Test Cross
Tall (dominant phenotype) plants of the F1
generation will have one of the following
genotypes:
DD or Dd
Test Cross
By crossing to a homozygous recessive individual,
the genotype of an unknown individual will be
revealed in the offspring.
Test Cross
If the tall plant of unknown genotype is heterozygous
and is crossed to a homozygous recessive plant:
D
d
Dd
d
Dd
d
dd
dd
Ratio=1:1
tall:dwarf
Test Cross
If the tall plant of unknown genotype is homozygous
and is crossed to a homozygous recessive plant:
D
D
d
Dd
Dd
d
Dd
Dd
Resulting ratio=
All Tall
Dihybrid Cross
Dihybrid Cross: A cross involving two pairs of
contrasting traits.
For example, cross true breeding plants with yellow,
round seeds to true breeding plants with green,
wrinkled seeds will yield an F1 generation of all
yellow, round seeds.
Dihybrid Cross: First Cross
P1
P1
F1
Dihybrid Cross: First Cross
What does the phenotype of the F1 generation tell
you?
Yellow is dominant to green
Round is dominant to wrinkled
What happens when the F2 is created?
Dihybrid Cross: Second Cross
F1
F1
9/16 yellow, round
3/16 green, round
3/16 yellow, wrinkled
1/16 green, wrinkled
Dihybrid Cross
This dihybrid cross is best considered as two
independent monohybrid crosses.
Each trait is inherited independently; the
probability of inheriting a particular color is not
influenced by the probability of inheriting a
particular color, and vice versa.
Punnett Square
Genotype for color:
Yellow = G
Green = g
Genotype for shape:
Round = W
Wrinkled = w
Punnett Square of Dihybrid Cross: F2
GW
GW
Gw
gW
gw
Gw
gW
gw
Punnett Square of Dihybrid Cross: F2
GW
Gw
gW
gw
GW GGWW GGWw GgWW GgWw
Gw
gW
gw
Punnett Square of Dihybrid Cross: F2
GW
Gw
gW
gw
GW GGWW GGWw GgWW GgWw
Gw GGWw GGww GgWw Ggww
gW GgWW GgWw ggWW ggWw
gw
Punnett Square of Dihybrid Cross: F2
GW
Gw
gW
gw
GW GGWW GGWw GgWW GgWw
Gw GGWw GGww GgWw Ggww
gW GgWW GgWw ggWW ggWw
gw GgWw Ggww ggWw ggww
9 yellow, round
3 yellow, wrinkled
3 green, round
1 green, wrinkled
GW
Gw
gW
gw
GW GGWW GGWw GgWW GgWw
Gw GGWw GGww GgWw Ggww
gW GgWW GgWw ggWW ggWw
gw GgWw Ggww ggWw ggww
Independent Assortment
Two monohybrid crosses:
Yellow x green yields all yellow F1
Round x wrinkled yields all round F1
F2 of yellow F1 yields ¾ yellow, ¼ green
F2 of round F1 yields ¾ round, ¼ wrinkled
Dihybrid Cross: The Product Law
When two independent events occur simultaneously,
the combined probability of the outcomes is equal
to the product of the individual probabilities of
occurrence.
What is the probability that an F2 plant of a dihybrid
cross will be yellow and round?
p= (3/4)(3/4)=9/16
Dihybrid Cross: The Product Law
What is the probability that a given F2 plant would
be yellow, wrinkled?
p= (3/4)(1/4)=3/16
Remember, this probability calculation rests on the
assumption that the traits are independent,
therefore the probability of all gamete
combinations is equal.
Trihybrid Cross
What if you added back stem height?
Treat it as three monohybrid crosses and use the
probability of each outcome.
DDGGWW x ddggww
You do the math for these outcomes:
Tall, green, round
Short, yellow, wrinkled