HUMAN BIOLOGY

Chromosomes and human GenetiCs 391

wHat can we learn from pedigree analysis?

• Pedigree analysis may reveal basic inheritance patterns that
  provide information about the probability that certain genes
  may be transmitted to children.


• A genetic abnormality is an uncommon version of an
  inherited trait.


• A genetic disorder is an inherited condition that produces
  mild to severe health problems.

taKe-home messaGe

Various types of genetic tests are available for

prospective parents. One, called carrier identification

testing, does what its name suggests. Using the

Web or library, find out who is most likely to receive

this type of test. Which diseases are the most

common concerns?

thInk outSIDe the book

Genetic analysis may predict disorders

Some prospective parents suspect that they are likely to

produce a severely afflicted child. Their first child, a close

relative, or they themselves may have a genetic disorder,

and they wonder how likely it is that future children

also will be affected. Psychologists, geneticists, and other

specialists may be called in to pro-

vide answers.

A common first step is deter-

mining the genotype of each par-

ent. Family pedigrees can aid the

diagnosis. For disorders that follow

one of the basic inheritance patterns

described in Chapter 19, it’s possible

to predict the chances a given child

will be affected. But not all follow

basic patterns. And those that do

can be influenced by other factors,

some identifiable, others not. Even when the extent of risk

has been determined with confidence, prospective parents

must understand that the risk is the same for each preg-

nancy. If a pregnancy has one chance in four of producing a

child with a genetic disorder, the same odds apply to every

s u b s e q u e nt p r e g n a n c y.

In thinking about genetics, it’s good to keep in mind
the difference between an abnormality and a disorder.
A genetic abnormality is simply a deviation from the
average, such as having six toes on each foot instead of
five. An abnormality doesn’t necessarily cause health prob-
lems. A genetic disorder causes mild to severe medical
problems. We use the word syndrome to mean a set of
symptoms that usually occur together and char acterize
a disorder.
Each of us carries an average of three to eight harm-
ful recessive alleles. Why don’t alleles that cause severe
disorders simply disappear from human populations? One
reason is that new gene alleles can come about by way of
mutation, as described in Section 20.8. Also, in heterozygotes,
a recessive allele is paired with a normal dominant one that
prevents the recessive phenotype from showing up. Even so,
the recessive allele still can be transmitted to offspring.
It’s not uncommon for some genetic disorders to be
described as diseases (for example, Huntington’s disease
and cystic fibrosis), but in other situations the terms are not
interchangeable. For instance, genes may increase a per-
son’s susceptibility or weaken the response to infection by
a virus, bacterium, or some other pathogen. Strictly speak-
ing, however, the resulting illness isn’t a genetic disease.
A common example is a genetic predisposition to develop
allergies or asthma that runs in some families.

Figure 20.7 Genetic researcher Nancy Wexler made a pedigree for Huntington’s disease (HD). Wexler has a special interest in
HD because it runs in her family.

Steve Uzzell

genetic abnormality A

deviation from the usual

phenotype that does not

harm health.

genetic disorder Genetic

condition that causes medi-

cal problems.

syndrome A set of symp-

toms that usually occur

together and that are asso-

ciated with a disorder.

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