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Probability and Using a
Punnett Square
Probability
• The likelihood that a particular event
will occur is called probability.
• If you flip a coin, what is the likelihood
that it will land heads up?
1 in 2
1:1
1/2
Probability
If you flip a coin three times in a row,
what is the probability that it will land
heads up every time (3 times in a row)?
½ x ½ x ½ = 1/8
That the individual probabilities are
multiplied together illustrates the point
that past outcomes do not affect
future outcomes.
Probability
The principles of probability can be used
to predict the outcomes of genetic
crosses.
What is a PUNNETT SQUARE?
• A tool to predict the probability of
certain traits in offspring by showing
the different ways alleles can combine
• A way to predict possible phenotype &
genotype
• A chart that shows all the possible
combinations of alleles that can result
when genes are crossed
Remember:
• Phenotype is a physical characteristic or
trait (i.e. tall pea plant).
• Genotype is genetic makeup (i.e. Tt or
TT).
Organisms that have two identical alleles
for a particular trait are called
homozygous
Organisms that have two different alleles
for a particular trait are called
heterozygous
Remember
• Letters stand for dominant and
recessive alleles
• An uppercase letter stands for a
dominant allele
• Lowercase letters stand for recessive
alleles
Using a PUNNETT SQUARE
To set up a simple Punnett square, draw
a large square, and then divide it into
4 equal sections (also squares). It
should look something like this:
Using a PUNNETT SQUARE
Now you need two parents to mate, ones
with a known or likely genotype.
For example, a red flower with the
genotype Rr and a white flower with
the genotype rr. Rr x rr
Using a PUNNETT SQUARE
Place one of the parents on top, and one
on the left. You should get a something
similar to this:
Using a PUNNETT SQUARE
Finally, take each letter in each column and
combine it with each letter from each
row. Write the combination in the
corresponding square. You should now
have a picture close to this:
Using a PUNNETT SQUARE
• The two-letter combinations are the
possible genotypes of offspring
• They are: Rr, Rr, rr, and rr genotypes
• From this it is possible to determine the
probability (chance) that a flower will
have a red phenotype (2/4 or 50%) or a
white phenotype (2/4 or 50%)
Sample question
1. The house plant Coleus has curves on
the edge of its leaves. Deep curves (D)
are dominant over shallow curves (d).
Cross a homozygous plant with deep
curved leaves with one that has shallow
curved leaves. What are the possible
phenotypes and genotypes of the
offspring?
Questions
1. An F1 plant that is homozygous for
shortness (t) is crossed with a
heterozygous F1 plant. What is the
probability that a seed from the cross
will produce a tall plant?
Use a Punnett square to explain your
answer and to compare the probable
genetic variations in the F2 plants.
Questions continued
2. Straight wings are dominant over curly
wings in fruit flies. How would you
determine whether a straight-winged fly is
heterozygous or homozygous?
3. In guinea pigs, a black coat is dominant
over a white coat. Can two white-coated
parents produce offspring with a black
coat? Why or why not?
4. In certain cattle, the polled (hornless)
variety is dominant (P) over the horns (p).
A polled bull is mated to a cow with horns.
Their calf is horned. The bull is then mated
to another cow and their calf develops
horns. Identify the possible genotypes of
all the individuals.