Incomplete Dominance
Codominance
Multiple Alleles
Review: Dominant/Recessive
• One allele is dominant over the other
(capable of masking the recessive allele)
– This is called complete dominance.
*All the problems done in class thus far have
been with complete dominance.*
PP = purple
pp = white
Pp = purple
Review Problem:
Dominant/Recessive
• In pea plants, purple flowers (P) are dominant
over white flowers (p) show the monohybrid
cross between two heterozygous plants.
GENOTYPES:
PHENOTYPES:
Incomplete Dominance
• A third (new) phenotype appears in the
heterozygous condition.
• Instead of dominant and recessive,
there is more of a “blending” of alleles
• Ex: Flower Color in 4 O’clocks
RR = red
rr = white
Rr = pink
Problem: Incomplete
Dominance
• Show the cross between a pink and a
white flower.
GENOTYPES:
R
r
PHENOTYPES:
r
r
Problem: Incomplete Dominance
Hair Texture in humans is a result of
incomplete dominance. What would be
the outcome of crossing a person who
is homozygous for straight hair with a
person who is homozygous for curly
hair?
Codominance
• The heterozygous condition, both
alleles are expressed equally
• No recessive condition
• Ex: Sickle Cell Anemia in Humans
NN =
normal cells
SS = sickle cells NS = some of
each
Problem: Codominance
• Show the cross between an individual with
sickle-cell anemia and another who is a
carrier but not sick.
N
S
GENOTYPES:
S
PHENOTYPES:
S
Problem: Codominance
The roan pattern in a cow’s coat is a result of
codominance. Using the square below, what would
be the outcome of crossing a roan bull (Rr) with a
white cow (rr)?
Multiple Alleles
• There are more than two alleles for
a single trait
• Ex: Blood type in humans
• Blood Types?
– Type A, Type B, Type AB, Type O
• Blood Alleles?
– A, B, O (in book – IA, IB, I)
Rules for Blood Type
• A and B are codominant
– AA = Type A
– BB = Type B
– AB = Type AB
• A and B are dominant over O
– AO = type A
– BO = type B
– OO = type O
Problem: Multiple Alleles
• Show the cross between a mother who has
type O blood and a father who has type AB
blood.
O
O
GENOTYPES:
A
PHENOTYPES:
B
Problem: Multiple Alleles
• Show the cross between a mother who is
heterozygous for type B blood and a father who is
heterozygous for type A blood.
GENOTYPES:
PHENOTYPES:
ORIGINAL SLIDES CREDITED TO: PALATINE
HIGH SCHOOL