Q.66 In sweet pea, the development of purple colored flowers requires the presence of
two dominant genes, C and R, e.g. CCRR. When either C (e.g. ccRR) or R (e.g.
CCIr) or both the genes (e.g. ccrr) are present in homozygous state, the purple
color of flowers CANNOT be produced. If CCrr (white) is crossed with ceRR
(white), what will be the modified dihybrid ratio at F2 generation.

1.15:1
2. 13:3
3. 9:3:4
4.9:7

Sweet pea flower color genetics involves complementary genes C and R, where both dominant alleles (at least one C and one R) are required for purple flowers; absence of either results in white. Crossing CCrr (white) with ccRR (white) produces an F1 of all CcRr (purple), and the F2 generation yields a modified 9:7 dihybrid ratio of purple to white.

Genetic Basis

Purple flowers develop only with at least one dominant C and one dominant R (C_R_). Genotypes cc (any R) or rr (any C) or ccrr produce white flowers due to complementary gene action. This is a classic epistasis example where both genes must function for pigment.

Parental Cross and F1

CCrr (white, lacks R) crossed with ccRR (white, lacks C) yields F1: all CcRr. All F1 plants show purple flowers since both dominants present. Gametes: CCrr gives CR only; ccRR gives cR only.

F2 Generation Punnett Square

Selfing F1 (CcRr x CcRr) follows standard dihybrid: 16 combinations.

• Purple (C_R_): 9/16 (CCRR, CCRr, CcRR, CcRr x2 each appropriately).

• White: 7/16 (CCrr 1, ccRR 1, ccRr 2, ccrr 3; all lack either C or R or both).
  Ratio: 9 purple : 7 white.

Option Analysis

Correct Answer

Option 4: 9:7. This modified dihybrid ratio arises because 7/16 genotypes fail pigment synthesis.