The model for the genetic control of mitochondrial malate dehydrogenase (mMDH) postulates two diallelic sets of duplicated loci (Yang, Sorenson and Scandalios, PNAS 74:310-314, 1977). The duplicated sets of loci and the isozyme products each codes for are:
 

Gene Set	Genotype	Isozymes Expressed
Mdh1 Mdh2	Mdh1-m1, Mdh2-m3	MDH1, MDH3
	Mdh1-m2, Mdh2-m5	MDH2, MDH5
Mdh3 Mdh4	Mdh3-m1, Mdh4-m3	MDH1, MDH3
	Mdh3-m3a, Mdh4-m7	MDH3a, MDH7

The gene Mdh1 codes for the isozymes MDH1 or MDH2 while Mdh2 codes for the isozymes MDH3 or MDH5. The gene Mdh3 codes for MDH1 or MDH3a, while Mdh4 codes for MDH3 or MDH7. The isozyme MDH4 is a hybrid product of MDH3 and MDH5. MDH6 is a hybrid between MDH5 and MDH7. The isozyme MDH5 can be formed as a hybrid between MDH3 and MDH7.

However, the isozyme MDH3a does not form detectable hybrids. This is exemplified by the discovery of an inbred line in our laboratory which expresses only mitochondrial MDH3a and MDH7. In this line no hybrid is seen between MDH3a and MDH7. The model is supported by genetic, biochemical, and developmental data, and verified by other competent investigators elsewhere. We have undertaken a study to map the mMDH structural genes using trisomic analysis and B-A translocations. Using trisomic analysis, Mdh3 and Mdh4 were located on chromosome 6. B-A translocations have established that the structural genes Mdh3 and Mdh4 are on the long arm of chromosome 6. Additional experiments now being completed have allowed us to tentatively locate the Mdh1 and Mdh2 structural genes on another chromosome. The entire set of data will be shortly reported in a forthcoming detailed manuscript.

D. E. McMillin and J. G. Scandalios

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