Mechanisms of Meiotic Non-disjunction in Man

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IT is useful at this stage of human cytogenetics to keep in mind the whole spectrum of possibilities of chromosomal behaviour. In this sense Dr. Stewart's emphasis on the possibility of non-disjunction during the second meiotic division is valuable. His discussion rests on the assumption that non-disjunction is preceded by crossing-over between homologous chromosomes. In my discussion of second division non-disjunction I had specified absence of crossing-over (and, thus, made no assumption as to closeness of the locus for colour blindness to the kinetochore). According to Dr. Stewart's scheme it would follow that genes distantly located would less often become homozygous than proximal genes. This is the opposite to what is well established in Drosophila, although it cannot be excluded, of course, that human chromosomes may behave differently. In Drosophila the more frequent homozygosis for distal than proximal genes signifies that non-disjunction preceded by crossing-over occurs during the first meiotic division.

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    Ford, C. A., Amer. J. Hum. Genet., 12, 104 (1960).

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STERN, C. Mechanisms of Meiotic Non-disjunction in Man. Nature 187, 805 (1960) doi:10.1038/187805a0

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