Tuesday, 7 February 2012

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:

 a+ b+ c+
 a  b  c

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

 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).