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# Random chance and probability reading - Biol 302 Fall 2013...

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Biol 302, Fall 2013 Mendelian/non-Mendelian 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.

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