The 3:1 ratio of purple-flowering plants to white flowering plants that Mendel observed in the F2 generation is best explained by ______.

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When we examine Mendel's F2 3:1 ratio, several ideas come into play,
The answer choices can be evaluated one by one to see how well they align with the observed data.
Option 1: The Law of Independent Assortment. This law states that alleles of different genes assort independently during gamete formation. While this helps explain dihybrid crosses yielding a 9:3:3:1 ratio, it does not directly account for the 3:1 phenotypic ratio seen in a monohybrid cross, which is the context here. Therefore, this option does not best explain the 3:1 ratio for a single trait.
Option 2: The Law of Dominant Characters. Mendel did not formalize a “Law of Dominant Characters” as a standalone principle. Dominance describes how alleles interact in a heterozygote, but it is not a named fundamental law that predicts the 3:1 phenotypic ratio. This makes this choice inaccurate as the primary explanation for the observed ratio.
Option 3: The Law of Alternate Alleles. This phrasing is not part of Mendel’s recognized laws. While the concept that alleles can alternate in expression is related to dominance, there is no formal Law of Alternate Alleles in Mendelian genetics, so this option is not a correct framework for the 3:1 baseline ratio.
Option 4: The Law of Segregation. This law states that two alleles for a given gene segregate (separate) into different gametes, so offspring inherit one allele from each parent. This segregation during gamete formation explains why, in a monohybrid cross, the offspring appear in a 3:1 phenotypic ratio for a trait with complete dominance. This option provides the direct mechanism behind the observed ratio.
Further nuance: The 3:1 ratio emerges when heterozygous individuals (Aa) produce gametes (A and a) that combine with gametes from the other parent, yielding AA, Aa, Aa, and aa genotypes, with three showing the dominant phenotype and one showing the recessive phenotype. Thus, the segregation of alleles is the key explanatory principle for this specific observation.
In summary, Options 1–3 either address a different scenario or lack a formal basis in Mendelian theory, while Option 4 directly encapsulates the mechanism that yields the 3:1 ratio in the F2 generation.

The 3:1 ratio of purple-flowering plants to white flowering plants that Mendel observed in the F2 generation is best explained by ______.

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