1100
UNIT 5
Continuity
29
oxygen levels are low, causing the erythrocytes to assume a
sickle shape (see Figure 17.8b, p. 639). Individuals with a dou-
ble dose of the sickling allele (
ss
) have
sickle-cell anemia
, and
any condition that lowers their blood oxygen level, such as
respiratory difficulty or excessive exercise, can precipitate a
sickle-cell crisis
. Te deformed erythrocytes jam up and frag-
ment in small capillaries, causing intense pain.
Individuals heterozygous for the sickling gene (
Ss
) have
sickle-cell trait
. Tey make both normal and sickling hemo-
globin, and as a rule, these individuals are healthy. However,
they can suffer a crisis if there is prolonged reduction in blood
oxygen levels, as might happen when traveling in high-altitude
areas, and they can transmit the sickling gene to their offspring.
Multiple-Allele Inheritance
Although we inherit only two alleles for each gene, some genes ex-
hibit more than two allele forms, leading to a phenomenon called
multiple-allele inheritance
. For example, three alleles determine
the ABO blood types in humans:
I
A
,
I
B
, and
i
. Each of us receives
two of these. Te
I
A
and
I
B
alleles are
codominant
, and both are
expressed when present, resulting in the AB blood type. Te
i
al-
lele is recessive to the other two. Genotypes determining the four
possible ABO blood groups are shown in
Table 29.2
.
Check Your Understanding
7.
Why are there so few genetic disorders caused by dominant
genes?
8.
How does incomplete dominance differ from codominance?
For answers, see Appendix H.
Sex-Linked Inheritance
Describe the mechanism of sex-linked inheritance.
Inherited traits determined by genes on the sex chromosomes are
said to be
sex-linked
. Te X and Y sex chromosomes are not ho-
mologous in the true sense. Te Y, which contains the gene (or
genes) that determines maleness, is much smaller than the X chro-
mosome
(Figure 29.5)
. Te X bears over 2500 genes, and a dis-
proportionately large number of them code for proteins important
to brain function. Because the Y carries only 78 genes as of the last
count, it lacks many of the genes present on the X. For example,
genes coding for certain clotting factors, cone pigments, and even
Recessive Traits
Some examples of recessive inheritance are the more desirable
genetic condition. For example, normal vision is dictated by
recessive alleles, whereas astigmatism is prescribed by domi-
nant alleles. However, many if not most genetic disorders are
inherited as simple recessive traits. Tese include conditions as
different as
albinism
(lack of skin pigmentation);
cystic fibrosis
,
a condition of excessively thick mucus production that impairs
lung and pancreatic functioning; and
Tay-Sachs disease
, a disor-
der of brain lipid metabolism, caused by an enzyme deficit that
shows itself a few months a±er birth.
Recessive genetic disorders are more frequent than those
caused by dominant alleles because those who carry a
single
re-
cessive allele for a recessive genetic disorder do not themselves
express the disease. However, they can pass the gene on to off-
spring and so are called carriers of the disorder. Conversely, as
we explained above, few individuals with dominant genetic dis-
orders get a chance to pass on the detrimental gene.
Incomplete Dominance
In dominant-recessive inheritance, one allele variant completely
masks the other. Some traits, however, exhibit
incomplete
dominance
. In such instances, the heterozygote has a pheno-
type intermediate between those of homozygous dominant and
homozygous recessive individuals. Incomplete dominance is
uncommon in humans.
Perhaps the best human example is inheritance of the
sick-
ling gene
(
s
), which causes a substitution of one amino acid
in the beta chain of hemoglobin. Hemoglobin molecules
containing the abnormal beta chains crystallize when blood
Table 29.1
Traits Determined by Simple
Dominant-Recessive Inheritance
PHENOTYPE DUE TO EXPRESSION OF:
DOMINANT GENES (
ZZ
OR
Zz
)
RECESSIVE GENES (
zz
)
Tongue roller
Inability to roll tongue into
a U shape
Astigmatism
Normal vision
Freckles
Absence of freckles
Dimples in cheeks
Absence of dimples
PTC* taster
PTC nontaster
Widow’s peak
Straight hairline
Double-jointed thumb
Tight thumb ligaments
Syndactyly (webbed digits)
Normal digits
Achondroplasia (heterozygous:
dwarfism; homozygous: lethal)
Normal endochondral
ossification
Huntington’s disease
Absence of Huntington’s
disease
Normal skin pigmentation
Albinism
Absence of Tay-Sachs disease
Tay-Sachs disease
Absence of cystic fibrosis
Cystic fibrosis
*PTC is phenylthiocarbamide, a harmless bitter chemical.
Table 29.2
ABO Blood Groups
 
FREQUENCY (% OF U.S. POPULATION)
BLOOD GROUP
(PHENOTYPE)
GENOTYPE
WHITE
BLACK
ASIAN
O
ii
45
49
40
A
I
A
I
A
or
I
A
i
40
27
28
B
I
B
I
B
or
I
B
i
11
20
27
AB
I
A
I
B
4
4
5
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