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Biology 40S
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PROBABILITY AND INHERITANCE – Lab 2: MENDELLIAN INHERITANCE
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Pre-Lab: A Brief Review Mendel’s Postulates
Law of dominance: one form of an allele masks a different allele of the gene when different alleles are
present for the same trait.
Law of segregation: during gamete formation (egg or sperm), the genes for a trait separate, so that each
gamete has only one allele form of the genes for the trait.
Law of independent assortment: as gametes form, the genes for various traits separate independently of
one another.
Mendel studied “factors” that caused traits. Today, we call these factors “genes” and versions of the genes
are called “alleles”.
The Law of Probability
We will be applying the law of probability in our study on heredity. This is a mathematical law that
expresses outcomes of an event as ratios or percent probability. It depends on the number of outcomes.
e.g. If there are two outcomes only, the law of probability states that the probability is ½ or 50%.
OBJECTIVES: In this lab you will
 Use pennies to simulate Mendel’s postulates of trait behaviour
 Apply Mendel’s postulates and the law of probability
 For each part, you will develop a written purpose
 Create an appropriate chart to display data meaningfully for Part I
MATERIALS:
2-pennies
masking tape
pen or pencil and paper
In your lab write up, you will need to compose a purpose for each part of this lab. The purpose should
include an explanation of how the procedure relates to one or more of Mendel’s postulates. Next is the data
(charts, tables, and/or anecdotes), followed by the results questions. Be sure to label charts, tables,
answers for each part, well. This lab handout goes on the FRONT of the lab.
Part I: Data table – 3 mks
Part II: Data table – 3 mks
Part III: Data Collection – 1 mk
Purpose – 2 mks
Purpose – 2 mks
Purpose – 2 mks
Questions – 3 mk
Questions – 5 mk
Questions – 9 mk
Total – 8 mks
Total – 10 mks
Total – 12 mks
TOTAL – 30 mks
PROCEDURES:
Part I. Occurrence of a Single Event
Read the instructions below. You may first wish to record RAW DATA, and then later create a suitable data
chart that records all the data you will be collecting. The data table should include headings for HEADS
TAILS. It should display the data for 20 tosses, 30 tosses and 50 tosses separately (yet together), as well
as a total. This requires planning.
1. Applying the law of probability, if you were to toss a penny 20 times, how many times do you expect
it to land on heads? tails? Record this data as EXPECTED OUTCOME.
2. Do step 1 for 30 tosses and 50 tosses. Record your expected outcomes.
3. Use a single penny and toss it 20 times. Have a partner keep track of the number of times it lands
tails up. Record the number of tails and heads data as OBSERVED OUTCOME.
4. Repeat step 3 for 30 tosses, and again for 50 tosses.
5. Calculate DEVIATION between your EXPECTED outcome and OBSERVED outcomes. To do this,
calculate the absolute value when you subtract the expected from the observed.
6. Total up all tosses (100 tosses) and record the number of times it landed tails up and heads up.
Record the expected outcome for 100 tosses. Calculate the total deviation.
Results Discussion Questions for Part I.
Purpose: (2)
Data Table (3)
#
Tosses
Expected
Outcome
Heads
Tails
Observed
Outcome
Heads
Tails
Deviation
Deviation
% Deviation = ------------ x 10
Expected
Expected – Observed
20
30
50
100
1. Did your observed data results always agree with the expected ratio? If not what would be a reason for
the deviation? (1)
2. Compare the deviations from the expected 20, 30, 50 tosses. What seems to be the relationship
between the sample size and the deviation? If the sample size where larger (class totals), what would
you expect the deviation to approach? (2)
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Part II. Independent Evens Occurring Simultaneously
Using the lab of probability, when there are four equally likely outcomes from a procedure, the probability
that one of the outcomes will occur is ¼ or 25%.
Determine the EXPECTED outcome of 40 tosses of two coins that you will get heads-heads, heads-tails,
tails-head, and tails-tails. Use a table similar to the one suggested below. Give it a suitable title.
1. Toss 2 pennies simultaneously for 40 tosses. Have your partner keep track of how many times you
get heads-heads, heads-tails (tails-head will also occur but it will be counted the same as heads-tails
for data), and tails-tails.
2. Calculate the percent probabilities you OBSERVED by taking your observed number divided by 40
times 100.
3. Calculate the deviation by subtracting the expected from the observed.
Data Table (3)
Combinations
Heads-Heads
Heads-Tails/Tails-Heads
Tails-Tails
TOTAL
Expected
%
Observed
%
Deviation
40
100
40
100
----------------
Results Discussion Questions for Part II.
Purpose: (2)
1. How closely did your results agree with the expected percentage? Account for possible reasons for a
high deviation from expectation. (1)
2. If you were to toss the coins simultaneously 400 times, would you expect the deviation to be greater or
less than it was in tossing them 40 times? Explain why. (2)
3. How do Parts I and II relate to Mendel’s postulates? Discuss. (2)
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Part III. Probability and Mendel Ian Genetics
The law of probability is used to predict the probability of given genetic traits appearing in offspring of
particular parents. Punnett squares are often used to show these predictions.
When gametes are formed the pair of genes that determine a particular trait separate, and one form of the
gene goes to each gamete. When fertilization occurs, a male and female gamete fuse. The resulting
zygote now contains two genes (alleles) for the trait. Which two of the parent’s genes appears in the zygote
is a result of chance.
 For our example, we will consider the inheritance in pea plants of round and wrinkled peas.
o R = round and r = wrinkled.
1. Place a small piece of masking tape on each side of two pennies. Each penny represents a single
parent. Each side of one penny represents one allele on one gene.
2. On one penny write R on each side. i.e. “RR” homozygous dominant
3. On the other penny write “r” on each side. i.e. “rr” homozygous recessive
4. Toss the pennies ten times. In your data collection, write down your observations.
5. Replace the tape with new tape. On each penny, write R on one side and “r” on the other so that each
“parent” is heterozygous.
6. Toss the pennies 10 times. In your observations, write the combinations of genes that appear.
7. For each combination of genes (genotype) write the phenotype and its ratio.
Results Discussion Questions for Part III.
Purpose: (2)
Data Collection: (1)
RR x rr outcomes: Genotypes
Phenotypes
Genotypic Ratio
Phenotypic Ratio
Rr x Rr outcomes: Genotypes
Phenotypes
Genotypic Ratio
Phenotypic Ratio
1. What was the genotypic ratio when an RR plant was crossed with an rr plant? Explain. (2)
2. Would the offspring have been round peas or wrinkled peas? Explain. (2)
3. Describe the genotypic ratio you would expect from crossing an Rr plant with an Rr plant. If you tossed
your coins 40 times, how many would you expect to be RR, Rr, rr? Explain. (2)
4. Complete a Punnett square like the one below for the cross of Rr and Rr plants. Explain how the
Punnett square relates to our lab results. (1)
R
r
R
r
5. Discuss how Part III relates to Mendel’s Law of Dominance and Independent Assortment. (2)
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