Determining Probability of Genetic

Outcomes

Featuring the Punnett Square

8th Grade Science – K. Franklin – 8/25/10

Problem: What will the outcome be if a tall plant is crossed with a short plant?

http://www.emc.maricopa.edu/faculty/farabee/biobk/biobookgenintro.html#The%20Monk%20and%20his%20peas

PhenotypePhenotype• Is the physical

appearance of a trait.• The plant on the left

has a Tall phenotype.• The plant on the right

has a Short phenotype.

Alleles or Genes• Are inherited “factors”

(Mendel’s term) that control traits for each characteristic.

• Living things have 2 for each trait. (One from each parent)

• We use letters to represent alleles or genes.

(t)

lower case letters =

recessive traits

(T)

capital letters =

dominant traits

“Tallness” is a dominant

trait

“shortness” is a recessive trait

T = Tall

t = short

Genotype• Is the combination of alleles (type of genes).

TT 2 dominant alleles (purebred or “true

breeder”)

Homozygous Dominant

Tt 1 dominant and 1 recessive (hybrid like f1)

Heterozygous

tt 2 recessive alleles (purebred or “true

breeder”)

Homozygous Recessive

We’ll use a Punnett Square as a graphic organizer to help us figure

out the probability of outcomes.

1st: The Parent’s alleles Go on the outside

Parent 1

Par

ent

2

1st: Write the parent’s alleles correctly.Parent 1

Par

ent

2

T T

t

t

Purebred tall

Purebred short

2nd: Bring the alleles from Parent 1 down.Parent 1

Par

ent

2

T T

t

t

T

T

T

T

3rd: Bring the alleles from Parent 2 over.Parent 1

Par

ent

2

T T

t

t

T

T

T

Tt t

tt

4th: Look at the different possibilities of outcomes.

Parent 1

Pa

ren

t 2

T T

t

t

T

T

T

Tt t

tt

What is the Probability of a tall plant?

T T

t

t

T

T

T

Tt t

tt

1. Look for the

capital T

2. Count the number of squares with a dominant allele.

4 out of 4 possibilities

= a 100% chance of a

tall plant.

The phenotype for this genotype is

TALL

Heterozygous Heterozygous

Heterozygous Heterozygous

What is the Probability of a short plant?

T T

t

t

T

T

T

Tt t

tt

1. Look for a double recessive

(tt)

2. Count the number of squares with double recessive alleles.

0 out of 4 possibilities

= a 0% chance of a short plant.

Heterozygous Heterozygous

Heterozygous Heterozygous

What are the possible outcomes if the hybrids are crossed?

T

T

t

t

Now the first generation will serve as “parents” of the

next generation.

2nd: Bring the alleles from parent 1 down.

T

T

t

t

T

t

t

T

3rd: Bring the alleles from Parent 2 over to the right.

T

T

t

t

T

T

TT

t t

t

t

4th: Check the probability

T

T

t

t

T

T

TT

t t

t

t

Homozygous Dominant

Homozygous Recessive

Heterozygous

Heterozygous

1 out of 4=25%

1 out of 4=25%

2 out o

f 4=50%

What is the probability of a short Phenotype?

T

T

t

t

T

T

TT

t t

t

t

Tall

Homozygous Recessive

Heterozygous

Heterozygous

1 out of 4=25%3 out o

f 4=75%

Tall

Tall

Short

Homozygous Dominant

What is the probability of a Homozygous Recessive

Genotype?T

T

t

t

T

T

TT

t t

t

t

Homozygous Dominant

Homozygous Recessive

Heterozygous

Heterozygous

1 out of 4=25%

More Practice

• Read all about it! Textbook pp. 62-64

• Use one of these websites for additional practice as a class or for enrichment at home:– http://www2.edc.org/weblabs/Punnett/punnettsqua

res.html

– http://www2.edc.org/weblabs/Mendel/MendelMenu.html

Now You Try!

Complete the worksheet that your teacher gave you!