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Incomplete Dominance
Incomplete Dominance: The heterozygous
genotype produces a phenoytype that falls in
between the dominant trait and the recessive trait.
Example:
In flowers, petal color demonstrates incomplete dominance.
Red results when a flower has homozygous dominant alleles
for the trait. White results when a flower has homozygous
recessive alleles for the trait. A flower that is heterozygous
for this trait will be pink. Predict the genotypic and
phenotypic ratios of the offspring for the following:
1. Red and white
R
RR x rr
R
Genotypic Ratio:
100% Rr
r
Rr
Rr
Phenotypic Ratio:
r
Rr
Rr
100% pink flowers
Example:
In flowers, petal color demonstrates incomplete dominance.
Red results when a flower has homozygous dominant alleles
for the trait. White results when a flower has homozygous
recessive alleles for the trait. A flower that is heterozygous
for this trait will be pink. Predict the genotypic and
phenotypic ratios of the offspring for the following:
2. Red and pink
R
r
RR x Rr
Genotypic Ratio:
50% RR
50% Rr
R
R
RR
RR
Phenotypic Ratio:
Rr
50% Red flowers,
50% Pink flowers
Rr
White and Pink
rr x Rr
Genotypic Ratio:
r
r
R
Rr
r
Pink and Pink
Rr
50% Rr
50% rr
Phenotypic Ratio:
50% Pink flowers
50% White flowers
rr
rr
Rr x Rr
Genotypic Ratio:
R
r
RR
Rr
R
r
Rr
rr
25% RR
50% Rr
25% rr
Phenotypic Ratio:
25% Red flowers
50% Pink flowers
25% White flowers
Codominance
Codominance: condition in which both alleles of
a gene are expressed in heterozygous offspring.
Example 1:
In cows black coat color and white coat color are
codominant. Heterozygous offspring will be spotted.
1. Black cow and White cow
B
BB x WW
B
Genotypic Ratio:
W
100% BW
BW
BW
Phenotypic Ratio:
W
BW
BW
100% Spotted cow
Example 1:
In cows black coat color and white coat color are
codominant. Heterozygous offspring will be spotted.
2. Black cow and Spotted cow
B
B
W
BB
B
BB
BB x BW
Genotypic Ratio:
50% BB
50% BW
Phenotypic Ratio:
BW
BW
50% Black cow
50% Spotted cow
Example 1:
In cows black coat color and white coat color are
codominant. Heterozygous offspring will be spotted.
3. Both Spotted cows
BW x BW
Genotypic Ratio:
B
B
BB
W
25 % BB
50% BW
25% WW
BW
Phenotypic Ratio:
W
BW
WW
25% Black cow
50% Spotted cow
25% White cow
Example 2:
In blood typing, the gene for type A and the gene for type B
are codominant. The gene for type O is recessive.
Blood Type (phenotype)
Genotype
A
AA or AO
B
BB or BO
AB
AB
O
OO
Determine the possible blood types of the offspring when:
1. Mother is type O, Father is type A (homozygous)
O
O
Genotypic Ratio:
A
AO
AO
100% AO
Phenotypic Ratio:
A
AO
AO
100% Type A Blood
Determine the possible blood types of the offspring when:
2. Mother is type AB, Father is Type A (heterozygous)
A
A
B
Genotypic Ratio:
25 % AA
25% AB
AA
25% AO
25% BO
AB
Phenotypic Ratio:
O
AO
BO
50% Type A Blood
25% Type AB Blood
25% Type B Blood
Determine the possible blood types of the offspring when:
3. Mother is type A (heterozygous), Father is type B (heterozygous)
A
B
O
Genotypic Ratio:
25 % OO
25% AB
AB
25% AO
25% BO
BO
Phenotypic Ratio:
O
AO
OO
25% Type O Blood
25% Type A Blood
25% Type AB Blood
25% Type B Blood
Determine the possible blood types of the offspring when:
4. Mother is type B (homozygous), Father is type A (homozygous)
B
B
Genotypic Ratio:
A
AB
AB
100% AB
Phenotypic Ratio:
A
AB
AB
100% Type AB Blood