Using the Punnett Square and Calculating Probability
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Introduction
Ever wonder how your sister came out with blue eyes, when both of your parents
have brown eyes? Was it the mailman? No, it was genetics! Today you will learn
how to use a Punnett Square to figure out the probability of certain
genotypes, and therefore, how likely someone is to have a certain trait.
Some Ideas Refreshed:
 Genotype: combination of alleles in your DNA (i.e. GG)
 Phenotype: the trait that shows up based upon the genotype (i.e.
green peas)
For each trait you have 2 alleles, one comes from each parent:
 homozygous: two of the same alleles.
 heterozygous: two different alleles.
The dominant allele BLOCKS the recessive allele (for ALL cases, unless labeled
"Mated dominance")
Part I: First, lets practice some vocabulary.
Practice using these terms. For all questions, use these facts: the trait is
fur color (f). Black fur is dominant over white fur.
 Write the letter of the dominant allele. _________
 Write the letter of the recessive allele. _________
 Write the genotype for white fur (2 alleles!). _________
 Write the genotype for black fur (2alleles!). _________ or _________
 Write out the homozygous dominant genotype. _________
 Write out the heterozygous genotype. _________
 Write out the homozygous recessive genotype. _________
 Write the phenotype for #5. _________
 Write the phenotype for #6. _________
 Write the phenotype for #7. _________
Part II: How to make a Punnett Square. Here
is an example:
(Black fur is dominant over white)
Stinky
Lucy

When Stinky and Lucy mate, they will each give one allele from their
gene for fur color. SO, on to the question.
Problem: Mate a heterozygous mouse (mom, Lucy) with a
homozygous recessive (dad, Stinky) mouse.
How will the kids come out? Follow the steps below to find out.

 Make a tictactoe board.
 X out the top left corner.
 Write the mom’s alleles across the top.
 Write the dad’s alleles down the side.
 Match the column & rows for each box.
 Write the capital letter first.


 List probabilities of different genotypes
· 2 Ff = 1 Ff
4 total 2
· 2 ff = 1 ff
4 total 2

 List probabilities of different phenotypes:
˝ Black Fur ˝ White Fur

Here are some problems for practice: (still for mouse fur color)

Part III: Now its you’re turn to make a Punnett
Square.
 If the mother is homozygous recessive and the father is homozygous
dominant.
 Write the genotype probabilities.
 Write the phenotype probabilities.
 If the mother is heterozygous, and the father is heterozygous.
Write the genotype probabilities.
 Write the phenotype probabilities.
 If the mother is heterozygous, and the father is homozygous dominant.
 Write the genotype probabilities.
 Write the phenotype probabilities.
 If the mother is homozygous recessive, and the father is heterozygous.
 Write the genotype probabilities.
 Write the phenotype probabilities.
Part IV: Lets do some more Punnett Square
practice using flower traits.
If you are experimenting with orchids, where red flowers are dominant
over white flowers. (Use the letter "R"). Make the following crosses:
 Cross red homozygous orchids with white homozygous orchids.
 Write the genotype probabilities.
 Write the phenotype probabilities.
 Cross red heterozygous orchids with homozygous orchids.
 Write the genotype probabilities.
 Write the phenotype probabilities.
 Cross homozygous recessive orchids with red heterozygous orchids.
 Write the genotype probabilities.
 Write the phenotype probabilities.
 Cross red homozygous orchids with red heterozygous orchids.
 Write the genotype probabilities.
 Write the phenotype probabilities.
Part V: Now for more practice using traits from
people.
Now look lets look at people, using eye color. Brown eyes are dominant over
blue. (Use the letter "B"). Make the following crosses:
 Mate two homozygous dominant parents together.
 Write the genotype probabilities.
 Write the phenotype probabilities.
 Mate two heterozygous parents together.
 Write the genotype probabilities.
 Write the phenotype probabilities.
 Mate a homozygous dominant mom with a heterozygous dad together.
 Write the genotype probabilities.
 Write the phenotype probabilities.
 If Kevin has blue eyes, and both his parents have brown eyes, what must
their genotypes be? (Look at Part III #13 for help)
 Write the genotype probabilities.
 Write the phenotype probabilities.
Part VI: Are you now ready for a challenge? Pick
a trait to practice Punnett squares, such as a bird's wing color. Decide upon
the dominant and recessive allele. Then solve the following four Punnett squares.
 Cross homozygous dominant orchids with homozygous recessive
orchids.
 Write the genotype probabilities.
 Write the phenotype probabilities.
 Cross heterozygous orchids with homozygous recessive orchids.
 Write the genotype probabilities.
 Write the phenotype probabilities.
 Cross homozygous recessive orchids with heterozygous orchids.
 Write the genotype probabilities.
 Write the phenotype probabilities.
 Cross homozygous dominant orchids with heterozygous orchids.
 Write the genotype probabilities.
 Write the phenotype probabilities.
Part VII: Are you now ready for a challenge?
"Sexlinked traits" come from genes that are located on the X chromosome, and
not on the y. (Remembers women are XX and men are Xy).
For example, baldness is a recessive sexlinked trait. Now you write the allele
letter as a part of the X chromosome, like this:
X^{B} or X^{b};
y stays blank.
The players: B is not bald, b is bald.
Genotype:

X^{B}X^{B}

X^{B}y

X^{B}X^{b}

X^{b}y

X^{b}X^{b}

Phenotype:

Not bald female

Not bald male

Not bald "carrier" female

Bald male

Bald female

Example:
˝ X^{B}X^{b} — Not bald carrier
female.
˝ X^{B}y — Not bald male.
 Cross (mate) a bald male to a carrier female.
 Write the genotype probabilities.
 Write the phenotype probabilities.
 Cross a not bald male to a carrier female.
 Write the genotype probabilities.
 Write the phenotype probabilities.
 If you’re a guy, which member of your family should you look at
to tell if you are going to go bald? Why? (Use a Punnett Square in your answer).