Final answer:
To determine gene order in a 3 point testcross, the no-crossover progeny need to be compared with the double crossover progeny, which helps identify the middle gene based on the differences in allele combinations.
Step-by-step explanation:
To determine the gene order in a 3 point testcross involving 3 linked genes, one needs to compare the no-crossover progeny to the double crossover progeny. This comparison is pivotal because double crossovers reveal the gene that is in the middle. In general, double crossover offspring have the lowest frequency among the progeny and they display the combination of alleles that differ from both the parental (no-crossover) and single-crossover progeny. By identifying which gene differs in the double crossover progeny as compared to the no-crossover progeny, geneticists can establish the correct order of the three linked genes.
Understanding Recombination Frequency and Gene Order
The recombination frequency between linked genes indicates their relative physical distances on a chromosome: a higher frequency suggests that the genes are farther apart and thus more likely to experience crossover events. Determining the gene order is an essential step in constructing genetic maps, which use recombination frequencies to predict genetic distances. Such genetic maps provide important insights into linkage and inheritance patterns by revealing how genes are arrayed on chromosomes.