Test Crosses

Mono and Di hybrid crosses with test cross

1. Test Cross
Tall (dominant phenotype) plants of the F1
generation will have one of the following
genotypes:
DD or Dd

2. Test Cross
By crossing to a homozygous recessive individual,
the genotype of an unknown individual will be
revealed in the offspring.

3. Test Cross
If the tall plant of unknown genotype is heterozygous
and is crossed to a homozygous recessive plant:
D d
d
d
Ratio=1:1
tall:dwarf
Dd
Dd
dd
dd

4. Test Cross
D D
d
d
If the tall plant of unknown genotype is homozygous
and is crossed to a homozygous recessive plant:
Dd Dd
Dd Dd
Resulting ratio=
All Tall

5. 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 F
1generation of all
yellow, round seeds.

6. Dihybrid Cross: First Cross
P
1
P
1
F
1

7. Dihybrid Cross: First Cross
What does the phenotype of the F
1generation tell
you?
Yellow is dominant to green
Round is dominant to wrinkled
What happens when the F
2is created?

8. Dihybrid Cross: Second Cross
F
1 F
1
9/16 yellow, round 3/16 green, round
3/16 yellow, wrinkled 1/16 green, wrinkled

9. Dihybrid Cross
This dihybrid cross is best considered as two
independentmonohybrid 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.

10. Punnett Square
Genotype for color:
Yellow = G
Green = g
Genotype for shape:
Round = W
Wrinkled = w

11. Punnett Square of Dihybrid Cross: F
2
GW Gw gW gw
GW
Gw
gW
gw

12. Punnett Square of Dihybrid Cross: F
2
GW Gw gW gw
GW
Gw
gW
gw
GGWWGGWwGgWWGgWw

13. Punnett Square of Dihybrid Cross: F
2
GW Gw gW gw
GW
Gw
gW
gw
GGWWGGWwGgWWGgWw
GGWwGGwwGgWwGgww
GgWWGgWwggWWggWw

14. Punnett Square of Dihybrid Cross: F
2
GW Gw gW gw
GW
Gw
gW
gw
GGWWGGWwGgWWGgWw
GGWwGGwwGgWwGgww
GgWWGgWwggWWggWw
GgWwGgwwggWwggww

15. GW Gw gW gw
GW
Gw
gW
gw
GGWWGGWwGgWWGgWw
GGWwGGwwGgWwGgww
GgWWGgWwggWWggWw
GgWwGgwwggWwggww
9 yellow, round
3 yellow, wrinkled
3 green, round
1 green, wrinkled

16. Independent Assortment
Two monohybrid crosses:
Yellow x green yields all yellow F
1
Round x wrinkled yields all round F
1
F
2of yellow F
1yields ¾ yellow, ¼ green
F
2of round F
1yields ¾ round, ¼ wrinkled

17. 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

18. 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.

19. 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