Correct Option: A. 3 : 1 Explanation of the Correct Answer To understand why the genotypic ratio of the second filial generation (F2 generation) is 3:1, we need to delve into basic Mendelian genetics, specifically focusing on the inheritance of a single trait.

1. Understanding the Genes:
2. In this scenario, we have two alleles for the trait of plant height:
   • T: Dominant allele for tallness
   • t: Recessive allele for dwarfness
3. Parental Generation (P):
4. Let's assume we start with two homozygous parents:
   • One parent is homozygous tall (TT).
   • The other parent is homozygous dwarf (tt).
5. When these two parents are crossed, all offspring in the first filial generation (F1) will be heterozygous (Tt), exhibiting the dominant trait (tallness).
6. First Filial Generation (F1):
7. All F1 plants will have the genotype Tt (tall).
8. Genotypic ratio of F1: 100% Tt
9. Second Filial Generation (F2):
10. When the F1 generation (Tt) is self-fertilized (Tt x Tt), we can use a Punnett square to determine the genotypes of the F2 generation:
    • The Punnett square will look like this:
    | T | t | |-------|-------| | T | TT | Tt | |-------|-------| | t | Tt | tt |
11. From the Punnett square, we can see the possible genotypes for the F2 generation:
    • TT: 1
    • Tt: 2
    • tt: 1
12. Calculating the Genotypic Ratio:
13. The genotypic ratio from the Punnett square is:
    • 1 TT : 2 Tt : 1 tt
14. This can be simplified to:
    • 1 (homozygous tall) + 2 (heterozygous tall) + 1 (homozygous dwarf) = 4 total plants
15. Therefore, the ratio of tall to dwarf plants in terms of phenotype is:
    • Tall (TT + Tt) = 3
    • Dwarf (tt) = 1
16. Thus, the phenotypic ratio is 3:1.

Explanation of Incorrect Options

• B. 1 : 2 : 1:
• This option represents the genotypic ratio of the F2 generation, which is indeed 1 TT : 2 Tt : 1 tt. However, the question specifically asks for the phenotypic ratio, which is 3:1 (tall to dwarf). Therefore, while this option is correct for genotypes, it does not answer the question correctly.
• C. 2 : 2:
• This option suggests an equal distribution of two genotypes, which does not reflect the actual outcomes from the cross of Tt x Tt. The correct ratios are 1 TT, 2 Tt, and 1 tt, not 2:2.
• D. 4 : 0:
• This option implies that all offspring are of one genotype (which would be TT), suggesting that there are no dwarf plants at all. This is incorrect because the presence of the recessive allele (t) in the F1 generation allows for the possibility of dwarf plants in the F2 generation.

Summary for Revision

• The genotypic ratio of the F2 generation from a Tt x Tt cross is 1 TT : 2 Tt : 1 tt.
• The phenotypic ratio of the F2 generation is 3 tall (TT + Tt) : 1 dwarf (tt).
• The dominant trait (tallness) masks the recessive trait (dwarfness) in heterozygous individuals.
• Understanding Punnett squares is crucial for predicting the outcomes of genetic crosses.