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Chapter 6: Introduction to Genetics
Investigating Probability
BACKGROUND
Gregor Mendel used mathematical analysis or garden pea plants to develop the laws of
genetics. In this investigation, you will mimic his experiments by studying traits carried on
human chromosomes. Recall that each sperm cell and egg cell carried only one genes for any
trait, and when these cells fuse during fertilization, the result is a fertilized egg with two genes
for each trait, one from each parent.
Because the separation of each homologous chromosome pair during meiosis is a random
event, the particular allele that a gamete carries for any single trait cannot be predicted.
Therefore, the chromosome pair that an offspring receives for that trait cannot be predicted.
Using known genetic information about an offspring’s parents, however, the principles of
probability can be applied to determine the offspring’s probable genetic inheritance. The
simplest and most reliable calculations can be made if the parental genotypes are known. In a
monohybrid cross, only a single pair of genes is involved. A dihybrid cross involves two traits,
each determined by pairs of genes located on different pairs of chromosomes.
In this investigation, you will examine chance genetic events to determine the probability
of specific outcomes.
PROBLEM
How can you determine the probability of a genetic outcome? (FORMULATE A
HYPOTHESIS TO ANSWER THIS QUESTION AND INCLUDE IN THE “PROBLEM”
SECTION OF YOUR LAB REPORT)
MATERIALS
Bag containing 50 large red beans and 50 large white beans
Bag containing 50 small red beans and 50 small black beans
2 glass containers
PROCEDURE
Part A: Monohybrid Cross
1. The possible phenotypes for eye shape are almond-shaped and round. Almondshaped eyes are dominant over round eyes. Use the small beans to represent the two
alleles for the gene for eye shape. The red beans represent alleles for almond-shaped
eyes – A – and the black beans represent alleles for round eyes – a. Pour the bag of
beans into the glass dish. Be very careful not to lose any beans!
2. Without looking, have one partner randomly select pairs of beans from the glass dish.
If two red beans are chosen, place a check in the AA column in Data Table 1. If one
red and one black bean are chosen, place a check in the Aa column. If two black
beans are chosen, place a check in the aa column.
3. Repeat this process for 25 trials for that partner, making sure to return the beans to the
dish after each trial.
4. Have the other partner repeat steps 2 and 3.
5. Record your group results on the board and calculate the class totals.
Data Table 1
Trial Number
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
Total (your group)
Class total
AA
Aa
aa
Part B: Dihybrid Cross
1.
The phenotypes for eye position are straight and slanted upward. Straight eyes
– S – are dominant to slanted upward eyes – s.
2.
Using the bag of large beans to represent the alleles for eye position, let the
large white bean equal S and the large red bean equal s.
3.
Not looking in the dishes, have one partner select two beans from each dish to
represent the results of a dihybrid cross following the traits of eye position and
eye shape (from Part A).
4.
Repeat for 25 trials.
5.
Have the second partner repeat steps 3 and 4.
6.
Record your results in Data Table 2. Once again compare your data with class
data.
Data Table 2
Trial Number
Almond-shaped
and straight eyes
Almond-shaped
and slanted eyes
Round and
straight eyes
Round and
slanted eyes
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
Total (your
group)
Class totals
Analysis Questions: (to be answered in complete sentences – use CLASS DATA)
Part A:
1. What was the genotypic ratio of your selections? The phenotypic ratio?
2. What is the probability that an offspring of this cross would have almond-shaped eyes?
Round eyes?
Part B:
3. What was the phenotypic ratio of your selections?
4. What are the possible genotypes for each of the four phenotypes?
5. What is the probability that an offspring of this cross would have almond-shaped, straight
eyes? Round, upwardly slanted eyes?
Both A and B:
6. Why is it important to conduct a large number of trials?
7. Explain how independent assortment applies to Part B of this investigation.
8. What is your phenotype for eye shape and position?