Fork-line Method
A method that uses genotypic ratios to predict the
outcomes of genetic crosses.
Based on the idea that, in a dihybrid, the two
traits sort independently of one another
* What happens with one trait is completely
unrelated to what happens with the other
trait.
Monohybrid Example 1
AA x Aa
Allele options for 1st parent
Genotypic Ratios
½ A
½ AA
½ a
½ Aa
A
Allele options for 2nd parent
Monohybrid Example 2
aa x Aa
Allele options for 1st parent
Genotypic Ratios
½ A
½ Aa
½ a
½ aa
a
Allele options for 2nd parent
Monohybrid Example 3
Aa x Aa
Allele options for 1st parent
Genotypic Ratios
½ A
¼ AA
½ a
¼ Aa
½A
½ A
¼ Aa
½ a
¼ aa
½a
Allele options for 2nd parent
½ Aa
Dihybrid Example 1
AABb x Aabb
First, you need to do a Punnet Square for each trait
in order to figure out your trait options
A
A
A
AA
AA
a
Aa
Aa
B
b
b
Bb
bb
b
Bb
bb
Dihybrid Example 1
AABb x Aabb
1st trait options
Genotypic Ratios
½ Bb
¼ AABb
½ bb
¼ AAbb
½ Bb
¼ AaBb
½ bb
¼ Aabb
½ AA
½ Aa
2nd trait
Dihybrid Example 2
AaBB x AAbb
First, you need to do a Punnet Square for each trait
in order to figure out your trait options
A
a
A
AA
Aa
A
AA
Aa
B
B
b
Bb
Bb
b
Bb
Bb
Dihybrid Example 1
AABb x Aabb
1st trait options
Genotypic Ratios
½ Bb
¼ AABb
½ bb
¼ AAbb
½ Bb
¼ AaBb
½ bb
¼ Aabb
½ AA
½ Aa
2nd trait
Dihybrid Example 3
AaBb x AaBb
First, you need to do a Punnet Square for each trait
in order to figure out your trait options
A
a
A
AA
Aa
a
Aa
aa
B
b
B
BB
Bb
b
Bb
bb
Dihybrid Example 3
AaBB x AAbb
1st trait options
¼ BB
¼ AA
½ Aa
¼ aa
Genotypic Ratios
1/16 AABB
½ Bb
1/8
AABb
¼ bb
1/16
AAbb
¼ BB
1/8
AABB
½ Bb
1/4
AABb
¼ bb
1/8
AAbb
¼ BB
1/16
aaBB
½ Bb
1/8
aaBb
¼ bb
1/16
aabb
2nd trait
Dihybrid Example 4
In rabbits, black fur (B) is dominant to white fur (b) and long hair (L)
dominates short hair (l). What is the chance of getting a black, shorthaired rabbit from a cross between a rabbit that is heterozygous for
both traits and a rabbit that is white and heterozygous for long hair?
B b, L l
x
b b, L l
(first do your punnet squares)
B
b
b
Bb
bb
b
BB
Bb
L
l
L
LL
Ll
l
Ll
ll
Dihybrid Example 4
In rabbits, black fur (B) is dominant to white fur (b) and long hair (L)
dominates short hair (l). What is the chance of getting a black, shorthaired rabbit from a cross between a rabbit that is heterozygous for
both traits and a rabbit that is white and heterozygous for long hair?
B b, L l
½ Bb
½ bb
x
b b, L l
¼ LL
1/8
BbLL
½ Ll
1/4
BbLl
¼ ll
1/8
Bbll * black, short-haired
¼ LL
1/8
bbLL
½ Ll
1/4
bbLl
¼ ll
1/8
bbll