What will be the phenotypic ratio in the f1 generation, if a dominant red flowered plant (RR) is crossed with recessive white flowered plant (rr)?

Analyze the question: This is a monohybrid cross between a homozygous dominant parent (RR) and a homozygous recessive parent (rr). We need the phenotypic ratio of the F1 generation.

Concept: Each parent gives one allele per gamete. RR can only give R, and rr can only give r. Every offspring is therefore heterozygous (Rr). Since R is dominant, every Rr plant shows the red phenotype.

Punnett square:

| | R | R | |---|---|---| | r | Rr | Rr | | r | Rr | Rr |

• Genotypic ratio: 4 Rr (all heterozygous)
• Phenotypic ratio: 4 red : 0 white (all red)

Checking the options:

• A (1:1) would come from a test cross, Rr × rr. It does not apply here.
• C (3:1) is the F2 ratio from Rr × Rr, not the F1 generation.
• B (2 red : 0 white) and D (4 red : 0 white) both express "all red". The Punnett square has 4 boxes, so 4 red : 0 white matches it directly. Ratio B is the same proportion reduced, but D is the one that reflects the four offspring in the square.
• E (4:4) is incorrect.

Answer: D. All the F1 offspring are red (Rr).

Exam tip: Whenever you cross homozygous dominant (RR) × homozygous recessive (rr), the F1 is 100% heterozygous and shows the dominant trait. The 3:1 ratio only shows up in the F2, when you cross two F1 hybrids (Rr × Rr).