This problem leads you through the derivation of a corrected equation for RF in yeast tetrad analysis that takes into account double crossover (DCO) meioses. A yeast 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 (lys−). Among the 400 unordered tetrads resulting from this mating, 233 were PD, 11 were NPD, and 156 were T. a. What types of spores are in the PD, NPD, and T tetrads?

b. Are the his and lys genes linked? How do you know?

c. Using the simple equation RF = 100 × [NPD +(1/2)T]/total tetrads, calculate the distance in mapunits between the his and lys genes.

d. If you think about all the kinds of meiotic eventsthat could occur (refer to Fig. 5.24), you can see that the calculation you did in part (c) may substantially underestimate RF. What kinds of meioses (NCO, SCO, or DCO) generated each of the tetrad types in this cross?

e. What incorrect assumptions does the simple RFequation you used in part (c) make about themeiotic events producing each type of tetrad? Whencould these assumptions nevertheless be correct?

f. Use your answers to part (d) to determine thenumber of NCO, SCO, and DCO meioses thatgenerated the 400 tetrads.

g. Use your answers to part (f) to write a generalequation that relates the number of DCO meiosesto the number of the various tetrad types. Thenwrite another general equation that computes thenumber of SCO meioses as a function of the number of the various tetrad types.

h. Based on your answer to part (f), calculate theaverage number of crossovers per meiosis (m)between his and lys.

i. Use your answer to (h) to write an equation for min terms of NCO, SCO, and DCO meioses.

j. What is the relationship between RF and m?

k. Use your answer to part (j) to write a corrected equation for RF in terms of SCO, DCO, and NCO meioses. l. Using your answer to part (g), rewrite the corrected RF equation from part (k) in terms of the numbersof the various tetrad types.

m. The equation you just wrote in part (l) is a corrected equation for RF that takes into account doublecrossovers that would otherwise have been missed. Use this improved formula to calculate a moreaccurate distance between the his and lys genes than the one you calculated in part (c).