Incomplete Dominance &
Codominance
Complete Dominance
• Ameoba Sisters Video
• Recall that in Mendel’s pea plant crosses, one
allele was completely dominant over another,
a relationship called complete dominance.
– Heterozygous and dominant homozygous plants
have the same phenotype.
– Example:
PP
Pp
Incomplete Dominance
• Occurs when two or more alleles influence
the phenotype, resulting in a phenotype
intermediate between the dominant and
recessive traits.
– Example: Four o’clock flowers
Codominance
• Occurs when both alleles for a gene are
expressed in a heterozygous offspring.
• Neither allele is dominant or recessive, nor do
the alleles blend in the phenotype.
– Example: Horse coat color R = red; W = white
Red = RR
Roan = RW
White = WW
Roan = Coat consists of both white and red hairs.
Example
•
Snapdragons are incompletely dominant for
color; they have phenotypes red, pink, or white.
The red flowers
are homozygous dominant,
the white flowers are homozygous recessive, and
the
pink flowers are heterozygous.
Give
the genotypes for each of the phenotypes, using
the letters
“R” and “ r ” for alleles:
• a. Red snapdragon _______
• b. Pink snapdragon _______
• c. White snapdragon _______
• a. pink x pink
• _________ Genotypic ratio
• _________ Phenotypic ratio
• Red x White
__________ genotypic ratio
__________ phenotypic ratio
Blood Type
• Blood type is controlled by
multiple alleles.
– 3 alleles = A, B, and O
– The alleles A and B are
codominant when expressed
together.
– O is recessive to A and B.
• Two of these alleles form an
individuals genotype (see
chart).
Blood Type
Genotype
Type A
AA, AO
Type B
BB, BO
Type AB
AB
Type O
OO