Incomplete Dominance and Codominance

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Incomplete Dominance and Codominance

• Some things Mendel didnt know about

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Mendelian genetics

• Mendel only talked about two kinds of
  allelesdominant and recessive
• Remember, Mendel died in 1884
• No idea what the factors were
• Pea plants have mostly traits determined by one
  gene which occurs in dominant and recessive
  alleles

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Incomplete Dominance

• A condition where the characteristics defined by
  the alleles are blended together to create an
  intermediate phenotype
• Instead of capital and lower case, we use two
  capital letters
• Colors often show incomplete dominance because
  there are two differently colored proteins which
  mix together like in art

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Four-o-clock flowers

• Can be red, white or pink
• Rallele for red
• Wallele for white

RRred WWwhite WR or RW--pink
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Incomplete dominance, cont.

• Punnett squares work the same way

WW X RR
Red flowers crossed with white flowers
W
W
WR
WR
R
4/4 WR Pink
WR
R
WR
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Incomplete in humans

• The best example of incomplete dominance in
  humans is sickle cell anemia

Normal red blood cells are smooth and have the
inner tube look so they can carry more oxygen--A
Sickle cells are shaped like sickles, carry less
oxygen and can form clots when they shouldnt--S
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Sickle cell

• AAnormal red blood cells
• SSSickle cell anemia or disease
• ASSickle cell traitonly a small number of their
  blood cells are affected

If two people with sickle cell trait have
children, what are the genotypes and phenotypes
of their children?
A
S
¼ AANormal 2/4 ASSickle cell
trait 1/4SSSickle cell disease
A
AA
AS
SS
S
AS
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Codominance

• Works like incomplete dominance except that there
  is a third possibility that is recessive to the
  other two
• Human blood type
• A and B have incomplete dominance but o is
  recessive to both

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Blood type

• AAtype A
• Aotype A

BBtype B Botype B
ootype O
ABtype AB