CBSE · Class 10 · Science · Heredity and EvolutionIn a genetic cross between a pure-breeding tall pea plant (TT) and a pure-breeding dwarf pea plant (tt), work out the phenotypic and genotypic ratios for the F1 and F2 generations. Also state the law derived from this monohybrid cross.

To determine the phenotypic and genotypic ratios of the monohybrid cross between a pure tall pea plant (TT) and a pure dwarf pea plant (tt), we can follow the step-by-step procedure below:

Step 1: Identify Parental Generation (P)

• Pure tall plant genotype = $TT$
• Pure dwarf plant genotype = $tt$

Step 2: Determine Gametes

• The tall plant ($TT$) produces gametes carrying the allele: $T$
• The dwarf plant ($tt$) produces gametes carrying the allele: $t$

Step 3: F1 Generation Cross and Results

• Fertilization between gamete $T$ and gamete $t$ results in the F1 generation offspring with the genotype $Tt$.
• Phenotype of F1 generation: All plants are tall because the 'T' (tall) allele is completely dominant over the 't' (dwarf) allele.
• F1 Genotypic Ratio: 100% $Tt$ ( heterozygous tall )
• F1 Phenotypic Ratio: 100% Tall

Step 4: F2 Generation Cross (Selfing of F1)

• Two F1 plants ($Tt \times Tt$) are crossed.
• Gametes from each parent: $T$ and $t$.
• Using a Punnett square:
  • Top row gametes: $T$ (50%), $t$ (50%)
  • Left column gametes: $T$ (50%), $t$ (50%)
  • Resulting offspring genotypes: $TT$ (1), $Tt$ (2), $tt$ (1).

Step 5: F2 Ratios Calculation

• Genotypic Ratio: $1 (TT) : 2 (Tt) : 1 (tt)$ or $1:2:1$
• Phenotypic Ratio: $3 \text{ Tall} : 1 \text{ Dwarf}$ or $3:1$

Conclusion & Law:\nFrom this monohybrid cross, Mendel formulated the Law of Segregation (First Law), which states that alleles of a gene separate during gamete formation so that each gamete carries only one allele for each gene.