Determining Gene Order

I had a lot of office visits today regarding how to set up the F₁ chromosomes in order to figure out the gene order.  The textbook uses examples where all the wild-type alleles are on one chromosome and all the mutants are on the other.  This is called a coupling arrangement:

e.g.

 a+ b+ c+

==========

 a  b  c

However, it's certainly permissible to have an F₁ organism that has some alleles in repulsion:

e.g. 

 a+ b   c

==========

 a  b+  c+

You should note that in both cases, the genotypes of the F₁ are the same:  they both represent heterozygous creatures.  This will dramatically change the ratios from your testcross and which numbers represent the "parentals" (which actually just give the chromosomes for your F₁).

For the first case (all in coupling), if the het is derived from two true-breeding parentals, they might have the genotypes of:

 a+ b+ c+          a  b  c

==========   x    =========

 a+ b+ c+          a  b  c

The double crossover class from the testcross would be:

 a+ b  c+         a  b+ c

==========  or   ==========

 a  b  c          a  b  c      <=== This came from the testcross parent

For the second (some repulsion), the double crossover classes from the testcross would be:

 a+ b+ c         a  b  c+

=========  or   ==========

 a  b  c         a  b  c      <=== This came from the testcross parent

Here's an exercise to help you with this concept.

A solution is shown below (click on the YouTube icon to go to the YouTube site so you can view it in full screen and high definition).