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, so 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.

The mutations arising in the two yeast strains cannot be considered to be allelic. This is because we can observe the patterns of complementation in the scenario depicted in the question.

Strain A ans Strain B contains recessive mutations at different genetic loci. Strain A contains a mutation at gene A where as the strain B will contain a mutation at a different gene say gene B. Both the mutations will be recessive.

Hence, when these strains are crossed, wild type genes correspond the mutant alleles in the offsprings.