Correct Option: B. 3 tall : 1 short Detailed Explanation: To understand the genetic ratio of the F₂ generation when members of the F₁ generation are allowed to self-pollinate, we need to delve into basic Mendelian genetics, particularly focusing on traits governed by a single gene with two alleles.

1. Understanding the Generations:
2. P Generation (Parental Generation): This is the original generation that is crossed. For example, let’s say we have a tall plant (TT) and a short plant (tt). Here, "T" represents the dominant allele for tallness, and "t" represents the recessive allele for shortness.
3. F₁ Generation: When the P generation is crossed (TT x tt), all offspring in the F₁ generation will be heterozygous (Tt) and exhibit the dominant trait, which is tallness. Therefore, all F₁ plants are tall.
4. F₂ Generation: When the F₁ generation (Tt) is allowed to self-pollinate (Tt x Tt), we can predict the genotypes of the F₂ generation using a Punnett square.
5. Using a Punnett Square:
6. Set up a Punnett square for the cross Tt x Tt: T t ------------ T | TT | Tt | ------------ t | Tt | tt |
7. From the Punnett square, we can see the possible genotypes of the offspring:
   • TT (homozygous tall) = 1
   • Tt (heterozygous tall) = 2
   • tt (homozygous short) = 1
8. This results in a total of 4 offspring with the following phenotypes:
   • 3 tall (TT + Tt + Tt)
   • 1 short (tt)
9. Calculating the Ratio:
10. The phenotypic ratio of the F₂ generation is therefore 3 tall : 1 short.

Why Other Options Are Incorrect:

• Option A: 1 tall : 3 short: This is incorrect because it suggests that there is a higher number of short plants than tall plants, which contradicts the results from the Punnett square. The correct ratio shows more tall plants.
• Option C: 1 tall : 1 short: This option implies that there are equal numbers of tall and short plants. This would only be true if the parents were both heterozygous for a trait that exhibits incomplete dominance or if the cross was between a homozygous tall and a homozygous short plant, which is not the case here.
• Option D: 4 short : 0 tall: This option suggests that all offspring are short, which is impossible given that the F₁ generation consists entirely of tall plants. The presence of the dominant allele (T) ensures that at least some offspring will be tall.

Common Pitfalls:

• Misunderstanding Dominance: Students often confuse dominant and recessive traits. Remember, the presence of at least one dominant allele (T) results in the tall phenotype.
• Ignoring the Genotype: It’s crucial to differentiate between genotype (TT, Tt, tt) and phenotype (tall or short). The ratio refers to phenotypes.
• Not Using a Punnett Square: Some students may skip using a Punnett square, which is a helpful tool for visualizing genetic crosses and ensuring accurate predictions.

Revision Summary:

• The F₂ generation from a Tt x Tt cross results in a phenotypic ratio of 3 tall : 1 short.
• Use a Punnett square to visualize and calculate genetic ratios.
• Dominant alleles mask the expression of recessive alleles in heterozygous individuals.
• Always differentiate between genotype and phenotype when discussing genetic ratios.