to the Bc, even if the expected closer recombination frequency between the nearest marker and the QTL in the [F.
1BL) and one normal chromosome 1B were identified, for a recombination frequency of 6%.
The recombination frequency between adjacent loci was chosen at random from a log normal distribution generated as [2.
The estimate of recombination frequency was made by the method of maximum likelihood (Mather, 1951: Allard, 1956).
Following Mather (1951), we derived the maximum likelihood estimator for recombination frequency (r) from this six-class segregation as the value of r that satisfied the following equation:
2] populations among which no tall plants were observed and the recombination frequency (r-value) calculated for that population size for which with 95% probability at least one tall plant would be expected if the two parent lines contained different dominant dwarfing genes.