Ayeast strain that cannot grow in the absence of the amino acid histidine (his—) is mated with a yeast strain that cannot grow in the absence of the amino acid lysine ( among the 400 unordered tetrads resulting from this mating, 233 were rd, 11 were npd, and 156 were t. a. what types of spores are in the pd, npd, and t tetrads? b. what is the distance in map units between the his and lys genes? c. assuming that none of these tetrads was caused by more than two crossovers between the genes, how can you estimate the number of meioses that generated these 400 tetrads in which zero, one, or two crossovers took place? d. based on your answer to part c, what is the mean number of crossovers per meiosis in the region between the two genes? e. the equation rf 100 x (npd + 1/2 t) / total tetrads accounts for some, but not all, double crossovers between two genes. which double crossovers are missed? can you extrapolate from your answer to part d to obtain a more accurate equation for calculating map distances between two genes from the results of tetrad analysis? f. using your corrected equation from part e, what is a more accurate measurement of the distance in map units between the his and lys genes?