Codominance
…it’s not like mixing paint!
Definitions
•  Incomplete dominance situation in which one allele is
not completely dominant over
another (a blending). Use a
capital letter and capital letter
prime (P, P’).
•  Codominance - situation in
which both alleles of a gene
contribute to the phenotype of
the organism. Use the same
capital letter but with different
superscripts (HR, HW).
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1. Carnation Example
(incomplete dominance)
•  Carnations: R - red, R’ - can’t make pigment
–  Always make a genotype key, first.
•  RR: red
•  R’R’: white (no pigment)
•  RR’: pink (not enough pigment)
• RR red
• R’R’ white
• RR’ pink
Q. If a Pink carnation is crossed with another
pink carnation, what are the possible offspring?
If it was like mixing paint, we would get all pink.
If it was absolute dominance, we would get 3/4 red, 1/4 white.
•  Parents Phenotypes Pink, pink
•  Parents Genotypes RR’, RR’
•  Punnett Square
R
R’
RR
RR’
R’ RR’
R’R’
R
•  Offspring Genotypes
1/4 RR, 1/2 RR’, 1/4 R’R’
•  Offspring Phenotypes
1/4 red, 1/2 pink, 1/4 white
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2. Cattle Example
(codominance)
•  Cattle: HR - red hair, HW - white hair
–  Make a genotype key.
•  HRHR: red
•  HWHW: white
•  HRHW: roan (a mix of white hairs & red hairs)
X
• HRHR: red
• HWHW: white
• HRHW: roan
=
Q. If a Red Bull is crossed with a white cow,
what are the possible offspring?
If it was like mixing paint, we would get all tan.
If it was absolute dominance, we would get all red.
•  Parents Phenotypes Red,white
•  Parents Genotypes HRHR, HW HW
•  Punnett Square
HR
HR
H W H RH W H RH W
H W H RH W H RH W
•  Offspring Genotypes 100% HRHW
•  Offspring Phenotypes 100% roan
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What about Palominos?
•  The palomino horse
is a hybrid exhibiting
a golden color with a
lighter mane and tail.
•  A pair of alleles (D1
& D2) is known to be
involved in the
inheritance of these
coat colors.
What do you write in the key?
•  Genotypes
homozygous for
the D1 allele are
Chestnut colored
(reddish)
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What do you write in the key?
•  Genotypes
homozygous for the
D2 allele are
Cremello (almost
white).
What do you write in the key?
•  Heterozygous
genotypes are
Palomino colored
•  What does this trait show:
codominance or incomplete
dominance?
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Try the problems!
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