Dihybrid Crosses and Gametes

Monohybrid and Dihybrid Crosses

• Monohybrid Cross:

  • Focuses on one trait.
  • Example: Crossing Aa with Aa (only one gene).

• Dihybrid Cross:

  • Involves two traits.
  • Both traits are hybrid or heterozygous.
  • Example: Crossing AABB with aabb.

Gamete Formation

• Gametes Production:

  • Each parent can produce gametes containing one allele from each gene.
  • For dihybrid crosses, calculate possible combinations of alleles.

• Example with Pea Plants:

  • Traits: Yellow (dominant, represented as Y) vs Green (recessive, y).
  • Traits: Round (dominant, R) vs Wrinkled (recessive, r).

Possible Gametes for Dihybrid Cross

• For a plant with genotype YyRr, possible gametes:

  • YR, Yr, yR, yr

• Total gametes produced by each parent results in a four by four Punnett square, leading to 16 possible combinations.

Punnett Square and Ratios

• Punnett Square Setup:

  • Both parents produce 4 gametes leading to a 4 x 4 grid.

• Phenotypic Ratio in Dihybrid Crosses:

  • Result typically yields a 9:3:3:1 ratio when both traits are examined.
  • Example ratio breakdown:
  • 9 plants with both dominant traits (Round and Yellow)
  • 3 plants with Round, Green
  • 3 plants with Wrinkled, Yellow
  • 1 plant with both recessive traits (Wrinkled and Green)

Important Notes on Crosses

• When not heterozygous, determine possible gametes before constructing the Punnett square.

  • Example: one parent with genotype RRYY only produces one type of gamete: RY.

• When to Simplify:

  • Halving the size of a Punnett square is often more efficient than creating an unnecessary full grid.

Probability Rules in Genetics

• Alternatively, use probability rules for calculating expected outcomes instead of Punnett squares:

  • Multiplication Rule: Used for determining the probability of both events occurring.
  • Addition Rule: Used for determining the probability of either event occurring.

• This approach can be more straightforward for those less inclined towards using Punnett squares.