A Yeast Geneticist Isolates Two Different Haploid Mutant Yeast Strains

A yeast geneticist isolates two different haploid mutant yeast strains, strain A and strain B, which cannot grow unless the amino acid leucine is added to the growth media. Wild-type yeast strains can make their own leucine and do not require that it be added to the growth media. The geneticist discovers that each mutant yeast strain contains a single recessive mutation that leads to the observed leucine-requiring phenotype. When she crosses the two mutant strains together, she observes that the resulting diploid can grow without leucine added to the growth media. Explain the allelic relationship between the mutations in these two strains.