Final answer:
A 3:1 ratio for a trait in offspring indicates that the parents were both heterozygous. This Mendelian inheritance pattern is evident when using a Punnett square to predict the outcomes of a genetic cross, leading to the expected phenotypic ratio.
Step-by-step explanation:
When two plants are crossed and produce offspring with a 3:1 ratio for a specific trait, this indicates that the parents were both heterozygous for the gene controlling that trait. According to Mendel's laws of inheritance, this ratio results from crossing two heterozygous parents (e.g., Bb x Bb) — where 'B' is the dominant allele and 'b' is the recessive allele. Mendel's experiments demonstrated that traits could be dominant or recessive, and the 3:1 phenotypic ratio in the F2 generation confirms that both parents had to contribute a dominant and a recessive allele to their gametes, resulting in offspring with either homozygous dominant, heterozygous, or homozygous recessive genotypes. However, only the heterozygous or homozygous dominant display the dominant trait, leading to the observed ratio.
This finding is consistent with the use of a Punnett square, which is a predictive tool to determine the genotypic and phenotypic outcomes of a genetic cross. Mendel postulated that each parent contributes one allele to the offspring, so the gametes from a heterozygous parent would contain either a dominant or a recessive allele. The combination of gametes results in a ratio where approximately three-quarters of the offspring exhibit the dominant phenotype, while one-quarter display the recessive phenotype, assuming that one allele is dominant over the other.