Heredity ❮ 109
few years. Carriers of this disease do not show any of the effects of the disease, and thus the
allele is preserved in the population because carriers usually live to reproduce and poten-
tially pass on the recessive copy of the allele. This disease is found in a higher than normal
percentage of people of eastern European Jewish descent.
Cystic fibrosis(CF), a recessive disorder, is the most common fatal genetic disease in
this country. The gene for this disease is located on chromosome 7. The normal allele for
this gene is involved in cellular chloride ion transport. A defective version of this gene
results in the excessive secretion of a thick mucus, which accumulates in the lungs
and digestive tract. Left untreated, children with CF die at a very young age. Statistically,
one in 25 Caucasians is a carrier for this disease.
Sickle cell anemiais a common recessive disease that occurs as a result of an improper
amino acid substitution during translation of an important red blood cell protein called
hemoglobin.It results in the formation of a hemoglobin protein that is less efficient at car-
rying oxygen. It also causes hemoglobin to deform to a sickle shape when the oxygen con-
tent of the blood is low, causing pain, muscle weakness, and fatigue.
Sickle cell anemia is the most common inherited disease among African Americans. It
affects one out of every 400 African Americans, and one out of 10 African Americans is a
carrier of the disease. The recessive trait is so prevalent because carriers (who are said to have
sickle cell “trait”) have increased resistance to malaria. In tropical regions, where malaria
occurs, the sickle cell trait actually increases an individual’s probability of survival, and thus
the trait’s presence in the population increases (heterozygote advantage).
Phenylketonuria(PKU) is another autosomal recessive disease caused by a single gene
defect. Children with PKU are unable to successfully digest phenylalanine (an amino acid).
This leads to the accumulation of a by-product in the blood that can cause mental retarda-
tion. If the disease is caught early, retardation can be prevented by avoiding phenylalanine
in the diet.
Dominant disorders are less common in humans. One example of a dominant disorder
isHuntington disease,a fatal disease that causes the breakdown of the nervous system. It
does not show itself until a person is in their 30s or 40s, and individuals afflicted with this
condition have a 50 percent chance of passing it to their offspring.
Why are lethal dominant alleles less common than lethal recessive alleles? Think about
how recessive alleles often are passed on from generation to generation. An individual can
be a carrier of a recessive condition and pass it along without even knowing it. On the other
hand, it is impossible to be an unaffected carrier of a dominant condition, and many lethal
conditions have unfortunately killed the individual before reproductive maturity has been
achieved. This makes it more difficult for the dominant gene to be passed along. To remain
prevalent in the population, a dominant disorder must not kill the individual until repro-
duction has occurred.
Chromosomal Complications
We have spent a lot of time discussing how genes are inherited and passed from generation
to generation. It is also important to discuss the situations in which something goes wrong
with the chromosomes themselves that affects the inheritance of genes by the offspring.
Nondisjunctionis an error in homologous chromosome separation. It can occur during
meiosis I or II. The result is that one gamete receives too many of one kind of chromosome,
and another gamete receives none of a particular chromosome. The fusing of an abnormal
gamete with a normal one can lead to the production of offspring with an abnormal
number of chromosomes (aneuploidy).
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