Download Punnett Squares

Probability and Punnett
Squares
Probability
• Probability-the chance a particular
outcome will occur, usually
expressed in a percent, like there is
a 40% chance of snow today.
–Example- when you flip a coin,
there is a 50% chance of getting
heads and a 50% chance of
getting tails
Mendel and Probability
• Mendel was the first to realize
that he could use probability to
predict the chances a particular
genetic outcome could occur,
like the chances of two plants
producing tall or short offspring.
• When Mendel crossed his
purebred tall plants and his
purebred short plants, his F1
generation were 100% tall
plants.
• He was surprised that when he
crossed the F1 generation, he
got 75% tall plants and 25%
short plants.
Punnett Squares
• A tool to use to predict the
probabilities of possible genotypes
of offspring
• Parent genotypes are put above
the square and on the side
• Fill in letters across and down to
show the possible probabilities of
offspring allele combinations.
Dad Pea Plant: Homozygous dominant (TT)
Mom Pea Plant: Homozygous recessive (tt)
First, fill in Dad’s alleles across top of the square
Then, fill Dad 
in
Mom
Mom’s 
alleles
t
down
the side
of the
square
t
T
T
First, fill the dad’s alleles down
the columns.
T
T
Then, fill
the
t
mom’s
alleles
across
the
t
rows.
T t
T t
T t
T t
*Remember-an organism gets two copies of every
gene, one from their dad and one from their mom.
Also, Tt is the same as tT, but try to get in the habit of
writing the capital letter first.
T
t
t
T t
T t
T
T t
Tt
Genotype:
100% Tt
Phenotype:
100% tall
Results: All 4 of the squares have the
heterozygous combination. So, there is a 100%
chance of having a plant with a heterozygous
genotype. There is also a 100% chance to have
an offspring with the tall phenotype.
Now try to fill in the Punnett Square for
Mendel’s F1 Generation on your handout:
Remember, the genotypes for all the plants
was Tt. So fill in the Punnett Square and
predict the possible outcomes of this cross.
Give % for both genotype and phenotype.
T
t
T
T T
Tt
t
T t
t t
Click to check
your answersWere you right?
Genotypes: 25% TT, 50% Tt,
and 25% tt
Phenotypes: 75% tall, 25% short
Now you are Punnett
Square Experts! Good
luck predicting the
outcomes of genetic
crosses!!!