Q.64 When a true breeding tall plant containing red flowers was crossed with the true
breeding dwarf plant containing white flowers, all F1 plants were tall with red
flowers. When the F1 plant was self-pollinated, considering independent assortment
of plant height and flower colour traits, the calculated percentage probability of
dwarf plants bearing red flowers in the F2 generation is ________ percent (round
off to 2 decimal places).
Tall red-flowering plants crossed with dwarf white-flowering plants demonstrate Mendel’s law of independent assortment in a dihybrid cross, yielding a 9:3:3:1 phenotypic ratio in F2. The probability of dwarf plants with red flowers is 18.75 percent.
Parental Genotypes
True-breeding tall red plants have genotype TT RR, where T (tall) and R (red) are dominant. True-breeding dwarf white plants have genotype tt rr, where t (dwarf) and r (white) are recessive.
F1 Generation
The cross TT RR × tt rr produces all F1 offspring as Tt Rr, showing dominant tall stature and red flowers due to complete dominance.
F2 Generation Punnett Square
Self-pollination of F1 (Tt Rr × Tt Rr) yields 16 gamete combinations under independent assortment. Phenotypes appear as 9 tall red : 3 tall white : 3 dwarf red : 1 dwarf white.
Probability Calculation
Dwarf plants (tt) occur in 4/16 offspring (25%). Red flowers (RR or Rr) occur independently in 3/4 of progeny. Dwarf red combination probability is (1/4) × (3/4) = 3/16, or 18.75%.
Introduction to Dihybrid Cross F2 Probability
In dihybrid crosses like true-breeding tall red (TT RR) × dwarf white (tt rr) plants, F1 shows all tall red (Tt Rr). F2 selfing follows Mendel’s independent assortment, producing dwarf red flowers at 18.75% probability – key for CSIR NET genetics questions.
Step-by-Step Dihybrid Cross Solution
- Parents to F1: Gametes TR × tr yield Tt Rr (100% tall red).
- F1 Gametes: TR, Tr, tR, tr (each 25%).
- F2 Ratios: 9/16 tall red, 3/16 tall white, 3/16 dwarf red, 1/16 dwarf white.
- No other ratios apply; linkage would alter 9:3:3:1, but question specifies independent assortment.
Why 18.75% for Dwarf Red Flowers?
Dwarf (tt) = 1/4; red (R-) = 3/4. Multiply: 1/4 × 3/4 = 3/16 = 0.1875 or 18.75% (rounded to 2 decimals). This matches standard Punnett square outcomes across sources.
