HUMAN BIOLOGY

Chromosomes and human GenetiCs 393

wHicH genes cause genetic disorders?

• Recessive and dominant genes on autosomes both may cause
  genetic disorders.


• If both parents are heterozygous carriers of a recessive allele
  on an autosome, there is a 25 percent chance that a child
  of theirs will be homozygous for the trait and exhibit the
  recessive phenotype.


• A dominant autosomal trait may appear in each generation,
  because the dominant allele is expressed even in heterozygotes.

taKe-home messaGe

cases, expression of a dominant allele may not prevent
reproduction, or affected people have children before the
disorder’s symptoms become severe.
Huntington’s disease (Section 20.4) is a prime example
of an autosomal dominant disorder that doesn’t cause
symptoms until well into adulthood. In about half the
cases, symptoms emerge after age 30, when the person may
already have had children (so the allele may be passed on).
Homozygotes for the Huntington’s allele die as embryos, so
affected adults are always heterozygous. Today testing can
reveal the disease-causing allele, which is on chromosome 4.
There is no cure for HD. Some at-risk people opt not to be
tested for the disease, and many don’t have children to
avoid passing on the disorder.
Marfan syndrome (Figure 20.9B) is another autosomal
dominant condition. The responsible allele codes for a
defective form of the protein fibrillin, which is found in
connective tissue. Abnormal fibrillin causes a range of
problems, including disrupting both the structure and the
function of smooth muscle cells in the wall of the aorta,
the large artery that carries blood away from the heart.
Over time the wall weakens, and it can rupture suddenly
during strenuous exercise. Marfan syndrome affects 1 in
10,000 people throughout the world. Until recent medical
advances, it killed most affected people before age 50.
Another example, achondroplasia, also affects about
1 in 10,000 people. Homozygous dominant infants usually
are stillborn. Heterozygotes can reproduce, but while they
are young and their limb bones are forming, the cartilage

Figure 20.9 Animated! Autosomal dominant inheritance underlies a variety of genetic disorders. A Here the phenotype for the dominant
allele (red) is expressed in the carriers. B College basketball star Haris Charalambous, who died when his aorta burst during warmup exercises
at the University of Toledo in 2006. Charalambous was very tall and lanky, with long arms and legs, traits that are associated with Marfan
syndrome. C Achondroplasia affects Ivy Broadhead (left) as well as her brother, father, and grandfather.

A

elements of those bones cannot form properly. For that

reason, at maturity affected people have abnormally short

arms and legs (Figure 20.9C). Adults with achondroplasia

don’t grow taller than about 4 feet, 4 inches. In many cases,

the dominant allele has no other effects.

A dominant autosomal allele also causes familial

hypercholesterolemia. This allele leads to dangerously

elevated blood cholesterol because it fails to encode the

normal number of cell receptors for LDLs (low-density

lipoproteins). You may remember from Chapter 7 that

LDLs bind cholesterol in the blood, the first step in remov-

ing it from the body. A person who is homozygous for the

allele may develop severe cholesterol-related heart disease

as a child. About 1 person in 500 has familial hypercholes-

terolemia. Unless the disorder is diagnosed early in life, an

affected person may die quite young of heart disease, often

before the age of 40.

B

© Cengage Learning B: Courtesy of the family of Haris Charalambous. University of Toledo; C: © Newcastle Photos and Ivy & Violet Broadhead and family

C

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