Genetic dominance isn't always straightforward. Sometimes genes mix and match in surprising ways. Incomplete dominance and codominance shake up the classic Mendelian patterns, creating new possibilities for how traits are expressed.

These inheritance patterns add spice to genetic diversity. They're why snapdragons can be pink and why you might have AB blood. Understanding them helps us grasp the full spectrum of genetic expression in living things.

Incomplete Dominance and Codominance

Types of genetic dominance

• Complete dominance occurs when one allele completely masks the expression of the other allele
  • In pea plants, the allele for purple flowers (P) completely masks the allele for white flowers (p) resulting in purple-flowered plants in the presence of at least one P allele
• Incomplete dominance occurs when neither allele is completely dominant over the other
  • The heterozygous phenotype appears as an intermediate between the two homozygous phenotypes
  • In snapdragons, the allele for red flowers (R) and the allele for white flowers (W) exhibit incomplete dominance resulting in pink flowers in heterozygous (RW) individuals
• Codominance occurs when both alleles are expressed equally in the heterozygous individual
  • The heterozygous phenotype displays characteristics of both alleles simultaneously
  • In human blood types, the alleles for A and B antigens are codominant resulting in the AB blood type expressing both A and B antigens in heterozygous individuals

Offspring ratios in non-dominant inheritance

• Incomplete dominance in a monohybrid cross between two heterozygous individuals (RW x RW in snapdragons)
  • Genotypic ratio: 1 RR (red) : 2 RW (pink) : 1 WW (white)
  • Phenotypic ratio: 1 red : 2 pink : 1 white
• Codominance in a monohybrid cross between two heterozygous individuals (AB x AB in human blood types)
  • Genotypic ratio: 1 AA (type A) : 2 AB (type AB) : 1 BB (type B)
  • Phenotypic ratio: 1 A : 2 AB : 1 B

Effects of non-dominant inheritance

• Incomplete dominance results in the expression of intermediate phenotypes in heterozygous individuals
  • Increases phenotypic variation in a population
  • In a population of snapdragons with incomplete dominance, there will be red, pink, and white flowers present
• Codominance results in the expression of both alleles in heterozygous individuals
  • Increases phenotypic variation in a population
  • In a human population, codominance of the A and B blood type alleles results in the presence of A, B, AB, and O blood types

Problem-solving for non-dominant inheritance

• Incomplete dominance example: In snapdragons, the allele for red flowers (R) and the allele for white flowers (W) exhibit incomplete dominance. A cross between a heterozygous pink snapdragon (RW) and a white snapdragon (WW) would result in the following:
  • Punnett square:

    	W	W
    R	RW	RW
    W	WW	WW

  • Genotypic ratio: 1 RW (pink) : 1 WW (white)
  • Phenotypic ratio: 1 pink : 1 white
• Codominance example: In humans, the alleles for blood types A and B are codominant. A cross between an individual with blood type AB and an individual with blood type B would result in the following:
  • Punnett square:

    	B	B
    A	AB	AB
    B	BB	BB

  • Genotypic ratio: 1 AB (type AB) : 1 BB (type B)
  • Phenotypic ratio: 1 AB : 1 B