Earlier we (MGCNL 53:72-73) and Newton (MGCNL 53:16-21) reported independent discoveries (from different materials) of an electrophoretic "activity null" for Mdh2 on chromosome 6. This allele produces no apparent homodimer but does produce interallelic heterodimers with products of the nonnull alleles of Mdh2 as well as intergenic heterodimers with the nonnull alleles of Mdh1 (on chromosome 8--see Newton in this volume) and Mdh3 (on chromosome 3 near sh2). More recently we have discovered a similar allele for Mdh1 among several collections from Peru and Bolivia.
One of the most striking variants we have encountered in our isozyme survey of Latin American collections has been the D8.5 allele of Mdh4 (on chromosome 1). This variant, isolated from a Venezuelan collection, results in heterodimers which do not show the usual migration pattern of lying approximately half the distance between the corresponding homodimers. The interallelic heterodimer of D8.5 with D12 (the common variant) migrates only slightly beyond the homodimer of D8.5 itself, while the homodimer corresponding to D12 migrates substantially further on both our pH 5.0 and pH 5.7 histidine-citrate gels:
The interallelic heterodimer of D8 (another rare variant of Mdh4) and D8.5 is even more striking as it fails to migrate as far as either corresponding homodimer:
That this phenomenon is not unique to the D8 and D8.5 alleles at the Mdh4 locus is demonstrated by the fact that it also occurs for the interallelic heterodimer of D8.5 and D10.5 (another rare Mdh4 allele).
Recently we have shown that the same relative migration pattern (with the heterodimer lying outside the range of the corresponding homodimers) holds for the intergenic heterodimer of D8.5 and a rare allele at the Mdh5 locus (on chromosome 5).
M. M. Goodman and C. W. Stuber
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