PUNNETT SQUARES Part II
Name __________________
Class 2 3 4
If you understand and can do this activity, you have a good understanding of Punnett
squares!
1. Draw 2 blank Punnett squares below.
A.
B.
2. We will cross a tall pea plant with a purebred short pea plant. We do not know the
genotype of the tall pea plant.
(By the way, what is the genotype of the purebred short pea plant? ___________ .
What are possible genotypes of the tall pea plant? ____________
______________ .)
3. First, assume that the tall plant is purebred. In Punnett square A above, fill in the
genotype of each parent plant if the tall parent was purebred.
4. Now, assume that the tall plant is hybrid. In the other Punnett square B above, write
the genotype of each parent plant as they would appear if the tall parent was
hybrid.
5. Complete the Punnett Squares above correctly.
6. Let’s say we actually crossed these plants, took the resulting peas, and planted them to
see if they are tall or short plants. Here are the results:
Results
Tall
Short
Total
114 pea plants
99 pea plants
213 pea plants
Now, answer the following questions in complete sentences below. You may use an extra
piece of paper if you wish.
A. Are any short offspring shown in the first Punnett square you drew in step 3 above? If
so, what is the ratio of short and tall?
B. Are any short offspring shown in the second Punnett square you drew in step 4 above?
If so, what ratio?
C. Which square best fits the results in the table above?
D. What is the genotype of the tall parent plant? How do you know?
EXTRA CREDIT BRAIN-BUSTER
COLLEGE TEXTBOOK
PROBLEM
The following problem was taken directly from a college biology textbook. If you can
solve it, you will earn extra credit for your science grade and a college diploma from Mr.
Grogan. This could save your parents a lot of money!
Being a “vulcan” is a dominant trait. (“earthling” is recessive.) Being able to curl your
tongue into a U-shape is a dominant trait. (Being unable to curl your tongue is
recessive.) Suppose a woman who is a “vulcan” and can roll her tongue marries a
man who is “earthling” and who cannot roll his tongue. Their first child is an
“earthling” and cannot roll his tongue.
A. What are the genotypes of the mother, father, and the child?
mother ___________________________________________
father ___________________________________________
child
___________________________________________
B. If the same couple has a second child, calculate the following probability:
(Hint: find a way to make a bigger Punnett square, and staple it to this page!)
P(a child that is “vulcan” and a non-tongue-roller) =
Fraction =
_____________
Percentage = _____________