Punnett Square Crosses

26 Jun Punnett Square Crosses

Posted at 04:08h in College by Blogger

Question
Experiment 1: Punnett Square Crosses
In this experiment you will use monohybrid and dihybrid crosses to predict patterns of
inheritance.

Materials
Blue Beads
Green Beads
Red Beads

Yellow Beads
(2) 100 mL Beakers
Permanent Marker

Procedure:
Part 1: Punnett Squares
A. Set up and complete Punnett squares for each of the following crosses: (remember
Y = yellow, and y = blue)
Y Y and Y y

Y Y and y y

1. What are the resulting phenotypes?
2. Are there any blue kernels? How can you tell?
B. Set up and complete a Punnett squares for a cross of two of the F1 from Step 1
(above).
1. What are the genotypes of the F2 generation?
2. What are their phenotypes?
3. Are there more or less blue kernels than in the F1 generation?
C. Identify the four possible gametes produced by the following individuals:
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a) YY Ss:
______
______
______
______
b) Yy Ss:
______
______
______
______
c) Create a Punnett square using these gametes as P and determine the genotypes
of the F1:

1. What are the phenotypes? What is the ratio of those phenotypes?
Part 2 and 3 Setup
1. Use the permanent marker to label the two 100 mL beakers as “1” and “2”.
2. Pour 50 of the blue beads and 50 of the yellow beads into Beaker 1. Sift or stir
the beads around to create a homogenous mixture.
3. Pour 50 of the red beads and 50 of the green beads into Beaker 2. Sift or stir the
beads around to create a homogenous mixture.
Assumptions for the remainder of the experiment:

Beaker 1 contains beads that are either yellow or blue.
Beaker 2 contains beads that are either green or red.

Both beakers contain approximately the same number of each colored bead.

These colors correspond to the following traits (remember that Y/y is for
kernel color and S/s is for smooth/wrinkled):
1. Yellow (Y) vs. Blue (y)
2. Green (G) vs. Red (g).

Part 2: Monohybrid Cross
1. Randomly (without looking) take two beads out of Beaker 1. This is the genotype of
Individual #1. Record the genotype in Table 1. Do not put these beads back into the
beaker.

Table 1: Parent Genotypes: Monohybrid Crosses
Generation

Genotype of Individual 1 Genotype of Individual 2

P
P1
P2
P3
P4
2. Repeat Step 1 for Individual #2. These two genotypes represent the parents
(generation P) for the next generation.
3. Set up a Punnett square and determine the genotypes and phenotypes for this
cross. Record your data in Table 2
4. Repeat Step 3 four more times (for a total of five subsequent generations). Return the
beads to their respective beakers when finished.

Table 2: Generation Data Produced by Monohybrid Crosses
Parents

Possible
Offspring
Genotypes

Possible
Offspring
Phenotypes

Genotype Ratio Phenotype Ratio

P
P1
P2
P3
P4

Part 3: Dihybrid Cross
1. Randomly (without looking) remove two beads from Beaker 1 and two beads
from Beaker 2. These four beads represent the genotype of Individual #1. Record
this information in Table 3
2. Repeat Step 1 to obtain the genotype of Individual #2. Record the phenotypes of both
individuals in Table 3.

Table 3: Parent Genotypes: Dihybrid Crosses
Generation

Genotype of Individual 1 Genotype of Individual 2

P
P1
P2
P3
P4
3. Determine what the possible genotypes might be if each individual produced
gametes. Record these possible genotypes in Table 4.
4. Predict the ratio of possible genotypes which could be produced by a cross
between Individual #1 and Individual #2? Record your predictions in Table 4
Hint: Think back to the example the dihybrid cross in the Introduction. Record
your prediction in Table 2.
5. Set up a Punnett square and determine the genotypes and phenotypes for this
cross.
6. Repeat Step 5 four additional times (for a total of five subsequent generations).

Table 4: Generation Data Produced by Dihybrid Crosses
Parents

Possible
Offspring
Genotypes

Possible
Offspring
Phenotypes

Genotype Ratio Phenotype Ratio

P
P1
P2

P3
P4
Post-Lab Questions
Part 2: Monohybrid Cross
1. How much genotypic variation do you find in the randomly picked parents of
your crosses?
2. How much in the offspring?
3. How much phenotypic variation?
4. Is the ratio of observed phenotypes the same as the ratio of predicted
phenotypes? Why or why not?
5. Pool all of the offspring from your five replicates. How much phenotypic variation
do you find?
6. What is the difference between genes and alleles?
7. Organisms heterozygous for a recessive trait are often called carriers of that trait.

What does that mean?

8. In peas, green pods (G) are dominant over yellow pods. If a homozygous

dominant plant is crossed with a homozygous recessive plant, what will be the
phenotype of the F1 generation? If two plants from the F1 generation are crossed,
what will the phenotype of their offspring be?

1. How similar are the observed phenotypes in each replicate?
2. How similar are they if you pool your data from each of the five replicates?
3. Is it closer or further from your prediction when you pool the data, compared to
each replicate?
4. Did the results from the monohybrid or dihybrid cross most closely match your
predicted ratio of phenotypes?
5. In certain bacteria, an oval shape (O) is dominant over round (o) and thick cell
walls (T) are dominant over thin (t). Show a cross between a heterozygous oval,
thick cell walled bacteria with a round, thin cell walled bacteria. What are the
phenotype of the F1 and F2 offspring?
Feedback on this lab:
1. Why did you choose to do this assignment? (e.g., needed extra credit, liked topic,
looked fun, always do all the assignments, etc)
2. What is your opinion of the clarity of the lab instructions?
3. How difficult (in terms of content) did you find the lab to do?
4. What is your opinion of the educational value of the lab?
5. Should it be added as a regular assignment to BIO102 (instead of an extra credit
assignment)?
Thanks for your input!

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