Incomplete Dominance and Codominance

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Incomplete Dominance and Codominance
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• In Mendels work, alleles were either dominant or
  recessive.
• If green (G) is dominant to yellow (g), then
• GG will be green
• Gg will be green
• gg will be yellow
• Notice that we only got 2 different phenotypes
• However, sometimes we get 3 different phenotypes

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Incomplete Dominance

• In the case of incomplete dominance, two alleles
  are neither dominant or recessive
• They blend to produce a new phenotype.

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• For example, when a red snapdragon is crossed
  with a white snapdragon, all of the F1 offspring
  are an intermediate color pink.

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WW
RW
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Example

• A certain type of flower will produce green
  petals when a blue flower is crossed with a
  yellow flower. Cross two green flowers.
• B blue Y yellow BY green
• BY x BY

Genotype 25 BB 50 BY 25 YY
Phenotype 25 blue 50 green 25
yellow
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Codominance

• Codominance occurs when both alleles are equally
  dominant and both are expressed in a new
  phenotype.
• If a red cattle is crossed with a white one, the
  offspring are roan.
• A roan coat is a mixture of red and white hairs
  intermingled.
• Essentially both alleles are dominant.

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Multiple Alleles

• For each of the traits that Mendel studied, there
  were only 2 alleles (ex. round/wrinkled,
  tall/short, etc.)
• It is possible to have more than 2 alleles for a
  trait.
• Human blood type is an example of a trait that
  has multiple alleles (A, B, O)

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Blood Types

• Type O is recessive
• Types A and B are dominant to O, but are
  codominant to each other

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Blood Type Chart
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Example

• A woman whose blood type is pure A marries a man
  whose blood type is O. What are the possible
  blood types of their children?
• AA x oo

Genotype 100 Ao
Phenotype 100 type A