You crossed a yeast strain unable to grow in the absence of

You crossed a yeast strain unable to grow in the absence of arginine (arg7 mutant) to a yeast strain unable to grow in the absence of leucine (leu2 mutant). You sporulated the resulting diploid and obtained the following tetrads:

leu2 ARG7 LEU2ARG7 leu2 ARG7

leu2 ARG7 LEU2ARG7   leu2 arg7

LEU2 arg7 leu2 arg7 LEU2ARG7

LEU2 arg7 leu2 arg7 LEU2 arg7

38 4 8

how many cells have undergone meiosis to produce these tetrads? (PLEASE EXPLAIN)

What is the map Distance between LEU2 and ARG7 genes? (PLEASE EXPLAIN)

Solution

a) Answer : 50

Total tetrads = 38 + 4 + 8 = 50

Tetrad contains four haploid cells.

One cell undergo meiosis to produce 4 haploid cells.

Therefore, 50 cells undergo meiosis to produce 200 cells.

50 cells x 4 = 200 cells.

200 / 4 = 50 tetrads.

b) Answer : 16 map units

Parental genotypes: LEU2 arg7 x ARG7 leu2

Tetrad I is parental

Tetrad II is non parental (Recombinant)

Tetrad III is non parental (Recombinant)

Map distance between LEU2 and ARG7 genes

= [ (single crossover (1/2) + double crossover) / total ] x 100

= [((8/2) + 4 ) / 38+4+8] x 100 = 16 map units