7.1.3 Predicting Inheritance: Dihybrid Crosses

Example: Seed Shape and Colour in Pea Plants

• Seed Shape:
  • Round (R) is dominant.
  • Wrinkled (r) is recessive.
• Seed Colour:
  • Yellow (Y) is dominant.
  • Green (y) is recessive. When crossing a plant with round yellow seeds (RRYY) and one with wrinkled green seeds (rryy), the inheritance pattern is analysed.

First Cross (F₁ Generation)

1. Parental Genotypes: RRYY × rryy.
2. Gametes Produced:

• RRYY parent produces gametes: RY.
• rryy parent produces gametes: ry.

3. F₁ Offspring Genotypes: All RrYy.
4. F₁ Phenotypes: All offspring have round yellow seeds as the dominant alleles (R and Y) are expressed.

Second Cross (F₂ Generation)

Crossing two heterozygous individuals (RrYy × RrYy) results in a 9:3:3:1 phenotypic ratio in the offspring.

Steps:

5. Gametes Produced:

• Each parent produces four combinations of alleles: RY, Ry, rY, ry.

6. Punnett Square for Dihybrid Cross: | Parent Gametes | RY | Ry | rY | ry | |---|---|---|---|---| | RY | RRY Y | RRY y | RrY Y | RrY y | | Ry | RRY y | RR yy | RrY y | Rr yy | | rY | RrY Y | RrY y | rrY Y | rrY y | | ry | RrY y | Rr yy | rrY y | rr yy |

Phenotypes and Ratios:

• 9 Round Yellow (RRYY, RRYy, RrYY, RrYy).
• 3 Round Green (RRyy, Rryy).
• 3 Wrinkled Yellow (rrYY, rrYy).
• 1 Wrinkled Green (rryy).