919
CHAPTER TWENTY-FOUR
Genetics and Genomics
gous
(ho
mo-zi
gus) for that gene. A person with two differ-
ent alleles for a gene is
heterozygous
(het
er-o-zi
gus) for it.
A gene may have many alleles, but an individual person can
have a maximum of two alleles for a particular gene.
The allele that causes most cases of cystic F
brosis was discovered in
1989, and a test developed to detect it. However, other alleles were
soon discovered. Today, more than 1,000 mutations (changes) in the
cystic F
brosis gene are known. Di±
erent allele combinations produce
di±
erent combinations and severities of symptoms. Tests for the dis-
order detect ninety-six or more of the most common alleles.
determine sex. The other two chromosomes, the X and the
Y, include genes that determine sex and are called
sex chro-
mosomes.
They are discussed later in the chapter in the sec-
tion entitled “Matters of Sex.”
Each chromosome includes hundreds or thousands of
genes. We have two copies of each chromosome, so we also
have two copies of each gene. The two members of a gene
pair are located at the same position on homologous chromo-
somes. Sometimes the members of a gene pair are identical in
DNA sequence. However, because a gene consists of hundreds
of nucleotide building blocks, it exists in variant forms, called
alleles
(ah-le
¯lz
), that differ in DNA sequence. An individual
who has two identical alleles of a particular gene is
homozy-
FIGURE 24.2
²rom gene to protein to person. (
a
) The gene encoding the C²TR protein, and causing cystic F
brosis when mutant, is on the seventh
largest chromosome. C²TR protein folds into a channel that regulates the ³
ow of chloride ions into and out of cells lining the respiratory tract,
pancreas, intestines, and elsewhere. (
b
) In cystic F
brosis, the C²TR protein is abnormal, usually missing an amino acid. Its shape is altered, which
entraps the chloride ions inside cells. Water entering these cells leaves behind very thick mucus and other secretions in the places highlighted in
the illustration. The sticky secretions cause the symptoms of the illness. Source: Data from M.C. Iannuzi and ².S. Collins, “Reverse Genetics and Cystic
²ibrosis” in
American Journal of Respiratory Cellular and Molecular Biology,
2:309–316, 1990.
DNA
Chromosome 7
mRNA
Transcription
Cystic fibrosis
transmembrane
conductance
regulator (CFTR)
protein
Cell
membrane
(a)
Translation
Organs affected in cystic fibrosis
Airways
Mucus-clogged bronchi
and bronchioles.
Respiratory infections.
(Common)
Liver
Blocked small bile ducts
impair digestion. (Rare)
Pancreas
Blocked ducts prevent
release of digestive
enzymes, impairing
fat digestion. Diabetes
is possible. (Common)
Intestines
Hard stools may block
intestines. (Rare)
Reproductive tract
Absence of ductus deferens.
(Common)
Skin
Salty sweat. (Common)
(b)
previous page 949 David Shier Hole's Human Anatomy and Physiology 2010 read online next page 951 David Shier Hole's Human Anatomy and Physiology 2010 read online Home Toggle text on/off