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NEL The Basis of Heredity 615

Section18.5

In humans, free ear lobes are determined by the dominant allele E, and attached ear lobes

by the recessive allele e. The dominant allele Wdetermines a widow’s peak hairline and

the recessive allele wdetermines a straight hairline (Figure 5). The genes for these two

traits are located on different chromosomes. Suppose a man with the genotype EeWwand

a woman with the genotype EeWware expecting a child. What is the probability that the

child will have a straight hairline and attached ear lobes?

SAMPLE exercise 1

Figure 5

In humans, both ear lobe shape and hairline shape are inherited. The free ear lobe in

(a)is dominant to the attached ear lobe in (b), and the widow’s peak in (c)is

dominant to a straight hairline in (d).

(a) (b)

(c) (d)

Learning Tip

When thinking about

probability, keep the following

two rules in mind:

• When outcomes are
  independent, the probability
  of one outcome is not
  affected by the result of any
  other outcomes. For example,
  if you toss two heads in a
  row, the probability of
  tossing heads a third time is
  still 1 out of 2.


• The probability of
  independent events
  occurring together is equal to
  the productof those events
  occurring separately. The
  chances of tossing heads
  once is ^12 , the probability of
  tossing heads twice in a row
  is ^12 ^12 
   41 , and the
  probability oftossing heads
  three times in a row is
  ^12 ^12 ^12 
  ^18 .

Solution

To have attached ear lobes and a straight hairline, the child must have the genotype

eeww. Since the two genes are on separate chromosomes, the gene for ear shape and

hairline shape will assort independently. The outcome that the child will receive two

ealleles is, therefore, independent of the outcome that the child will receive two w

alleles.

First, determine the probability of each of these outcomes separately, using a separate

Punnett square for each gene. From Figure 6 (a), we see the probability that the child

will have attached ear lobes is one in four (^14 ). From Figure 6 (b), we see the

probability that the child will have a straight hairline is also one in four ( 41 ).

EE Ee

Ee ee

Ee

E

e

1

2

1

2

1

4

1

4

1

4

1

4

1

2

1

2

WW Ww

Ww ww

Ww

W

w

1

2

1

2

1

4

1

4

1

4

1

4

1

2

1

2

Figure 6

Punnett squares showing monohybridcrosses between heterozygous parents for

(a)free ear lobes and (b)for a widow’s peak

(a) (b)

Probability—The Sum and

Product Rules

This Audio Clip explores the use

of the sum and product rules of

probability.

http://www.science.nelson.com GO

+ EXTENSION