Beyond Dominant &
Recessive Alleles
Incomplete Dominance & Codominance
S
Learning Goals
S 1. Describe incomplete dominance. Describe codominance.
S 2. Compare and contrast incomplete dominance and
codominance. Give 2 similarities and 1 difference.
S 3. Explain how letters are chosen when doing punnett square
problems for incomplete dominant and codominant traits
S 4. Explain what multiple alleles are and give an example
S 5. Describe what a polygenic trait is and give an example.
Beyond Dominant and
Recessive Alleles
S Some alleles are neither dominant
nor recessive, and many traits are
controlled by multiple alleles or
multiple genes.
Incomplete Dominance
S Incomplete dominance - when one
allele is not completely dominant over
another.
S Heterozygous genotype = BLEND
S Since one allele is not completely dominant over
the other allele, we use two different capital letters
to represent the alleles
S Example: A cross
between red (RR)
and white (WW)
snapdragons
produces pink
flowers (RW).
Homozygous Heterozygous Homozygous
RR
RW
WW
(Red)
(Pink)
(White)
Codominance
S Codominance - both alleles contribute
to the phenotype.
S Heterozygous genotype = both traits appear
S Since one allele is not completely dominant over
the other allele, and both traits appear together, we
use two different capital letters to represent the
alleles
S Example: A cross between black (BB) and
white (WW) chickens produces chickens
speckled with black and white feathers (BW).
Multiple Alleles
S Multiple alleles -genes that are
controlled by more than two alleles.
S An individual can’t have more than two
alleles (Remember you get one copy from
each parent). However, more than two
possible alleles can exist in a population.
•Example: A
human’s blood
type is
determined by a
single gene that
has three different
alleles.
Polygenic Traits
S Polygenic traits -traits controlled by
two or more genes.
S Skin color in humans is a polygenic trait
controlled by more than four different genes.
Learning Goals
S 1. Describe incomplete dominance. Describe Codominance.
S 2. Compare and contrast incomplete dominance and
codominance. Give 2 similarities and 1 differences
S 3. Explain how letters are chosen when doing punnett square
problems for incomplete dominant and codominant traits
S 4. Explain what multiple alleles are and give an example
S 5. Describe what a polygenic trait is and give an example.
Whiteboard Practice
S Step 1: Incomplete Dominance or Codominance??????
S Step 2: You will be given the 3 phenotypes, Write the
corresponding genotypes.
S 2 alleles (1 from mom, 1 from dad)
S Use the 1st letter for each trait
S Both letters are capitol
S Step 3: Set up and fill-in a punnett square. Calculate
percent probability for ALL 3 TRAITS!!!
Let’s do the
st
1
one together 
S Birds can be blue, white, or white with blue-
tipped feathers
S 1. Incomplete Dominance or Codominance?
S 2. Genotypes:
bluewhite –
blue-tipped –
3. Show a cross between a white and blue-tipped
bird
Practice Identifying
S A cat can be black, tan, or tabby (black & tan)
S 1. Incomplete Dominance or Codominance?
S 2. Genotypes:
blacktan –
tabby –
3. Show a cross between a heterozygous tabby and tan cat
Practice Identifying
S A Who can have curly hair, spiked hair, or
wavy
S 1. Incomplete Dominance or Codominance?
S 2. Genotypes:
curlyspiked –
wavy-
3. Show a cross between a curly and wavy Who
Practice Identifying
S A sneech can be tall, short, or medium
S 1. Incomplete Dominance or Codominance?
S 2. Genotypes:
tallshort –
medium –
3. Show a cross between two heterozygous sneeches
Practice Identifying
S A horse can be black, white, or roan (black &
white)
S 1. Incomplete Dominance or Codominance?
S 2. Genotypes:
black –
white –
roan -
3. Show a cross between a heterozygous roan and black horse
Practice Identifying
S Flowers can be white, pink, or red
S 1. Incomplete Dominance or Codominance?
S 2. Genotypes:
whitepink –
red-
3. Show a cross between a white and red flower