Mendel, First Geneticist

Gene Squares
1-31-14 Do Now
Gene Squares
Key Question: How can you predict the
probability of traits appearing?
4 minutes
Initial Thoughts:
Evidence – Punnett Squares
Step by Step
1. Write the possible alleles donated by each parent
along the top and left side of the table?
*Each T along the top
represents an allele in
the sperm cell
produced by Skye.
Each t on the left
represents an allele in
the egg cell from
Evidence – Punnett Squares
Step by Step
2. Complete each box of the table by combining one
allele from the top and one allele from
the left.
* When you combine
one allele from each
parent into a box, you
are representing a
sperm cell fertilizing
an egg.
Evidence – Punnett Squares
Step by Step
3. Now you can use the Punnett square to
make some conclusions.
*All of the offspring of Skye and Poppy will have
one allele for blue tail color and one allele for
orange tail color. Their genotypes will be Tt. An
organism that has alleles for two different traits
is called heterozygous. Because blue tail color
is dominant over orange, the phenotype of all
offspring is blue tails, as found in the breeding
experiment between Skye and Poppy.
Evidence – Punnett Squares
The cross between the Generation 2 (Tt) critters
Ocean and Lucy is:
Ocean X Lucy
Tt X Tt
T = allele for blue tail color (dominant)
t = allele for orange tail color (recessive)
*Note that while Ocean and Lucy both have blue
tails, they are both heterozygous.
Evidence – Punnett Squares
1. Complete this Punnett square for the cross
between Ocean and Lucy?
a. Place Ocean’s
and Lucy’s alleles on
the dotted lined in
the Punnett square.
b. Complete the
Punnett square by
filling in each box
with the allele
above it and the
allele to its left.
Evidence – Punnett Squares
c. Use either a blue pencil or a regular pencil
to shade in the squares for offspring that
will have blue tails in your Punnett square.
d. About what fraction of the offspring of
Ocean and Lucy are predicted to have blue
tails, according to the Punnett square?
e. About what fraction are predicted to have
orange tails?
Evidence: Punnett Squares
• Paste the punnett square practice
problems into your notebook.
Analysis Questions
1. Why is it impossible for offspring to show
the recessive trait if one parent is
homozygous for the dominant trait?
2. What is the probability of having a boy (xy)
vs. a girl (xx)?
Analysis Questions contd.
3. A scientist has some purple-flowered pea plants.
She wants to find out if the pea plants are
homozygous dominant for the purple flower color.
White is recessive. Use P = purple p = white.
a) What cross will be best to find out if the purpleflowered peas are homozygous?
b) Use Punnett squares to show what will happen if
the plants are crossed with white-flowered
plants and
i. the purple-flowered plants do not have an
allele for the white trait.
ii. the purple-flowered plants do have an allele
for the white trait.
• What did you think about predicting the
probability of traits before?
• What did you learn from your evidence
about predicting the probability of traits?
• What are some further thoughts or
questions you have about predicting the
probability of traits?
• How can you tell if a flower is
homozygous dominant? What
experiment can you do?
Big Idea
• Punnett squares can be used to predict
the probabilities of certain dominant and
recessive traits.

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