Published online by Cambridge University Press: 14 April 2009
In tetrad data obtained from the Carbondale yeast stock, instances of greater than 50% recombination occur far more frequently than would be expected by chance. In the main, genes exhibiting this effect are located in the vicinity of the centromeres. Difficulties in assigning linear order also persist throughout the total data for certain gene combinations. The affinity hypothesis suffices to account for these two effects; the former resulting from ‘divergent’ combinations of affinity sites in the zygote, the latter, representing nonlinear quasi-linkages resulting from ‘convergent’ combinations.
Chromosome V (ur3-centromere-ch–hi1–is1–an) exhibits quasi-linkage with the gene th and reverse-linkage with the gene cu2, in Family 108. In each case ‘linkage’ is strongest with ch and becomes progressively weaker with the genes hi1, is1 and an, respectively, indicating that the preferentially segregating sites involved lie close to ch. It was impossible to determine whether the two sites were identical, as would be expected on an hypothesis of ‘polar’ rather than ‘mutual’ affinity.
Intra-ascal matings within tetrads NPD for cu2–ch, yielded quasi-linkage for these genes in F2, showing that the attraction is retained by the sites being segregated.
A second case of reverse linkage for cu1 and ar4 in Family 217, gave a similar effect—i.e. reverse linkage in F1, becoming quasi-linkage in the NPD—intra-ascal F2.
An intra-ascal mating between members of the single PD tetrad for cu1 and ar4 in Family 217 indicated that this tetrad did not result from ‘chance’ failure of otherwise active sites to preferentially segregate.