Title: Test Cross
1Test Cross
- Tall (dominant phenotype) plants of the F1
generation will have one of the following
genotypes - DD or Dd
2Test Cross
- By crossing to a homozygous recessive individual,
the genotype of an unknown individual will be
revealed in the offspring.
3Test Cross
If the tall plant of unknown genotype is
heterozygous and is crossed to a homozygous
recessive plant
Ratio11 talldwarf
Dd
dd
dd
Dd
4Test 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
5Dihybrid 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.
6Dihybrid Cross First Cross
P1
P1
F1
7Dihybrid 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?
8Dihybrid Cross Second Cross
9/16 yellow, round
3/16 green, round
1/16 green, wrinkled
3/16 yellow, wrinkled
9Dihybrid 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.
10Punnett Square
- Genotype for color
- Yellow G
- Green g
- Genotype for shape
- Round W
- Wrinkled w
11Punnett Square of Dihybrid Cross F2
gw
gW
Gw
GW
GW
Gw
gW
gw
12Punnett Square of Dihybrid Cross F2
gw
gW
Gw
GW
GgWw
GW
GGWW
GGWw
GgWW
Gw
gW
gw
13Punnett 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
14Punnett 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
159 yellow, round 3 yellow, wrinkled 3 green,
round 1 green, wrinkled
16Independent 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
17Dihybrid 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
18Dihybrid 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.
19Trihybrid 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