Mendel's dihybrid crosses examined two traits simultaneously: seed shape (round vs wrinkled) and seed color (yellow vs green). The cross RR yy à rr YY produced F1 offspring (Rr Yy) that were all round and yellow, demonstrating dominance. The F2 generation showed a 9:3:3:1 ratio, revealing independent assortment.
đŦ Key Concepts
Two traits examined: shape (R/r) and color (Y/y)
P generation: RR yy à rr YY
F1 generation: All Rr Yy (round, yellow)
F2 shows 9:3:3:1 phenotypic ratio
Dihybrid Cross: Rr Yy à Rr Yy
RY
Ry
rY
ry
RY
RRYY
RRYy
RrYY
RrYy
Ry
RRYy
RRyy
RrYy
Rryy
rY
RrYY
RrYy
rrYY
rrYy
ry
RrYy
Rryy
rrYy
rryy
F2 Generation 9:3:3:1 Ratio
RY
RY
RY
RY
RY
RY
RY
RY
RY
Ry
Ry
Ry
rY
rY
rY
ry
9 Round Yellow : 3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled Green
Actual Mendel's results: 315:108:101:32
Possible Gametes from Rr Yy
RY
Ry
rY
ry
Independent Assortment Discovery!
Mendel's actual F2 results were 315:108:101:32, remarkably close to the predicted 9:3:3:1 ratio! This proved that genes for different traits are inherited independently.
đ¯ Learning Objectives
Understand how two traits can be inherited independently, leading to new combinations in offspring and the mathematical 9:3:3:1 ratio that demonstrates independent assortment.