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How to do a linkage map

Time of entry mapping between a streptomycin-sensitive, prototrophic Hfr strain and a streptomycin- resistant Fminus strain that was Trp- Pyr- Cys- Nic- did not allow the order of the trp,pyr,cys genes to be determined, but these three genes were always transferred before nic. To determine the relative order of the trp,pyr,cys genes and the distances between them, the Hfr and Fminus strains were mated long enough to allow the transfer of the nic gene, after which Trp+ recombinants were selected. The unselected pyr and cys genes were then scored in the Trp+ recombinants , yielding the following results:

phenotype number of recombinants
Trp+ Pyr+ Cys+ 790
Trp+ Pyr+ Cys- 145
Trp+ Pyr- Cys+ 60
Trp+ Pyr- Cys- 5

Draw a map of the trp, pyr, cys genes relative to each other.Express map distances between adjacent genes as the frequency of crossing over between them.

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Solution Preview

Firstly, you need to calculate which gene is in the middle. To do this, we must find the gene that recombines on its own with the lowest frequency (since this represents a double cross-over event). The rarest genotype is Trp+ Pyr- ...

Solution Summary

Provides the solution and shows you how to obtain it yourself for a classic linkage map genetics problem.