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
Ratio11 talldwarf
Dd
dd
dd
Dd
4
Test Cross
If the tall plant of unknown genotype is
homozygous and is crossed to a homozygous
recessive plant
Dd
Dd
Resulting ratio All Tall
Dd
Dd
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 F1 generation
  of all yellow, round seeds.

6
Dihybrid Cross First Cross
P1
P1
F1
7
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?

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

10
Punnett Square

• Genotype for color
• Yellow G
• Green g
• Genotype for shape
• Round W
• Wrinkled w

11
Punnett Square of Dihybrid Cross F2
gw
gW
Gw
GW
GW
Gw
gW
gw
12
Punnett Square of Dihybrid Cross F2
gw
gW
Gw
GW
GgWw
GW
GGWW
GGWw
GgWW
Gw
gW
gw
13
Punnett Square of Dihybrid Cross F2
gw
gW
Gw
GW
GgWw
GW
GGWW
GGWw
GgWW
GgWw
Ggww
GGWw
GGww
Gw
GgWW
ggWW
ggWw
GgWw
gW
gw
14
Punnett Square of Dihybrid Cross F2
gw
gW
Gw
GW
GgWw
GW
GGWW
GGWw
GgWW
GgWw
Ggww
GGWw
GGww
Gw
GgWW
ggWW
ggWw
GgWw
gW
ggww
GgWw
ggWw
gw
Ggww
15
9 yellow, round 3 yellow, wrinkled 3 green,
round 1 green, wrinkled
16
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

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