CBSE · Class 12 · Biology · Principles of Inheritance and VariationA homozygous tall pea plant with red flowers (TTRR) is crossed with a homozygous dwarf pea plant with white flowers (ttrr). Determine the genotype and phenotype of the F1 generation. If the F1 generation is self-pollinated, calculate the phenotypic ratio of the F2 generation and explain the inheritance pattern.

Problem Statement and Given Data

• Parent 1 (Homozygous Tall & Red): Genotype = $\text{TTRR}$ (Phenotype = Tall, Red)
• Parent 2 (Homozygous Dwarf & White): Genotype = $\text{ttrr}$ (Phenotype = Dwarf, White)

Step 1: Gamete Formation

• Parent 1 (TTRR) produces gametes with alleles: TR
• Parent 2 (ttrr) produces gametes with alleles: tr

Step 2: F1 Generation Determination

• Cross: $\text{TR} \times \text{tr}$
• F1 Genotype: $\text{TtRr}$
• F1 Phenotype: Since 'T' is dominant over 't' and 'R' is dominant over 'r', all plants in the F1 generation will be Tall with Red flowers.

Step 3: F2 Generation Calculation (Selfing of F1)

• F1 Cross: $\text{TtRr} \times \text{TtRr}$
• Gametes produced by F1: Each parent produces 4 types of gametes in equal proportions: TR, Tr, tR, tr.

Step 4: Punnett Square Analysis for F2 Generation\nUsing a $4 \times 4$ Punnett square for a dihybrid cross involving 16 total combinations:

• Tall and Red (T_R_): 9 offspring
• Tall and White (T_rr): 3 offspring
• Dwarf and Red (ttR_): 3 offspring
• Dwarf and White (ttrr): 1 offspring

Step 5: Final Phenotypic Ratio

$$\text{Phenotypic Ratio} = 9 : 3 : 3 : 1$$

Explanation of Inheritance Pattern\nThis follows Mendel's Law of Independent Assortment. The alleles for height (T/t) and flower color (R/r) assort independently during gamete formation because they are located on different non-homologous chromosomes, leading to the classic 9:3:3:1 phenotypic ratio in the F2 generation.