534
chapter 23
Structure and Properties of DNA
of genes that play a vital role in the rejection of tissue
transplants consists of hundreds of different alleles (Chap-
ter 35). No two individuals, except for identical twins,
share exactly the same set of HLA alleles. For a gene
to be polymorphic, alternative alleles must be present
in at least 1% of individuals in a population. The rel-
ative frequencies of the different alleles at a polymor-
phic locus usually vary from one human population to
another.
The remaining 75% of human genes consist of a sin-
gle allelic form and are said to be
monomorphic.
For ex-
ample, all human beings have the same
a
and
(3
genes
for hemoglobin, presumably because the protein structure
has been optimized by millions of years of human evo-
lution. All persons, irrespective of race or ethnicity, have
hemoglobin genes that are identical in sequence. The only
exceptions to the monomorphic state of the hemoglobin
genes are the rare individuals who inherit or acquire a
mutation in the
a
or
(3
genes that gives rise to the sev-
eral hundred characterized human hemoglobinopathies
(Chapter 28).
Restriction enzyme sites in DNA also are highly poly-
morphic in human populations. The pattern of DNA
fragments produced by digesting DNA from different indi-
viduals will usually show a difference if a sufficient num-
ber of fragment sizes are examined; again, only identical
twins have identical restriction enzyme sites. The different
fragments produced by digesting DNA with one or more
restriction enzymes are called
restriction fragment length
polymorphisms (RFLPs).
RFLPs have been extremely useful in constructing a re-
striction map of the human genome (which can be corre-
lated with the genetic and physical maps), in screening hu-
man populations for the presence of mutant alleles, and for
diagnosing hereditary disorders in fetuses and prospective
parents. An early success in mapping the human genome
was determining the location of more than 10,000 RFLPs
on the 23 human chromosomes.
Most mutant genes that cause hereditary disorders are
linked to specific RFLP differences among affected and
unaffected persons that can be detected by using DNA
probes carrying the gene in question. For example, the
single-base change found in all cases of sickle cell anemia
also changes a restriction site located within the /3-globin
gene that is recognized by the restriction enzyme Mstll
(Figure 23-14). When DNA from unaffected and affected
individuals is digested with Mstll and a Southern blot anal-
ysis is performed with the appropriate probe, affected in-
dividuals show a different pattern of DNA fragments as
compared with unaffected individuals.
Huntington’s disease (HD)
is an autosomal dominant
disorder characterized by progressive chorea, dementia,
FIGURE 23-14
Detection of sickle cell anemia by RFLP analysis and Southern blot.
and ultimately death. It is a late-onset hereditary disease
that usually manifests after age 40. Any son or daughter
who had a parent with Huntington’s disease has a 50%
probability of inheriting the mutant allele from the af-
fected parent. A Hindlll restriction site is closely linked
to the
HD
gene in asymptomatic family members who
carry the mutant allele but is absent in family members
who have not inherited the mutant allele. Southern blot
analysis can determine which family members are des-
tined to die of the disease later in life. (Not all families
that carry a
HD
gene mutation have this particular mu-
tation that was characteristic of the original large family
investigated.)
Genetic testing for susceptibility to Huntington’s dis-
ease (and many others) raises profound ethical questions
for society, for families, and for individuals who may or
may not have inherited disease-causing mutant alleles, and
who may or may not want to know their disease status.
Persons diagnosed with a gene for an inherited disorder
may face discrimination in obtaining health insurance, life
insurance, job opportunities, and so path. Hundreds of
Mendelian (single-gene) disorders can now be detected
with appropriate DNA probes (Table 23-2). However, ge-
netic screening generally is not advised, particularly if no
medical treatment is available for the particular disease.
