Partial genetic maps of the chromosomes in an Hfr strain and F- strain are shown below:
pro- lac- ade+ pro+ lac- ade-
A series of conjugational crosses between these strains is done in order to map four different lac- alleles (#1-4). In each cross, the parents carry a different combination of lac alleles, but in every case pro+ lac+ ade+ recombinants are selected. (Student note: I'm not sure if there's a typo here or above, since there doesn't appear to be a lac+ allele in the above chromosomes. (But my confusion about this problem goes far beyond a possible typo.)) The Table below shows a number of such recombinants obtained in crosses involving each combination of parental lac alleles.
Recipient lac allele
Donor lac allele Lac-1 Lac-2 Lac-3 Lac-4
Lac-1 0 24 18 236
Lac-2 206 0 21 190
Lac-3 192 179 0 214
Lac-4 37 26 23 0
Analyze the data in the Table and determine the relative order of the four lac alleles with respect to the flanking pro and ade loci.
The answer is pro-3-2-1-4-ade, but I have no idea how it was obtained. I don't even know where to get started on a problem like this. I know what Hfr and F- cells are, but that's about the extent of my knowledge regarding this problem.
The expert examines conjugational cross and gene order. The relative order of the four lac alleles with respect to the flanking pro and ade loci is determined.