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Wize AP Biology Textbook > Mendelian Genetics

Incomplete Dominance, Co-Dominance

Genotype and PhenotypePrevious Section
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Incomplete Dominance and Co-Dominance


Combinations of alleles (genotype) result in different traits or physical features (phenotype) being expressed.
  • Remember that when one allele masks the other, the allele that is seen in the phenotype is called dominant. The other allele is called recessive.
  • Sometimes expression is not as cut and dry, resulting in incomplete dominance or co-dominance phenotypes.

Incomplete Dominance

  • Neither allele is dominant nor recessive.
  • Both traits come together to contribute equally to an intermediate trait.
  • Sometimes not an obvious mixture of the two traits. Example: In flowers, allele R encodes Red and allele r encodes white. What phenotype might result from a heterozygous genotype Rr?
    Pink
Photo by Adabow / CC BY

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Co-Dominance

  • Neither allele is dominant nor recessive.
  • Both traits are expressed, not in a mixture but as a hybrid. Example: consider the previous example of flower color alleles. If co-dominance is occurring, then the Rr genotype results in both red and white showing up.
Photo by darwin cruz / CC BY
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Example: Incomplete vs Co-dominance

Coat color in a species of mice is controlled by a gene that produces a brown pigment giving the mice a brown coat. This gene has two alleles: C which is a functional allele, and C' which is a mutated allele that can not produce the pigment. What kind of dominance could you see in this scenario? And what would the coat color phenotypes be?

You could see either complete or incomplete dominance.
1) With complete dominance you would see brown and white mice.
2) With incomplete you would see brown (CC), white (C'C') and an intermediate tan/beige (CC').

Petal color in a species of peony is controlled by a gene that produces pigments ranging from blues to reds. This gene has two alleles: Xr which produces a reddish pigment and Xb which produces a blueish pigment. What kind of dominance could you see in this scenario? And what would the coat color phenotypes be?

You could see either complete dominance or co-dominance.
1) With complete dominance, you would see red and blue petals.
2) With co-dominance, you would see all red, all blue or red/ blue hybrid.

Practice: Recessive Trait

In order for a recessive trait to be expressed you would need ____ allele(s) to be recessive.