In the cross Hfr aro + arg + ery r str s x F- aro - arg - ery s str r , the mark
ID: 57955 • Letter: I
Question
In the cross Hfr aro+ arg+ eryr strs x F- aro- arg- erys strr, the markers are transferred in the order given (with aro+ entering first) mbut the first three genes are very close together. Exconjugants are plated on a medium containing Str, Ery, Arg, and Aro. The following results are obtained for 300 colonies isolated from these plates and tested for growth on various media: on Ery only, 252 strains grow; on Ery + Arg, 253 strains grow; on Ery + Aro, 279 strains grow; on Ery + Arg + Aro, 300 strains grow.
a. Draw up a list of genotypes, and indicate the number of individuals in each genotype.
b. Calculate the recombination frequencies.
c.Calculate the ratio of the size of the arg-to-aro region to the size of the ery-to-aro region.
Explanation / Answer
a) The colonies which grew on Ery only is 252 strains, Hence these are arg+, aro+, eryr. The remaining 48 colonies are mutant for one or both genes.
The additional colony which grew up on addition of arg (253-252 = 1); This colony is arg- aro+ eryr.
279 are arg+ since they grew only in the presence of erythromicin and aromatic amino acid. 26 is aro- (279-253).
The genotype and their frequencies are given below.
252 eryr arg+ aro+
26 eryr arg+ aro-
1 eryr arg- aro+
21 eryr arg- aro-
b) Recombinant frequency for aro-arg can be calculated from aro+ arg- and aro- arg+.
= 100 % (26+1)/300 = 9.0% (m.u)
Recombinant for ery- arg can be calculated from aro+arg- and aro- arg-
+ 100% (1+21)/300 = 7.3% (m.u)
Recombinant frequency for ery-aro region can be calculated from arg+aro-, arg-aro- and arg- aro+
= 100% (26+21+2)/300 = 16.3% (m.u)
c) The ratio 27:22 or 1.2:1.0