Some genes have alleles that are dominant and recessive, but some do not. Genes with alleles that are not dominant or recessive create more than two phenotypes.

Simple Dominance – One allele of the gene is dominant, one is recessive
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With this type of inheritance, there can only be
2 possible phenotypes
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Ex. Eye color (B – non-blue eyes, b – blue eyes)
BB = non-blue eyes
bb = blue eyes
Bb = non-blue eyes

Incomplete Dominance – Neither allele is dominant so heterozygous organisms have a mixed phenotype
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With this type of inheritance, there are 3 possible phenotypes
Since neither allele is dominant, lowercase letters are used to represent the genes
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Ex. Flower color in snapdragons (r – red flowers, w – white flowers)
rr = red flowers ww = white flowers
rw = pink flowers

Codominance – Both alleles are dominant, heterozygous organisms show both traits in their phenotype
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With this type of inheritance, there are 3 possible phenotypes
Since both alleles are dominant, uppercase letters are used to represent the genes
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Ex. Feather color in Erminette chickens (B – black feathers, W – white feathers)
BB = black feathers
BW = black & white feathers
WW = white feathers