106 ❯ STEP 4. Review the Knowledge You Need to Score High
Heads or Tails?
Probabilityis a concept important to a full understanding of heredity and inheritance.
What is the probability that a flipped coin will come up heads? You can answer that easily:
1 ⁄ 2. What is the probability that two coins flipped simultaneously will bothbe heads? This
is a little harder—it is 1 ⁄ 2 ×1⁄2 =1 ⁄ 4. Take a look at Figure 10.5. The first time you toss
the coin, there is a probability of 1 ⁄ 2 that it will land heads and 1 ⁄ 2 that it will land tails.
When you toss it again, it again has a probability of 1 ⁄ 2 that it will land heads, and 1 ⁄ 2
that it will land tails. So in the figure, just concentrate on the 1 ⁄ 2 of the tosses that land
heads. Of those, 1 ⁄ 2 of them will land heads the second time—or 1 ⁄ 2 of 1 ⁄ 2. Multiplied
together, this results in the 1 ⁄ 4 chance of getting heads twice with two coin tosses. This
example illustrates the law of multiplicationwith probabilities. This law states that to
determine the probability that two random events will occur in succession, you simply mul-
tiply the probability of the first event by the probability of the second event.A (20) CA(5)B (15) C or A (20) C ( 15) B
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A(5)B(5)D (10) C Actual sequence of genesFigure 10.4 A genetic linkage map.Toss 1 Toss 2(^1) ⁄ 2 Heads (^1) ⁄ 4 = Heads twice
(^1) ⁄ 2 = One heads, one tails
(^1) ⁄ 4 = Tails twice
(^1) ⁄ 2 Heads
(^1) ⁄ 2 Tails
(^1) ⁄ 2 Heads
(^1) ⁄ 2 Tails
(^1) ⁄ 2 Tails
Figure 10.5 Probability in the law of multiplication.
This is the same thought process that we follow to understand Mendel’s law of segregation.
If you are Aa for a trait, what is your chance or passing on the A? That’s right—1 ⁄ 2. If you
are AaBb, what is the chance you pass on both A and B? Clever you are—you multiply 1 ⁄ 2 ×
1 ⁄ 2 to get 1 ⁄ 4.
Pedigrees
Pedigreesare family trees used to describe the genetic relationships within a family.
Comprehension of the probability concept is important for a full understanding of pedi-
gree analysis. Squares represent males, and circles are used for females. A horizontal line