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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
Phenotype
• 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
T = Tall
“shortness” is a
recessive trait
t = short
Genotype
• Is the combination of alleles (type of genes).
TT
Tt
tt
2 dominant alleles
(purebred or “true
breeder”)
1 dominant and 1
recessive (hybrid like f1)
2 recessive alleles
(purebred or “true
breeder”)
Homozygous
Dominant
Heterozygous
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 2
Parent 1
1st: Write the parent’s alleles correctly.
Parent 1
T
Parent 2
t
Purebred
short
t
T
Purebred
tall
2nd: Bring the alleles from Parent 1 down.
Parent 1
T
T
T
t
T
T
Parent 2
t
T
3rd: Bring the alleles from Parent 2 over.
Parent 1
T
T
Tt
t
T t
Tt
Parent 2
t
T t
4th: Look at the different possibilities of
outcomes.
Parent 1
T
T
Tt
t
T t
Tt
Parent 2
t
T t
What is the Probability of a tall
plant?
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.
T
T
Heterozygous
t
T t
Heterozygous
Tt
The phenotype for
this genotype is
TALL
t
T t
Heterozygous
Tt
Heterozygous
What is the Probability of a short
plant?
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.
T
Heterozygous
t
T t
Heterozygous
t
T t
T
Heterozygous
Tt
Heterozygous
Tt
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 t
t
T t
t t
4th: Check the probability
T
Homozygous Dominant
T
T T
t
Heterozygous
T t
1 out of 4=25%
Heterozygous
t
T t
Homozygous Recessive
t t
1 out of 4=25%
What is the probability of a
short Phenotype?
T
Homozygous Dominant
T
T T
Tall
Heterozygous
t
t
Heterozygous
T t
Tall
Homozygous Recessive
T t
t t
Tall
Short
1 out of 4=25%
What is the probability of a
Homozygous Recessive
Genotype?
T
Homozygous Dominant
T
T T
Heterozygous
t
T t
t
Heterozygous
T t
Homozygous Recessive
t t
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!