Final answer:
The phenotypic ratio of 9:3:4 in the F2 generation indicates that the flower color is controlled by two genes with some form of gene interaction or epistasis, differing from Mendel's typical 3:1 monohybrid ratio.
Step-by-step explanation:
The question concerns the phenotypic ratios expected in the F2 generation of a cross between a pure-breeding red plant and a pure-breeding white plant, suggesting that flower color is controlled by two genes. From the given options, a 9:3:4 ratio is indicative of a dihybrid cross, where two genes control the trait in question. When dealing with such a cross, we typically see this ratio when there is epistasis, or interaction between genes, such that one gene modifies the phenotypic expression of another gene.
In Gregor Mendel's classic monohybrid crosses, as with his experiment on pea plants, the expected F2 phenotypic ratio for a trait controlled by one gene with complete dominance is 3:1, as observed with purple and white flower color. Three of every four plants exhibited the dominant phenotype (purple flowers), while one in four displayed the recessive phenotype (white flowers). This finding formed the basis of Mendelian inheritance and contradicts the expected ratios for a two-gene control system.
Thus, for a flower color trait controlled by two genes, the 9:3:4 ratio accounts for the various combinations of two gene interactions, where the first gene may either mask the expression of the second gene (epistasis) or they may interact to produce new phenotypes.
SUMMARY of key points: