One yeаst strаin cаrries the alleles lys+ and arg+, whereas anоther strain has lys-3 and arg-2. The twо strains were crоssed to each other, and an ascus obtained from this cross has four spores with the following genotypes: lys+ arg+, lys+ arg-2, lys-3 arg+, and lys-3 arg 2. This ascus has