Incomplete Dominance (Leaving Cert Biology): Revision Notes

Incomplete Dominance

• Incomplete dominance occurs when neither allele is completely dominant over the other.
• This results in a heterozygous phenotype that is somewhere between the two homozygous phenotypes.
• Example: In snapdragon flowers, a red-flowered plant crossed with a white-flowered plant produces offspring with pink flowers.
• Incomplete dominance can occur in plants and in the coat colour of certain breeds of animals like cattle.

Step-by-Step: Performing an Incomplete Dominance Cross

Example (a): Red-Flowered Snapdragons x White-Flowered Snapdragons

Key Alleles:

• RR: Red-flowered.
• rr: White-flowered.
• Rr: Pink-flowered (intermediate phenotype).

Step 1: Determine the Parent Genotypes

• Parents:
  • Parent 1: RR (red-flowered).
  • Parent 2: rr (white-flowered).

Step 2: Identify Gametes

• Each parent produces one type of gamete:
  • Parent 1 (RR) → R gametes.
  • Parent 2 (rr) → r gametes.

Step 3: Perform the Cross

	R	R
r	Rr	Rr
r	Rr	Rr

Step 4: F1 Generation

• Genotype of Offspring: 100% Rr.
• Phenotype of Offspring: 100% pink flowers.

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Example (b): Crossing Two Heterozygous (Pink-Flowered) Offspring

Step 1: Determine the Parent Genotypes

• Parents:
  • Parent 1: Rr (pink-flowered).
  • Parent 2: Rr (pink-flowered).

Step 2: Identify Gametes

Each parent produces two types of gametes:

• R and r.

Step 3: Perform the Cross

	R	r
R	RR	Rr
r	Rr	rr

Step 4: F2 Generation

• Genotypes:
• RR: 25% (red-flowered).
• Rr: 50% (pink-flowered).
• rr: 25% (white-flowered).
• Phenotypes:
• Red-Flowered: 25%.
• Pink-Flowered: 50%.
• White-Flowered: 25%.