Biol 302, Fall 2013
Mendelian/nonMendelian Genetics
Why do we study random chance and probability at the beginning of a unit on Mendelian
genetics?
Genetics is the study of inheritance, but it is also a study of probability. Chance plays
a major role in determining which alleles, and therefore which combinations of traits, end up in
each new individual.
The Laws of Probability
There are three Laws of Probability that are important in genetics and they can be easily
demonstrated using simple models like flipping a coin or choosing cards from a deck:
•
The Rule of Independent Events
: Past events have no influence on future events.
Question: If a coin is tossed 5 times, and each time a head appears, then what is the
chance that the next toss will be heads?
Answer: 1/2 (1 chance in 2), because coins have 2 sides.
•
The Rule of Multiplication
:
The chance that two or more independent events will
occur together is equal to the product of the probabilities of each individual event.
Question: What are the chances of drawing a red nine from a standard deck of cards?
Answer: 1/26 (1 chance in 26), because there is 1/2 chance of drawing a red card and 1
chance in 13 of drawing a nine. Therefore, 1/2 x 1/13 = 1/26 or 1 chance in 26 of
drawing a red nine.
•
The Rule of Addition
:
The chance of an event occurring when that event can occur two
or more different ways is equal to the sum of the probabilities of each individual event.
Question: If 2 coins are tossed, what is the chance that the toss will yield 2 unmatched
coins (1 head & 1 tail)?
Answer: 1/2 (1 chance in 2) because the combination of 2 unmatched coins can come
about in 2 ways: Result A (coin #1 heads, coin #2 tails) as well as Result B (coin #1 tails,
coin #2 heads). Therefore (1/2 x 1/2) + (1/2 x 1/2) = 1/2, or the chance of Result A plus
the chance of Result B.
Paired Coins and Genetics
Using paired coins, in fact, mimics genetics closely. Each coin can serve as the model for a
gamete during fertilization.
When you toss two coins, there are three possible outcomes:
• 2 heads
• 2 tails
• 1 head, 1 tail
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View Full DocumentBiol 302, Fall 2013
Mendelian/nonMendelian Genetics
Determining the Chisquare Value
The Chisquare test is a statistical test that makes a comparison between the data collected in an
experiment versus the data you expected to find. Genetics uses the Chisquare test to evaluate
data from experimental crosses to determine if the assumed genetic explanation is supported by
the data. The Chisquare test helps you to decide if the difference between your observed results
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 Fall '14
 Genetics, ChiSquare Test, Variance, Mendelian Genetics, Chisquare distribution, Coin flipping, random chance

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