The is 1-locus of the yeast Saccharomyces cerevisiae is the structural gene for threonine dehydratase. is 1-mutants require isoleucine for growth and do not have active threonine dehydratase. Interallelic complementation is frequent among is 1-mutants. This is indicative for an aggregate or multimeric structure of yeast threonine dehydratase. Complementing and non-complementing mutants were crossed to wildtype. Properties of threonine dehydratase were assayed in crude extracts of the resulting heterozygotes. Specific activities varied considerably between full wildtype activity and a level about 10% of that. The apparent Michaelis constants were increased in many heterozygotes. This effect was probably due to the aggregation of both mutant and wildtype subunits to form a hybrid threonine dehydratase with reduced substrate affinity in addition to pure wildtype enzyme. This notion is supported by the observation in one heterozygote of two enzyme fractions with increased Michaelis constants in addition to a wildtype-like fraction. The possible formation of hybrid enzymes with normal, reduced or no activity is considered to blur gene dosage relations. A given pair of alleles in a heterozygous cell can generate a new type of enzyme with properties not encountered in the corresponding two homozygous cells. This situation is not accounted for by the classical concepts of dominant-recessive or intermediate behaviour, because the difference between the heterozygotes and the homozygotes is not necessarily only quantitativ but also qualitative.