1. Biology Probability Worksheet
INTRODUCTION
The passing of traits from one generation to the next involves probability. Every normal
human, for example, has 46 chromosomes in each cell except for the gametes. The
gametes contain 23 chromosomes. This reduction in chromosome number results in
meiosis. The chromosomes in your cells come from two sources: your mother and your
father. The egg contains one set of 23 chromosomes and the sperm contains one set of
23 chromosomes. Upon fertilization, the usual chromosome number of 46 is restored.
But why is it that you look like your maternal great grandfather and your sister looks like
your paternal grandmother? In fact, why is it that you have a sister rather than a
brother anyway? This is due to probability.
In this exercise, you will work with probability and statistics using coins. Do not get
alarmed that math is involved. The rules are very simple. A large part of probability is
good old common sense and logic.
The Sum Rule: Mutually exclusive events
The sum rule is used when you are dealing with mutually exclusive events ie. Either/or
situations. For example, using your coin, you can only get a head or a tail at one given
toss. If you get a head, the tail has been excluded. The sum rule states that the
probability of two mutually exclusive events occurring is the sum of the individual
probabilities. The rule can be extended to more than two events as well.
The Product Rule: Simultaneous Independent Events
What happens when two different coins are tossed at the same time? What is the
probability that you will get two heads on a given toss? This is answered by the product
rule. This rule states that the probability of two independent events occurring
simultaneously is the product of the individual probabilities of the two events. This rule
can be extended to more than two events as well.
Before you go on, think about the following: what are the different combinations of
results? Obviously you can get two heads or two tail. You can also get a head and a
tail or a tail and a head. You must keep this in mind. The order of things might seem
trivial, but the fact is they are both different events. There are two ways to get a head
and a tail. This needs to be calculated into the probability. If there are two
combinations, you multiply the probability by two. If there are three combinations, you
multiply the probability by three, and so on and so forth.
PROCEDURE
Part One: Take one coin and flip it 50 times. Record the date in the chart. After each
ten flips, work out the percentage of heads and tails and see how far it is from the
expected outcome.

2. Part Two: Take two coins and toss them simultaneously 50 times. Record the data in
the chart. After each ten flips, work out the percentage of 2 heads, 1 head/1tail and 2
tails and see how far it is from the expected outcome.
Part Three: Determine the various categories and combinations of each category when
tossing three coins simultaneously. Record the probability for each in a chart in the
space below.
RESULTS
1. What is the probability of getting a head when you toss a coin?
How did you get this answer?
2. What is the probability of getting a tail when you toss a coin?
How did you get this answer?
Part One:
Heads Percentage Tails Percentage
Flips 1-10
Flips 11-20
Totals 1-20
Flips 21-30
Totals 1-30
Flips 31-40
Totals 1-40
Flips 41-50
Totals 1-50
Class Totals
3. How did the percentages change with more flips?

3. Part Two:
2 Heads Percentag 1 Head & 1 Percentag 2 Tails Percentage
e Tail e
Flips 1-10
Flips 11-20
Totals
1-20
Flips 21-30
Totals
1-30
Flips 31-40
Totals
1-40
Flips 41-50
Totals
1-50
Class Total
4. What is the probability of getting 2 heads when you toss a coin?
5. What is the probability of getting 2 tails when you toss a coin?
6. What is the probability of getting 1 head & 1 tail when you toss a coin?
7. How is this similar to a cross with two heterozygotes?
Part Three: