Incomplete & Codominance
Biology Honors
Exceptions to Mendel’s Work
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Pea plants show complete
dominance only
Ex: purple flowers are
dominant to white flowers
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PP=purple
Pp=purple
pp=white
Sometimes genetics are
more complex than that!
(actually, they usually are)
Incomplete Dominance
Incomplete Dominance
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Blending of traits
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Neither allele dominates the other
Three genotypes = three phenotypes
Example:
RED flower x WHITE flower  PINK flower
RR = red
rr = white
Rr = pink
Incomplete Dominance
Make a key to help you!
RR = Red
Rr = Pink
rr = White
Practice, practice, practice!
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Oompa Loompa can have red (R), blue (r)
or purple hair. Purple hair results from the
heterozygous condition. Make a "key"
showing the genotypes and phenotypes
for hair color.
RR = red
rr = blue
Rr = purple
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Olivia Oompa is married to Odo Oompa
and they both have purple hair. What
color hair and in what proportion would
you expect their children to have?
Codominance
Codominance
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Both traits are shown
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Both alleles are dominant together
red x white  red & white spotted
RR x rr  Rr
Think “co-” is together (cooperate, coexist)
Codominance
Make a key to help
you!
RR = Red
Rr = Roan (red & white)
rr = White
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Practice, practice, practice
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In a certain species of frogs, purple legs
(L) are dominant to black legs (l), but
heterozygous genotype results in purple
and black legs. Set up a key to show
this.
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A purple and black legged male mates
with a black legged female. What is the
chance of having purple legged offspring?
Show the cross.