Complex Patterns of
Inheritance
I. Incomplete Dominance
• An individual displays a trait that is
intermediate between the two parents
• Example: a red snapdragon
crossed with a white
snapdragon produces
pink offspring
Red = RR, White = WW, Pink = RW
Make a Punnett square for a red
plant crossed with a white plant
R
W
W
R
All offspring are pink, with
RW genotype
W
W
R
R
RW
RW
RW
RW
Show the cross between two
pink snapdragons
Offspring are 1 red, 2 pink, and
1 white
R
W
R
W
RR
RW
RW
WW
Incomplete
dominance
in horse
coat color
II. Codominance
• Two dominant alleles are expressed at the
same time
• Roan coat color in horses or cows is an
example
• A roan horse has both red and white hairs
• Roan 
has red
and white
hairs
Parents
 
Codominance
in chicken
feather color.
Black and
White feathers

III. Multiple Alleles
• Traits controlled by more than 2 alleles.
• Examples are pigeon feather colors
and fur color in many animals.
Coat color in rabbits
Phenotype
Allele
Pattern of
Inheritance
Dark gray coat
C
Chinchilla
(silver-gray)
cch
ch
Dominant to all
other alleles
Dominant to
Himalayan and
white
Dominant to
white
c
recessive
Himalayan
(white with dark
ears)
White
1. List all possible genotypes for a:
a) dark gray-coated rabbit
CC, Ccch, Cch, Cc
b) chinchilla rabbit
cchcch, cchch, cchc
c) Himalayan rabbit
c hc h, c h c
d) white rabbit
cc
2. Predict the phenotype for a rabbit with a
chcch genotype
chinchilla
Predict the phenotype for a rabbit with a
Cch genotype
dark gray
3. Would it be possible to obtain white
rabbits if one parent is white and the
other is chinchilla?
Yes, if the chinchilla parent has a white
allele
4. Would it be possible to obtain chinchilla
rabbits if one parent is Himalayan and
the other is white?
No, because chinchilla is dominant to
both Himalayan and white, so the allele
couldn’t be “hiding”.
5. A chinchilla rabbit is mated with a
Himalayan. Some offspring are white.
What are the parents’ genotypes?
Chinchilla is cchc and Himalayan is chc .
IV. Sex-linked traits
• A sex-linked trait is a trait whose allele is
located on either the X or Y chromosome.
A) Hairy Ears are caused by a gene on
the Y Chromosome
B) Most sex-linked traits on X chromosome
are recessive
• Examples include: hemophilia, red-green
colorblindness, and a form of muscular
dystrophy
People with red-green color
blindness see the number 35, while
people with normal vision see the
number 57
Hemophilia
• A disorder in which the blood does not clot
properly
• Sex-linked recessive (on the X chromosome)
• Males receive the hemophilia gene from their
mothers
Extensive
bruising of
the left
forearm and
hand in a
patient with
hemophilia.
Genotypes for females
XHXH = normal blood clotting, non-carrier
XHXh = normal blood clotting, carrier of gene
XhXh = female with hemophilia
Queen Victoria of the
United Kingdom was a
carrier of hemophilia
Genotypes for Males
• XHY = normal blood clotting
• XhY = male with hemophilia
• Tsarevich Alexei of Russia
had hemophilia
What is Alix’s genotype?
1. Cross a carrier mother with a
normal father.
XH
XH
Y
Xh
What percent of sons have hemophilia?
What percent of daughters have
hemophilia?
XH
Xh
XH
XHXH
XHXh
Y
XHY
XhY
50% of sons have hemophilia, 0%
of daughters have hemophilia.
XH
Xh
XH
XHXH
XHXh
Y
XHY
XhY
2. Cross a hemophiliac father with
a normal (non-carrier) mother.
XH
Xh
Y
XH
What percent of sons have hemophilia?
What percent of daughters are carriers?
XH
XH
Xh
XHXh
XHXh
Y
XHY
XHY
0% percent of sons have hemophilia.
100% percent of daughters are carriers.
XH
XH
Xh
XHXh
XHXh
Y
XHY
XHY
3. Cross a carrier mother with a
hemophiliac father.
XH
Xh
Y
Xh
What percent of sons have hemophilia?
What percent of daughters have hemophilia?
XH
Xh
Xh
XHXh
XhXh
Y
XHY
XhY
50% percent of sons have hemophilia.
50% percent of daughters have hemophilia.
XH
Xh
Xh
XHXh
XhXh
Y
XHY
XhY
4. Cross a hemophiliac mother
with a normal father
• What percent of sons have hemophilia?
• What percent of daughters have
hemophilia?
100% percent of sons have hemophilia.
0% percent of daughters have hemophilia.
Xh
Xh
XH
XHXh
XHXh
Y
XhY
XhY
V. Polygenic traits
• Traits that are influenced by 2 or more
genes
• Examples include human height, weight,
hair color, eye color, and skin color
Coat color in Labrador retrievers
Coat color in Labrador retrievers
Is it possible for two
chocolate lab
parents to have
black lab offspring?
VI. Traits influenced by the
environment.
• Color of hydrangea flowers (blue in acidic
soil, pink in basic soil)
• Color of arctic fox
• Human height, skin color, behavior