Final answer:
To determine the middle gene in a triple cross, one analyzes the recombinant frequencies from a trihybrid cross, uses a forked-line diagram or Punnett-square method, and calculates the phenotypic ratios to infer gene order among F2 generation offspring based on Mendelian principles.
Step-by-step explanation:
One would determine which gene is in the middle of a triple cross by analyzing the recombinant frequencies from a trihybrid cross. For example, if you are looking at three genes, let's say A, B, and C, and you find that the offspring have various combinations of these genes, those combinations that are least common are the result of double crossovers, which can help indicate the gene in the middle. If gene A and C recombine less frequently than A and B, or B and C, then gene B is likely the middle gene because it is involved in the more frequent recombinations.
The process often involves creating a forked-line diagram or using the Punnett-square method, which shows the independent assortment and various probabilities of offspring genotypes. By calculating the phenotypic ratio, which in a trihybrid cross is 27:9:9:9:3:3:3:1, you can infer the gene order based on the observed phenotypic frequencies among the F2 generation. These methods stem from principles such as independent assortment and dominance, as seen in Mendel's dihybrid cross exhibiting a 9:3:3:1 phenotypic ratio.