Incomplete Dominance Codominance Multiple Alleles

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Incomplete DominanceCodominanceMultiple Alleles
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Review

• Mendel studies peas, in which one trait was
  dominant over the other
• In such crosses, homozygous dominant and
  heterozygous have the same phenotype
• In other modes of inheritance, the phenotype of
  the heterozygote is different

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Incomplete Dominance

• A third (new) phenotype appears in the
  heterozygous condition.
• Flower Color in 4 Oclocks (Mirabilis jalapa)

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Problem Incomplete Dominance

• Show the cross between a pink and a white flower.

GENOTYPES

- Rr (2) rr (2) - ratio 11

PHENOTYPES
- pink (2) white (2) - ratio 11
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Codominance

• The heterozygous condition, both alleles are
  expressed equally
• Eg. Sickle Cell Anemia in Humans

sick
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Problem Codominance

• Show the cross between an individual with
  sickle-cell anemia and another who is a carrier
  but not sick.

N S
GENOTYPES
- NS (2) SS (2) - ratio 11
S S
PHENOTYPES
- carrier (2) sick (2) - ratio 11
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Multiple Alleles

• For many traits, there are more than two alleles
• Eg. Blood type in humans
• Blood Types?
• Type A, Type B, Type AB, Type O
• Blood Alleles?
• IA, IB, i (i the O allele)

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Rules for Blood Type

• A and B are codominant
• IAIA Type A
• IBIB Type B
• IAIB Type AB
• A and B are dominant over O
• IAi type A
• IBi type B
• ii type O

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Problem Multiple Alleles

• Show the cross between a mother who has type O
  blood and a father who has type AB blood.

i i
GENOTYPES
- AO (2) BO (2) - ratio 11
IA IB
PHENOTYPES
- type A (2) type B (2) - ratio 11
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Problem Multiple Alleles

• Show the cross between a mother who is
  heterozygous for type B blood and a father who is
  heterozygous for type A blood.