Final answer:
The 3:1 ratio in the offspring of a cross suggests that the parents were both heterozygous for the trait in question. This ratio follows Mendel's laws of inheritance, where the dominant trait is seen three times as often as the recessive trait among the offspring.
Step-by-step explanation:
When two plants are crossed, and the resulting offspring displays a 3:1 ratio for a particular trait, it suggests that the parents were both heterozygous for that trait. This ratio is typical of a Mendelian monohybrid cross in which both parents contribute different alleles for the gene being examined – one dominant allele and one recessive allele (Bb x Bb). When these heterozygotes self-pollinate or are crossed with each other, the resulting Punnett square would show three boxes with the dominant phenotype to one with the recessive phenotype, giving rise to the 3:1 phenotypic ratio in the F2 generation.
Mendel's work with pea plants demonstrated that this ratio represents a genetic pattern where the dominant trait is expressed three times more frequently than the recessive trait among the offspring. Such a cross yields approximately 75% dominant phenotype offspring and 25% recessive phenotype offspring, indicating simple Mendelian inheritance of traits where one allele is dominant over the other. Therefore, in this context, the correct suggestion is that the parents were both heterozygous for the particular trait.