Final answer:
In three-point testcrosses, the least frequent classes correspond to the double recombinant progeny, which are a result of crossing over events that form new allele combinations, helping map the distance and order of genes.
Step-by-step explanation:
The two least frequent classes in three-point testcrosses usually represent the double recombinant progeny. These progeny result from crossing over events during meiosis, which lead to the creation of new combinations of alleles that differ from both parental types. As such, these rare offspring combinations indicate the order and distance between genes on a chromosome.
Using Punnett squares and understanding linkage can help to predict the genotypic and phenotypic outcomes of genetic crosses. If the genes are linked, nonparental types are less common than parental types, which indicate which alleles were originally on the same chromosome in the parent. When conducting a testcross, for example between a heterozygous organism (AaBb) and a homozygous recessive individual (aabb), the least common offspring will be the double recombinants (Aabb and aaBb if A and B are on the same chromosome).