HUMAN BIOLOGY

392 Chapter 20

amino acid, tyrosine. Excess phenylalanine builds up and

may be used by cells to make phenylpyruvic acid. At high

levels, phenylpyruvic acid can cause mental retardation.

Fortunately, a diet low in phenylalanine will prevent PKU

symptoms. Many diet products are sweetened with aspar-

tame, which contains phenylalanine. They must carry a

warning label so people with PKU can avoid using them.

In some autosomal recessive disorders, the defective

gene product is an enzyme needed to metabolize lipids.

Infants born with Tay-Sachs disease lack hexosamini-

dase A, which is an enzyme required for the metabolism

of sphingolipids, a type of lipid that is especially abundant

in the plasma membrane of cells in nerves and the brain.

Affected babies seem normal at birth, but over time they

lose motor functions and also become deaf, blind, and

mentally retarded (Figure 20.8). Most die in early childhood.

Tay-Sachs disease is most common among children of

Eastern European Jewish descent. Biochemical tests before

conception can determine whether either member of a

couple carries the recessive allele.

some disorders are due to dominant genes

Other kinds of clues indicate that an autosomal dominant

allele is responsible for a trait. First, the trait usually appears

in each generation because the dominant allele generally

is expressed even in heterozygotes. Second, if one parent

is heterozygous and the other is homozygous for

the normal, recessive allele, there is a 50 percent

chance that any child of theirs will be heterozygous

(Figure 20.9A). A few dominant alleles that cause

severe genetic disorders persist in populations.

Some result from spontaneous mutations. In other

20.5 inheritance of Genes on autosomes

n Research on genetic disorders and abnormalities has

revealed patterns in the way dominant and recessive

genes on autosomes are inherited.

n Links to blood cholesterol 7.8, basal nuclei and Parkinson’s

disease 13.11, basic heredity concepts and probability

19.1, 19.3

inherited recessive traits on autosomes

cause a variety of disorders

For some traits, inheritance patterns reveal two clues that

point to a recessive allele on an autosome. First, if both

parents are heterozygous, any child of theirs will have a

50 percent chance of being heterozygous and a 25 percent

chance of being homozygous reces-

sive (Figure 20.8). Second, if both

parents are homozygous recessive,

any child of theirs will be too.

Cystic fibrosis, which you read

about in the chapter introduction,

is an autosomal recessive condi-

tion. So is phenylketonuria (PKU),

which results from an abnormal buildup of the amino

acid phenylalanine. Affected people are homozygous for

a recessive allele that fails to provide instructions for an

enzyme that is needed to convert phenyl alanine to another

Figure 20.8 Animated! Both parents may be carriers in autosomal recessive inheritance. A The pattern in which both parents are

heterozygous carriers of the recessive allele (red). B Conner Hopf, who was diagnosed with Tay-Sachs disease as an infant. He died at

22 months. (© Cengage Learning)

A

B

Conner’s Way Foundation

autosomal dominant

Caused by a dominant allele

on an autosome.

autosomal recessive

Caused by a recessive allele

on an autosome.

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