Incomplete Dominance Codominance Multiple Alleles

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Incomplete DominanceCodominanceMultiple Alleles

• In a nutshell!

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Review Dominant/Recessive

• One allele is dominant over the other (capable of
  masking the recessive allele)

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Review Problem Dominant/Recessive

• In pea plants, purple flowers (P) are dominant
  over white flowers (p) show the cross between two
  heterozygous plants.

P p
GENOTYPES

- PP (1) Pp (2) pp (1) - ratio 121
P p
PHENOTYPES
- purple (3) white (1) - ratio 31
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Incomplete Dominance

• A third (new) phenotype appears in the
  heterozygous that is a blend of the two traits
• Example are Flower Color in 4 oclock flowers
• R red allele
• W white allele (other designations R and r)

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Problem Incomplete Dominance

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

GENOTYPES

- RW (2) WW (2) - ratio 11

PHENOTYPES
- pink (2) white (2) - ratio 11
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Examples of Incomplete Dominance
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Codominance

• The heterozygous condition, both alleles are
  expressed equally
• See both traits expressed (spots/strips)

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Codominant Human Example

• Sickle Cell Anemia in humans
• Cells are sickle-shaped (half-moon)
• Do not carry oxygen well and can get stuck in
  veins

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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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Examples of Codominance in Animals
Appaloosa horse
Roan cow
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Multiple Alleles

• There are more than two alleles for a trait (not
  just H and h)
• Hair color
• HBr brown, Hbl blonde, hr red, hBk black
  
• Eye Color
• EBr brown , EHz hazel, eGr green,
• ebl blue

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An Example of Multiple Alleles Human Blood Type

• Blood Types?
• Type A, Type B, Type AB, Type O
• Blood Alleles There are three
• A, B, O (in book IA, IB, i)
• We all get a combination of two of these alleles

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Rules for Blood Type

• A and B are codominant and both are dominant over
  O
• IA IA (homozygous form) and IA i (heterozygous
  form) Type A
• IB IB (homozygous form) and IB i (heterozygous
  form) Type B
• IAIB Type AB
• ii Type O