The B-A translocations are currently used to locate factors to chromosome. Last year, in the 52 issue, we reported the results of a series of crosses of a recessive proline auxotroph (pro) not yet placed with a set of B-A translocations. The resulting hypoploids in the progeny of such crosses give normal 3:1 ratios, upon selfing, unless a hypoploid for the critical chromosome is involved. The only functional gametes yielded by the critical hypoploid in fact are those derived through crossing over between the locus and the break point as follows:

Figure.

The pro mutant has been located, on this evidence, on chromosome 8. Hypoploids for chromosome 8 segregate 1545 mutant out of 2935 seeds, while normal siblings show a 3:1 ratio (799 mutants out of 3024 seeds).

A measure of the distance from the mutant to the breakpoint can be readily calculated from the relative frequencies of pro and Pro. The recombination fraction p, given by the formula:
         
p = ÷1-(Pro/Pro+pro)

(Beckett, 1978, J. Heredity 69:27) amounts to 27.2%. On the basis of this estimate we can predict that 72.8% of the gametes produced from a hypoploid for chromosome 8 are parentals and 27.2% recombinants. 84.4% (39.6/46.9) of the nonmutant plants in the progeny of a hypoploid are thus expected to be heterozygous for pro. 26 out of 31 (83.9%) of such plants are in fact segregating for pro nicely fitting the expected ratio and confirming the validity of our calculation.

G. Gavazzi and G. Todesco

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