Final answer:
The least frequent F2 phenotypic class in a trihybrid cross between two true-breeding homozygous individuals with contrasting phenotypes is recessive for all three traits, with the genotype likelihoods following the laws of probability.
Step-by-step explanation:
The correct answer to the question regarding a trihybrid cross between two true-breeding homozygous individuals with contrasting phenotypes is that the least frequent F2 phenotypic class is recessive for all three traits.
When true-breeding, homozygous individuals with contrasting phenotypes for three different traits (A, B, and C) are crossed, all the F1 offspring will be heterozygous for those traits (AaBbCc). If these F1 heterozygous individuals are self-crossed to produce an F2 generation, the expected genotypic ratio would be much more complex due to the independent assortment of each trait. The phenotypic ratio extrapolates from the 3:1 ratio observed in a monohybrid cross and the 9:3:3:1 ratio observed in a dihybrid cross to a 27:9:9:9:3:3:3:1 ratio for a trihybrid cross in which three unlinked traits are segregating. Thus, the most frequent phenotypic classes would exhibit dominant phenotypes for all three traits, whereas the least frequent phenotypic class would be recessive for all three traits.
The laws of probability can be used in trihybrid crosses, as they are essential in predicting the genotypic and phenotypic ratios of offspring. These laws show that traits are inherited independently and that the individual chance of inheriting a dominant or recessive allele for each trait is consistent with Mendelian principles.