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In yeast the mating type alleles a and a also control sporulation; heterozygous strains (a/a) sporulate while homozygous strains (a/a or a/a) do not. Net DNA synthesis, an early event preceding sporulation, occurred normally in the heterozygous strains but did not occur in the homozygous strains. Therefore, the a/a allelles also influence early development well before spore formation begins.
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Roth et al. (1970) studied this question.
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