Direct Effect of Preferential Segregation on the Origin of Polysomy


INVESTIGATIONS of reverse linkage1,2 in Saccharomyces (excesses of non-parental ditype tetrads yielding a frequency of recombination greater than 50 per cent) have revealed that the centromeres of the adenine–inositol chromosome and the proline–lysine chromosome demonstrate affinities for each other at the first meiotic division3. (Reverse linkage is not detectable in organisms in which a long heterochromatic region intervenes between the centromere and the euchromatin, permitting random recombination between the centromere and the euchromatic genes.) The centromeres of both chromosomes are of two types, designated ‘up’ (U) and ‘down’ (D). In reverse linkage for genes closely linked to these centromeres (genes controlling the synthesis of inositol and proline, respectively) the mating is U1D2 × D1U2 (Fig. 1). Preferential segregation has been shown to occur in nearly every meiosis of this mating by the close agreement between the distances between inositol and proline obtained by (a) one mapping function assuming 100 per cent non-parental preferential segregation (which yielded the distance 36.25) and (b) another mapping function which utilized only the frequency of tetratype tetrads and thus was independent of assumptions involving preferential segregation (which yielded the distance 37.50). Thus in nearly all resultant tetrads the gametes contain U1U2 or D1D2 centromere combinations. Unless recombination occurs between the gene and its corresponding centromere, only non-parental ditype tetrads are obtained. The mechanism controlling the preferential segregation of these non-homologous centromeres is inferred to comprise their synapsis just prior to meiosis I and their passing together to the poles in that condition at the first division. Marker-pairs which are in reverse linkage in one family may appear in direct linkage in a related family. This is inferred to result from the new mating U1U2 by D1D2.

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  1. 1

    Shult, E. E., and Lindegren, C. C., Genetica, 28, 165 (1956).

  2. 2

    Shult, E. E., and Lindegren, C. C., “Mapping Methods in Tetrad Analysis. III. Reverse Linkage” (unpublished).

  3. 3

    Lindegren, C. C., and Shult, E. E., Experientia, 12, 177 (1956).

  4. 4

    Roman, H., Phillips, M. M., and Sands, S. M., Genetics, 40, 546 (1955).

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SHULT, E., LINDEGREN, C. Direct Effect of Preferential Segregation on the Origin of Polysomy. Nature 179, 683 (1957).

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