The measurement of linkage in heredity / by K. Mather.

  • Mather, Kenneth, 1911-1990.
Date:
[1938]
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: The measurement of linkage in heredity / by K. Mather. Source: Wellcome Collection.

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    The other situation often met with and also often important in genetical analysis is that of linkage: In some families it may be necessary to distinguish linkage and factor interaction, or it may be necessary to have exact knowledge of the linkage relations of two genes prior to their use in the analysis of quanti tative characters, or the information may be necessary for a number of other reasons. Now, linkage can only be observed in families segregating for each of the two characters observed, and, furthermore, only if at least one parent is doubly heterozygous. For example, a cross of the type Aabb X aaBb gives no information about linkage. If the two genes are not linked the gametic segrega tion of the double heterozygote will be j AB, j Ab, I aB, I ab, and this may be realized in tetrad analysis. Among the progeny of a cross it may not be found exactly, because chance deviations will again occur. If linkage exists between the two genes, the gametic output of the double heterozygote is i(l — p) AB, \p Ab, \p aB, 1(1 — p) ab, or \p ÄB, i(l — p) Äb, |(1 — p) aB, \p ab, where p is the recombina tion fraction and has the value 0-5 when there is no linkage. If after considering chance deviations and other complications p is demonstrably different from 0*5 the evidence for linkage is clear. There is another test for the presence of linkage. In a double heterozygote two relations in arrange ment may exist between the genes. Either A — B and a — b may be on the same chromosomes or A — b and a — B may be the arrangement found. In the former case A — b and a — B are the re combination types. In the latter they are A — B and a — b. If two different arrangements, bearing these relations, can be shown to exist, linkage is demonstrated. These arrangements are differenti ated under the names of coupling and repulsion , the arrangement to which a particular name is allocated