Name ______________________________
Mrs. Geithner-Marron (Biology)
Date __________
Period ________
Lab 26-1 How Can the Genes of Offspring Be Predicted? (revised)
Background:
The Punnett square is a “graphic organizer” that can be used to predict the expected
genotypes of offspring from a genetic cross of two parents. In mice, black coat color (B) is
dominant over white coat color (b). If two heterozygous (Bb) parents are crossed, we would
expect three black offspring to every one white offspring. A Punnett square (Figure 1) shows
these expected results of the Bb x Bb cross.
However, observed results, which are the results that actually occur, do not always agree
with the expected results (predicted mathematically using a Punnett square). Four offspring of
the Bb x Bb parents may really be all white. That is, the observed results may all be white. These
results are not what would be expected (based on the Punnett square). If the expected results
are not the same as the observed results, then, what good are expected results? Expected results
help us determine what the observed results may be. The larger the sample size, the closer the
expected and observed results will generally be.
Objectives:
In this lab you will:
 set up a model to compare expected and observed results.
 predict some genes in offspring of mice.
Vocabulary (Define each of the following words.):

expected results __________________________________________________________
_______________________________________________________________________

observed results __________________________________________________________
_______________________________________________________________________

Punnett square ___________________________________________________________
_______________________________________________________________________

homozygous (pure) dominant _________________________________________________
_______________________________________________________________________

homozygous (pure) recessive _________________________________________________
_______________________________________________________________________

heterozygous (hybrid) ______________________________________________________
_______________________________________________________________________
Name ______________________________
Mrs. Geithner-Marron (Biology)
Date __________
Period ________
Materials:

2 coins (or plastic chips)

marker

tape (optional)
Part A: Calculating Expected Results

Assume that a female mouse has several litters of young in one year. She is heterozygous (Bb)
for black coat color and mates with a male that is also heterozygous (Bb) for black coat color.
B
b
B
b
Procedure:
You can predict what kind of offspring she will have by constructing a Punnett square as shown in
figure 1 above to the left. Results from mating two mice can be shown by tossing and reading coins.
1. Obtain the coins that represent the parents above (see figure to the right).
2. Place both coins in your cupped hands. Shake the coins and drop them on the table.
3. “Read” the “gene combinations” for the “offspring” (the sides of the coins that are facing up).
4. In the “Observed Results” row of Table 1, make a tally mark for the “gene combination” that
you observed (in # 3).
5. Repeat steps 2-4 a total of 40 times to represent the combination of genes in several litters.
6. After 40 trials, count up the tally marks for each “gene combination” and write the number in
the “Total observed in 40 tosses” row below the corresponding “gene combination” in Table 1.
7. Calculate the “total expected in 40 tosses” for each gene combination by using the Punnett
square in Figure 1 and the equation in Table 1 and record the answer in Table 1.
8. Record the coat colors for each gene combination on Table 1.
Name ______________________________
Mrs. Geithner-Marron (Biology)
Date __________
Period ________
Part B: Predicting Mouse Offspring

Suppose you mate a female mouse that is heterozygous (Bb) with a male that his homozygous
(pure) recessive (bb). Predict what kind of offspring she will have by completing the Punnett
square shown in Figure 2.
b
b
B
b
Procedure:
1. Complete the Punnett square in Figure 2 above.
2. Obtain the coins that represent the parents above (see figure to the right).
3. Place both coins in your cupped hands. Shake the coins and drop them on the table.
4. “Read” the “gene combinations” for the “offspring” (the sides of the coins that are facing up).
5. In the “Observed Results” row of Table 2, make a tally mark for the “gene combination” that
you observed (in # 3).
6. Repeat steps 2-4 a total of 40 times to represent the combination of genes in several litters.
7. After 40 trials, count up the tally marks for each “gene combination” and write the number in
the “Total observed in 40 tosses” row below the corresponding “gene combination” in Table 2.
8. Calculate the “total expected in 40 tosses” for each gene combination by using the Punnett
square in Figure 2 and the equation in Table 2 and record the answer in Table 2.
9. Record the coat colors for each gene combination on Table 2.
Name ______________________________
Mrs. Geithner-Marron (Biology)
Date __________
Period ________
Table 1. Results of Coin Tosses (Coat color in offspring of Bb x Bb parents.)
Gene Combinations
BB
Bb
bb
( 4 ) x 40 =
( 4 ) x 40 =
( 4 ) x 40 =
Observed results
(tally as toss coins)
Total observed results
(count tally marks)
Total expected
results in 40 tosses
(based on Punnett
square in figure 1)
Coat color
Table 2. Results of Coin Tosses (Coat color in offspring of Bb x bb parents.)
Gene Combinations
Bb
bb
Observed results (tally as toss
coins)
Total observed results (count
tally marks)
Total expected results in 40
tosses
(based on Punnett square in
figure 2)
Coat color
( 4 ) x 40 =
( 4 ) x 40 =
Name ______________________________
Mrs. Geithner-Marron (Biology)
Date __________
Period ________
Analysis Questions
1. When tossing a coin, how often do you expect it to land on heads? _____________________
2. When tossing a coin, how often do you expect it to land on tails? ______________________
3. When tossing TWO coins, how often do you expect to get a heads/heads combination? (Use a
Punnett Square or tree diagram to help you figure out the combinations.) ________________
4. When tossing TWO coins, how often do you expect to get a heads/tails combination? (Use a
Punnett Square or tree diagram to help you figure out the combinations.) ________________
5. When tossing TWO coins, how often do you expect to get a tails/tails combination? (Use a
Punnett Square or tree diagram to help you figure out the combinations.) ________________
6. In general, how close were your observed results and your expected results? Circle one.
_____(exactly the same / very close / very far apart)_____
7. What would be the expected results if the parents were BB and bb? Use the Punnett square
below to show the cross.
Name ______________________________
Mrs. Geithner-Marron (Biology)
Date __________
Period ________
8. Suppose you mated two more mice and expected to get 1 black : 1 white mouse.
a. How many mice would be black if there were 100 offspring? ____________________
b. How many mice would be white if there were 100 offspring? ____________________
9. A scientist made a cross between two black mice but did not know their genotypes. The cross
was repeated several times and all of the offspring were black.
a. What are the two possible genotypes for a mouse with black fur? _______ & _______
b. What are the three possible parent crosses you could have based on the parental
genotypes in #9a.
i. _______ x _______
ii. _______ x _______
iii. _______ x _______
c. Use the three Punnett squares below to show each of the crosses from #9b.
d. CIRCLE the TWO Punnett squares that would result in all black offspring and CROSS
OUT the Punnett square that shows white offspring.