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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(
xy
).
To calculate this “x given y” probability, we start with a more general version of the product rule:
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 Spring '08
 Nehring
 Genetics

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