Additive Rule
The probability of two or more
mutually exclusive
events occurring is equal to the sum of their
probabilities.
What is the probability that a student will get an “A” or a “B” in a class if students generally earn
the following grades:
A = 10% (or 0.10)
B = 35% (or 0.35)
C = 45%
D = 10%
Solution: In this example, the two outcomes (getting an "A" or getting a "B") are mutually
exclusive because you can only get one or the other. The additive rule is used to determine the
overall probability of getting an "A" or a "B". 10% + 35% = 45% (or 0.10 + 0.35 = 0.45).
Consider Two Loci at the Same Time
Independent Assortment
Genes that are on different chromosomes assort independently. The following are four different
metaphase I allignment patterns that are possible for a hypothetical species with a diploid
chromosome number of 6.
The alignment pattern shown in the diagram below will produce Sy and sY gametes.
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The alignment pattern shown in this diagram will produce SY and sy gametes.
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 Fall '10
 GwenHauner
 Biology, aabb

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