The following asci were obtained in Neurospora a crassa when a wild-type strain

Question

The following asci were obtained in Neurospora a crassa when a wild-type strain (thi^+ 1eu^+) was crossed to a double mutant strain that cannot grow in the absence of thiamine or leucine (thi^- leu^-). Only one member of each spore pair produced by the final mitosis is shown since the two cells in a pair have the same genotype. What is the approximate map unit distance between thi and leu? 25.0 35.0 17.5 8.3 They are unlinked Using the data set from Question 9 above, what is the approximate map unit distance between thi and the centromere? 50.0 8.3 17.5 35.0 16.7

Explanation / Answer

9) Identify Paretal Ditype (PD) , Non Parental Ditype(NPD) and Tetrad

PD have the same genotype as the paretal class

NPD have different genotype that the paretal class

Tetrad- show all the possible combinations

therefore, A is PD, B is T, C is T , D is NPD and E is T

Since PD=NPD, they are unlinked . Ans- e

10) identify the first division segregation and the Second division segregation.

first division segregation (FDS)- no crossover therefore all the pattern same as the centromeric pattern

Second division segregation (SDS)- crossover occured therefore, all the ptterns are different.

From the above data , map distance between thi and centromere =1/2* (no of SDS / total no of asci )*100

SDS for thi is asci B and E- 18+2=20

therefore, map distance between thi and centromere=1/2*(20/120)*100=8.3

Ans-b

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