Name________________________________________________
Biology Lab
Mrs. Olmo
Laws of Probability: Coin Toss Lab
Few concepts have had greater effect on the science of genetics than the laws of probability. Probability refers to the
chance of something happening. Under normal conditions probability calculations can give us good ideas of what to
expect from different genetic combinations. A thorough understanding of probability was instrumental in leading Gregor
Mendel to his basic conclusions about genetics, and these same laws of probability play an essential role in genetics
today.
Objectives:
 Explain the role of sample size in estimating probability
 Calculate the probability of occurrence of a single event.
 Compute a percent deviation from expected values for data gathered
Materials:


2 coins (same size)
calculator
Procedure:
1. What percent do you EXPECTED to flip for each side of the coin: ___________% Heads and
___________% Tails
Part 1: The Coin Toss
2. Toss a single coin 10 times. Record the number of heads AND tails that result. Record your results below using
tick marks.
Heads
3. Calculate the percent heads you flipped:
Tails
# times flipped
total # of flips
4. Calculate the percent tails you flipped:
5. Based only on your observed (experimental) results from #2, what do you expect to flip next, a head or tail? Explain
your answer.
6. Compare what you expected (from #1) with your observed results (from #3). Did you observe what you expected?
7. Toss a single coin 50 times. Record the number of heads AND tails that result. Record your results below using
tick marks.
Heads
8. Calculate the percent heads you flipped:
Tails
# times flipped
total # of flips
9. Calculate the percent tails you flipped:
10. Compare what you expected (from #1) with your observed (from #6). Did you observe what you expected?
11. Compare your answers from number 6 and number 10. Did you get “better” (more accurate) data when you flipped
more times, did it not make a difference? Explain your answer.
Part 2: Calculating The Deviation
After predictions are made for a given event and actual data are gathered, the deviation, or difference between observed
and expected, can be figured. This is usually expressed as a percentage and is an indication of the degree of error. If the
percent deviation is small (approximately 10 % or less), we can say it is due to chance. If the value is large, other
unknown factors may have entered into the experiment.
11. Use the formula to compute the percent deviation for each trait.
% deviation = Sum of differences from expected X 100
Total occurrences
Example: A coin is tossed 10 times producing 7 heads and 3 tails. The deviation is computed as follows
Observed
Expected
Difference from expected
Heads
7
5
2
Tails
3
5
2 (disregard negative value}
Total
10
10
4 (sum of differences)
Occurrences
Deviation
_4
= .4 X 100 = 40%
10
10 Coin Flip Trial
Observed
Expected
Heads
____________
____________
Tails
____________
____________
Total
10
Difference from expected
____________
+
10
____________
____________
Occurrences
Deviation =
50 Coin Flip Trial
Observed
Expected
Heads
____________
____________
Tails
____________
____________
Total
Occurrences
50
50
Difference from expected
____________
+
____________
____________
Deviation =
12. What is the relationship between sample size and the degree of error for a chance occurrence? In other words, did
the number of times you flipped the coin affect the degree of error? Why or why not? EXPLAIN YOUR ANSWER
13. How do you think this lab connects to the study of genetics?