A pure breeding red flower variety of pea plant (RR) is crossed with a pure breeding white flower variety of pea plant (rr). Draw a Punnett square to find out the Phenotypic and Genotypic ratios of the progeny belonging to the F2 generation.
Topic: Monohybrid cross — Punnett square
Answer
PARENTS (P): RR (red) × rr (white) Gametes: R and r F1 GENERATION: all Rr — ALL RED FLOWERS
F1 selfed: Rr × Rr Gametes: R, r and R, r
PUNNETT SQUARE R r R RR (red) Rr (red) r Rr (red) rr (white)
F2 GENERATION: PHENOTYPIC RATIO = 3 RED : 1 WHITE GENOTYPIC RATIO = 1 RR : 2 Rr : 1 rr
Set the working out in full — parents, gametes, F1, then the square — because marks are given for the method as well as the ratios.
THE TWO RATIOS ARE DIFFERENT, and the question asks for both: PHENOTYPIC 3 : 1 — what they LOOK like. Three of the four (RR, Rr, Rr) appear red because R is dominant; only rr is white. GENOTYPIC 1 : 2 : 1 — what alleles they CARRY. A common error is giving 3:1 for both.
NOTE WHAT HAPPENS TO WHITE: it vanishes completely in F1 and returns in a quarter of F2. That reappearance proves the recessive allele was never lost, only MASKED, and is the observation on which Mendel built the idea of a gene as a discrete particle.
Draw the square with the gametes of one parent along the top and the other down the side.