Transcript Punnett Squares - Science in the Making

Punnett Squares
Dihybrid Cross
Each parent is heterozygous for TWO traits
 Example: AaBb
 So each parent can give either a dominant
or recessive allele for the first trait (A or
a) and a dominant or recessive allele for
the second trait (B or b)
 This leads to 4 possible gametes (2n) for
each parent

◦ AB, Ab, aB, or ab
Determination of Gametes
Dihybrid Punnett Square
Dihybrid Punnett Square
Dihybrid Punnett Square
Dihybrid Punnett Square
Dihybrid Punnett Square

A cross with two
heterozygous parents
always gives the same
phenotypic ratio
Determining Probability

A cross with two
heterozygous parents
always gives the same
phenotypic ratio
◦ Both dominant: 9/16
Determining Probability

A cross with two
heterozygous parents
always gives the same
phenotypic ratio
◦ Both dominant: 9/16
◦ 1st dom; 2nd rec: 3/16
Determining Probability

A cross with two
heterozygous parents
always gives the same
phenotypic ratio
◦ Both dominant: 9/16
◦ 1st dom; 2nd rec: 3/16
◦ 1st rec; 2nd dom: 3/16
Determining Probability

A cross with two
heterozygous parents
always gives the same
phenotypic ratio
◦
◦
◦
◦
Both dominant: 9/16
1st dom; 2nd rec: 3/16
1st rec; 2nd dom: 3/16
Both recessive: 1/16
Determining Probability


A cross with two
heterozygous parents
always gives the same
phenotypic ratio
9:3:3:1
Determining Probability
Due to independent assortment, there is
another way to determine probability
 The probability of two events occurring
together is the product of their individual
probabilities
 Example: the odds of flipping two heads
in a row on a coin toss is ½ x ½ = ¼

Another Way

If you breed two plants that are both
heterozygous for being tall and yielding
red flowers, what are the chances of
getting an offspring that is tall and white?
◦ Red flowers are dominant to white flowers
◦ Tall is dominant to short
Another Way – An Example

If you breed two plants that are both
heterozygous for being tall and yielding
red flowers, what are the chances of
getting an offspring that is tall and white?
◦ Red flowers are dominant to white flowers
◦ Tall is dominant to short
Parent 1: TtRr
 Parent 2: TtRr

Another Way – An Example
Parent 1: TtRr
 Parent 2: TtRr

Another Way – An Example
Parent 1: TtRr
 Parent 2: TtRr

Another Way – An Example
Parent 1: TtRr
 Parent 2: TtRr

Another Way – An Example
Parent 1: TtRr
 Parent 2: TtRr

Probability of tall: ¾
Another Way – An Example
Parent 1: TtRr
 Parent 2: TtRr

Probability of tall: ¾
Probability of white: ¼
Another Way – An Example
Parent 1: TtRr
 Parent 2: TtRr

Probability of tall: ¾

Probability of white: ¼
Probability of tall AND white: ¾ X ¼ = 3/16 = 18.75%
Another Way – An Example
Parent 1: TtRr
 Parent 2: TtRr

Compare
Parent 1: TtRr
 Parent 2: TtRr

Compare