_{1}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 F

_{1}organism that has some alleles in repulsion:

*e.g.*

a+ b c

==========

a b+ c+

_{1}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 F

_{1}).

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 parentFor 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.