Final answer:
In a three-point test cross, the double crossover progeny are critical for determining the order of three linked genes, as they show the rarest allele combinations and reveal the gene order by exhibiting two crossover events.
Step-by-step explanation:
In a three-point test cross used to determine the order of three linked genes, when compared to the no-crossover progeny, the double crossover (DCO) class needs to be considered to determine gene order. The DCO progeny are critical because they represent the least common class, which indicates the occurrence of crossing over between all three genes. These progeny provide the key to deciphering the gene order as they display the rarest combination of alleles, having experienced two crossover events.
Geneticists utilize the frequency of recombination to create maps of gene locations on chromosomes. The higher the frequency of crossover between two linked genes, the further apart they are estimated to be. Therefore, through examining the progeny of a three-point test cross, where genes that exhibit the least frequency of crossing over are considered to be the closest in linkage, the correct sequence of the genes can be inferred.