Linkage studies with isozymes revealed four genes linked to Adh1 on the long arm of chromosome 1. The arrangement of the loci is based on a 358-plant, five-point testcross. The standard errors for the map distances vary from 1% for Mdh4 mmm and 2% for mmm - Pgm1 to less than 3% for the entire span, Mdh4 to Phi:

Mdh4 4.4% mmm 19.0% Pgm1 5.8% Adh1 13.7% Phi

/------------------------------ 42.9% -------------------------------/

(Recombinant chromosomes = 41.9%, including double crossover types)

Segregation data for Mdh5, Pgm2 and a2 on chromosome 5 indicate about 16% (± 4%) recombination between Mdh5 and Pgm2 and about 32% (± 7%) recombination between a2 and Pgm2.

On chromosome 6 we have previously reported about 5% recombination between Ep and Pgd1 near Y; now we can report that, in the distal region of the long arm of chromosome 6, Idh2 and Mdh2 show less than 5% recombination.

Our preliminary data show about 19% (± 5%) recombination between Idh1 and Mdh1; the localization of the latter to chromosome 8 is reported elsewhere in this volume by Newton.

We thus have two parallel cases of linkage:
 

A	Mdh4 - Pgm1 on chromosome 1 with 23% (± 2%) recombination;
	Mdh5 - Pgm2 on chromosome 5 with 16% (± 4%) recombination;
B	Idh1 - Mdh1 on chromosome 8 with 19% (± 5%) recombination;
	Idh2 - Mdh2 on chromosome 6 with less than 5% recombination.

These data suggest that duplication of chromosome segments involving isozyme loci may not be as uncommon as the cytological evidence would imply.

M. M. Goodman, C. W. Stuber, K. J. Newton and H. H. Weissinger

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