WT yeast (TRP+)can synthesize the amino acid tryptophan and can grow on media plates lacking tryptophan. In contrast, mutant alleles (trp-)block tryptophan synthesis and result in an inability of mutant strains to grow on media lacking tryptophan. A diploid yeast was made through the cross MATa ADE+ × MATα ade-. Meiosis was induced in the diploid strain to produce an ascus with four haploid spores, which were separated and grown on plates lacking tryptophan. How many spores are expected to grow into colonies on the plate?
A)0
B)1
C)2
D)3
E)4

Question

Quizplus · Accepted Answer

One parent strain (MATa ADE+) can grow on media lacking tryptophan, indicating that it is WT yeast with the ability to synthesize tryptophan. The other parent strain (MATα adeF1F1F1S1F1F1F10) cannot grow on media lacking tryptophan, indicating that it is a mutant strain with a block in tryptophan synthesis.

During meiosis, the four haploid spores produced will each receive one copy of the TRP+ allele and one copy of the trpF1F1F1S1F1F1F10 allele. Only those spores that receive at least one copy of the TRP+ allele will be able to synthesize tryptophan and grow on plates lacking tryptophan.

Thus, out of the four haploid spores, two are expected to grow into colonies on the plate, as they will each receive one copy of the TRP+ allele.

WT Yeast (TRP+)can Synthesize the Amino Acid Tryptophan and Can