Incomplete Dominance
Multiple Alleles
In a nutshell!
Review: Dominant/Recessive
• One allele is dominant over the other
(capable of masking the recessive allele)
– Example: Purple flowers are dominant over white
PP = purple
pp = white
Pp = purple
Review Problem:
Dominant/Recessive
• In pea plants, purple flowers (P) are dominant
over white flowers (p) show the cross between
two heterozygous plants.
GENOTYPES:
- PP (1); Pp (2); pp (1)
- ratio 1:2:1
PHENOTYPES:
- purple (3); white (1)
- ratio 3:1
P
p
P
PP
Pp
p
Pp
pp
Review of Monohybrid Crosses
• Remember, monohybrid crosses involve only
ONE trait
• Practice… In fruit flies, red eyes are
dominant over white eyes.
– Cross a white-eyed fly with a homozygous
dominant red-eyed fly.
– Cross two heterozygous red-eyed flies.
Draw a Punnett square for each cross,
and determine the genotypic and phenotypic ratios.
r
r
R
r
R
Rr
Rr
R
RR
Rr
R
Rr
Rr
r
Rr
rr
• Can you determine the genotype
• of a white-eyed fly just by looking at it? Why?
– Yes- it’s rr
– There is only one genotype for white eyes
• Can you determine the genotype
• of a red-eyed fly just by looking at it? Why?
– No- it could be RR or Rr
– There are two genotypes for red eyes
• How could you determine the genotype of the red-eyed
fly?
– Perform a test cross
Review of Monohybrid Crosses
• What two organisms would you cross in the test
cross?
– The unknown red-eyed fly with a white eyed fly
• If the red-eyed fly was RR, what would the
offspring of the test cross look like?
– All of the offspring would have red eyes
• If the red-eyed fly was Rr, what would the
offspring of the test cross look like?
– Half of the offspring would have red eyes, half
would have white eyes
Incomplete Dominance
• A third (new) phenotype appears in the
heterozygous condition.
• 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:
- Rr (2); rr (2); RR (0)
- ratio 2:2:0
R
r
r
Rr
rr
PHENOTYPES:
- pink (2); white (2) Red (0)
r
- ratio 2:2:0
Rr
rr
Multiple Alleles
• There are more than two alleles for
a trait
• Blood type in humans
• Blood Types?
– Type A, Type B, Type AB, Type O
• Blood Alleles?
– A, B, O
Rules for Blood Type
• A and B are codominant
– AA = Type A Blood
– BB = Type B Blood
– AB = Type AB Blood
• A and B are dominant over O
– AO = Type A Blood
– BO = Type B Blood
– OO = Type O Blood
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:
- AO (2) BO (2)
A
AO
AO
PHENOTYPES:
- type A (2); type B (2)
- ratio 2:2:0:0
B
BO
BO
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:
-AB (1); BO (1);
AO (1); OO (1)
- ratio 1:1:1:1:0:0
PHENOTYPES:
-type AB (1); type B (1)
type A (1); type O (1)
- ratio 1:1:1:1
A
O
B
AB
BO
O
AO
OO