Final answer:
A complementation test is used to determine whether mutations occur at the same or different loci by crossing organisms with similar phenotypes and seeing if the offspring display a wild-type or mutant phenotype.
Step-by-step explanation:
To determine whether mutations occur at the same or different loci, one would use a complementation test. A complementation test involves crossing two organisms with similar mutant phenotypes. If the mutations are at different loci (meaning the mutations affect different genes), the offspring can show a wild-type phenotype because one organism can complement the mutation of the other. However, if the mutations are at the same locus (mutations in the same gene), complementation will not occur, and the mutant phenotype will be present in the offspring. This is different from a test cross, which is used to determine if an organism with a dominant phenotype is homozygous or heterozygous for that trait. A test cross involves crossing the phenotype-expressing organism with a homozygous recessive organism. If the test subject is homozygous dominant, all offspring will display the dominant trait; if it is heterozygous, there will be a 1:1 ratio of offspring with the dominant and recessive traits.