Practice: gene-linkage map: Diagram and give the distances and orders between th

Question

Practice: gene-linkage map: Diagram and give the distances and orders between the three fly genes, D, E

and F. Of 1000 offspring from the cross DdEe (dihybrid) x ddee (homozygous recessive; “tester”), 347 were

DdEd, 148 were Ddee, 152 were ddEe and 353 were ddee. Of 1000 offspring from the cross EeFf x eeff,

296 were EeFf, 197 were Eeff, 203 were eeFf, and 304 were eeff. Of 1000 offspring from the cross DdFf x

ddff, 455 were DdFf, 45 were Ddff, 55 were ddFf and 445 were ddff.

- What are the distances and orders between the D, E and F loci in fruit flies?

- How do you know, or do you know?, that these three genes are located on the same chromosome?

Explanation / Answer

Step 1: Determine the parental genotypes.

The most abundant genotypes are the partenal types. What is different from our first three-point cross is that one parent did not contain all of the dominant alleles and the other all of the recessive alleles.

Step 2: Determine the gene order

To determine the gene order, we need the parental genotypes as well as the double crossover geneotypes . From this information we can determine the order by asking the question: In the double-crossover genotypes, which parental allele is not associated with the two parental alleles it was associated with in the original parental cross.

Step 3: Determing the linkage distances.

·         E-D distance calculation. This distance is derived as follows: 100*((45+55)/1000) = 10 cM

·        

Step 4. Draw the map.

                      See the attached image

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