HUMAN BIOLOGY

390 Chapter 20

may yield clues to a trait’s genetic basis. For instance, they

might figure out that the allele that causes a disorder is

dominant or recessive or that it is located on an autosome

or a sex chromosome. Pedigrees often are used to identify

those at risk of transmitting or developing the trait in ques-

tion—including any children that a couple may have.

Gathering numerous family pedigrees increases the

numerical base for analysis. Figure 20.7 shows a series of

pedigrees for Huntington’s disease, in which the ner-

vous system progressively degenerates. Genetic researcher

Nancy Wexler constructed the pedigrees for a huge

extended family in Venezuela that includes more than

10,000 people.

Someone who is heterozygous for a recessive disease

trait can be designated as a carrier. A carrier shows the

dominant phenotype (no disease symptoms) but still

can produce sperm or eggs with the recessive allele and

potentially pass it on to a child. If both parents are carri-

ers for a disorder, a child has a 25 percent chance of being

homozygous for the harmful recessive allele.

1.120.4 human Genetic analysis

n In some cases prospective parents can assess their risk of

conceiving a child with an inherited disorder. A first step

can be to construct a genetic family history.

n Links to Chromosomes 18.1, Dominant and recessive

genetic conditions 19.1

a pedigree shows genetic connections

In nonhuman organisms, geneticists use experimental

crosses to do genetic analysis. Since we can’t experiment

with humans, however, a basic tool is a genetic family

history called a pedigree. This is a

chart that tracks several generations

of a family, showing who exhib-

ited the trait being investigated. The

example shown in Figure 20.6 includes definitions of some

of the symbols used.

When analyzing a pedigree, geneticists use their knowl-

edge of probability and of basic inheritance patterns, which

Figure 20.6 Animated! A pedigree is the family history of a genetic trait. A Here you see some symbols

used in constructing pedigree diagrams. B A pedigree for polydactyly, in which affected people have extra

fingers, extra toes, or both. As described in Section 19.5, expression of the gene governing polydactyly can

vary. Here, black numerals designate the number of fingers on each hand. Blue ones designate the number

of toes on each foot. (© Cengage Learning)

I

II

III

IV

V

5,5

6,6

6,6

5,5

6,6

5,5

5,5

6,6

5,5

6,6

*Gene not expressed in this carrier.

5,5

6,6

5,5

6,6

5,5

6,6

5,6

6,7

6,6

6,6

6 7

12

*

male

marriage/mating

offspring

individual showing

trait being studied

sex not

specified

I, II, III, IV... generation

female

A

B

Jim Stevenson/SPL/Science Source

pedigree Family history of
a genetic trait.

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