General cytogenetics / J. Sybenga.

  • Sybenga, J. (Jacob), 1926-
Date:
1972
Catalogue details

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

Credit: General cytogenetics / J. Sybenga. Source: Wellcome Collection.

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    §7.2 Deviant cytogenetic systems 313 fig.7. IK), but infrequent recombination between С and the knob. The factor sh behaved similarly as it also was rather far from the centromere. On the other hand, wx did not exhibit preferential segregation: it is relatively close to the ceriromere. In rye, Prakken and Miintzing in 1942 observed neocentric ac¬ tivity in the chromosomes of several inbred lines. They were called T-chromosomes (terminal centromere), since here the activity was localized at or quite near the ends (fig.7.1 F,H). Sometimes one chromosome, sometimes two of a bivalent and occasionally all chromosomes had T-activity. No special chromosome responsible for the effect has been found, but there is a correlation with the presence of heterochromatic knobs. The effect is influenced by internal and external factors. Preferential segregations have not been observed. Neocentric activity has been observed in several more organisms, but detailed studies like those in maize have not been reported. 7.2.2. Meiotic drive. Affinity. Distributive pairing There are many more cases of abnormal segregations that can be attributed to meiotic deviations, but for which the actual cause is not known. They are usually taken together under the name meio¬ tic drive. The term was used for the first time by Sandler and Novitski in 1957, for disturbed segregations of genes in chromo¬ some 2 of Drosuphila melaiwgaster, when male flies with an SD- factor (segregation distorter) in this chromosome were mated with normal females. In extreme cases 26:1 instead of 1:1 segregations were found. The effect depends on several conditions. It has been ascribed to deviations in the structure of heterochromatic seg¬ ments in the vicinity of the centromere, that may lead to breaks in the chromosome. Another explanation is preferential segregation of such segments to specific poles that are unable to organize functional cells. A different phenomenon is that of linkage between factors in different chromosomes not involved in a reciprocal translocation: affinity (Wallace, 1961). Apparently, non-homologous chromo¬ somes, derived from the same parent, tend to go to the same pole in anaphase I, resulting in pseudo-linkage of genes in different