Genetics Probability

Genetics Probability - BICD100 Genetics Winter 2008...

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BICD100 Genetics Winter 2008 1 Probability Rules In diploid genetics each individual inherits one of two possible alleles of a gene from a parent (Mendel’s first law). Thus, statements about the outcome of a cross or a human pedigree must be made in probabilistic not absolute terms. For example, in a cross of a heterozygous parents Aa x Aa, the probability of a homozygous F 1 offspring aa is 1/4. This is shorthand for saying is that given a sufficiently large number of progeny the fraction of homozygous animals would approach 1/4. All probabilities must thus be a fraction between 0 and 1. Probabilities for all outcomes must together equal 1. The textbook introduces the two fundamental rules of probability, the sum and product rules. This handout covers two important additional concepts: conditional probability and Bayes’ Theorem. These will be needed in some of the genetics problems and exams. 1. Sum (or addition) rule P( x or y ) = P( x ) + P( y ) where outcomes x, y are mutually exclusive events Corollary: P( x ) = 1- P( y ) (must add up to 1) 2. Product (multiplication) rule P( x and y ) = P( x ) x P( y ) where x, y are independent events 3. Conditional probability What is the probability of one event given that the other has occurred? The probability of event x given event y is notated P( x|y ). To calculate this “x given y” probability, we start with a more general version of the product rule:
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Genetics Probability - BICD100 Genetics Winter 2008...

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