This problem leads you through the derivation of acorrected equation for RF in yeast tetrad analysis thattakes into account double crossover (DCO) meioses. A yeast strain that cannot grow in the absence ofthe amino acid histidine (his−) is mated with a yeaststrain that cannot grow in the absence of the aminoacid lysine (lys−). Among the 400 unordered tetrads resulting from this mating, 233 were PD, 11 wereNPD, and 156 were T.a. What types of spores are in the PD, NPD, andT 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 seethat the calculation you did in part (c) may substantially underestimate RF. What kinds of meioses(NCO, SCO, or DCO) generated each of the tetradtypes 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 correctedequation for RF in terms of SCO, DCO, and NCOmeioses.l. Using your answer to part (g), rewrite the correctedRF equation from part (k) in terms of the numbersof the various tetrad types. m.The equation you just wrote in part (l) is a correctedequation 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 genesthan the one you calculated in part (c).
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 map
units between the his and lys genes.
d. If you think about all the kinds of meiotic events
that 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 RF
equation you used in part (c) make about the
meiotic events producing each type of tetrad? When
could these assumptions nevertheless be correct?
f. Use your answers to part (d) to determine the
number of NCO, SCO, and DCO meioses that
generated the 400 tetrads.
g. Use your answers to part (f) to write a general
equation that relates the number of DCO meioses
to the number of the various tetrad types. Then
write another general equation that computes the
number of SCO meioses as a function of the number of the various tetrad types.
h. Based on your answer to part (f), calculate the
average number of crossovers per meiosis (m)
between his and lys.
i. Use your answer to (h) to write an equation for m
in 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 numbers
of the various tetrad types.
m.The equation you just wrote in part (l) is a corrected
equation for RF that takes into account double
crossovers that would otherwise have been missed.
Use this improved formula to calculate a more
accurate distance between the his and lys genes
than the one you calculated in part (c).
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