To determine which gene is in the middle for three point crosses, you need to analyze the offspring’s phenotypes and genotypes resulting from the cross.
First, perform a three-point test cross involving three genes (let’s call them A, B, and C). You typically cross an organism that is heterozygous for all three genes with a triple recessive organism.
After performing the cross, you will need to classify the offspring based on their phenotypes. You should identify the parental (most common phenotype) and recombinant (less common phenotypes) types among the offspring.
The gene that is in the middle will exhibit the lowest frequency of recombination, which can be discovered by comparing the recombinant offspring. For example, if you find that the phenotypes that differ by one trait are present at a higher frequency, while the phenotype which differs by two traits is less common, then the gene that is responsible for that middle trait will be the one that is flanked by the other two genes.
After identifying the parental types and the recombinant types, you can look at the frequency of each type to determine the order of the genes on the chromosome. The gene whose phenotypes appear less frequently among the recombinants is the one that is situated in the middle.
In summary, analyze the phenotype ratios of the offspring, identify the parental and recombinant types, and determine which gene has the lowest recombinant frequency to find the gene in the middle.