AA X AA
A
A
A
A
AA
AA
AA
AA
Genotypic ratio: all AA
Phenotypic ratio: all red
AA X Aa
A
A
a
A
AA
AA
Aa
Aa
Genotypic ratio: 1AA:1Aa
Phenotypic ratio: all red
AA X aa
A
a
a
A
Aa
Aa
Aa
Aa
Genotypic ratio: all Aa
Phenotypic ratio: all show
dominant trait
Aa X Aa
A
A
a
a
AA
Aa
Aa
aa aa
Genotypic ratio: 1 AA:2Aa:1aa
Phenotypic ratio: 3 not red:1 red
Aa X aa
A
a
a
Aa
Aa
Aa
Aa
Genotypic ratio: 1 Aa:1 aa
a
aa
aa
Phenotypic ratio: 1 red: 1white
aa X aa
a
a
a
a
aa
aa
aa
aa
Genotypic ratio: all aa
Phenotypic ratio: all white
Complete Dominance vs.
Incomplete Dominance
Most traits display complete dominance
the presence of one dominant allele masks the
recessive allele
Some traits display incomplete dominance
the heterozygous condition results in a separate
phenotype, neither allele is completely
dominant-they blend together
In some flowers, AA is red, Aa is pink, and aa is white
Codominance
Some traits are controlled by
codominance:
both alleles for a gene are expressed in
heterozygous offspring.
neither allele is dominant or recessive, nor do
they blend; each is expressed equally.
the best example of this is in a horse’s coat color;
heterozygous = roan color where they have both
white and red hairs.
Dihybrid Cross
dihybrid cross
between individuals with two pairs of traits.
Dihybrid Cross Rules
First figure out what the gametes are that
the parents can make. Use the FOIL
method to do this.
Parents:
AaBb X AaBb
Gametes: AB, Ab, aB, ab X AB, Ab, aB, ab
Then place the gametes along the top and
sides of the square and do the cross.
Dihybrid Cross
Gametes from Parent One (Female)
Gametes from Parent Two
(Male)
AB
Ab
aB
ab
AB
AABB
AABb
AaBB
AaBb
Ab
AABb
AAbb
AaBb
Aabb
aB
AaBB
AaBb
aaBB
aaBb
ab
AaBb
Aabb
aaBb
aabb
Multiple Alleles
 Several alleles exist for a trait
Sex Influenced Traits
Traits expressed differently in males and
females
Example: baldness
male
female
BB
Bb
bb
bald
bald
not bald
bald
not bald
not bald