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One preventable
cause of mental retardation is Phenylketonuria (PKU). It
is caused by a failure to break down and eliminate phenylalanine
(PA) from the body. Thus PA builds up in the bloodstream. This
buildup causes problems in the development of the brain, which may
lead to mental retardation and sometimes to seizures. PA is an amino
acid found in many proteins. A person must inherit a defective gene
from both parents in order to have PKU. One defective gene is not
harmful. If an infant inherits a faulty gene from both parents, and
is not placed on a diet low in PA, mental retardation is certain or
nearly so. For this reason all hospitals in the United States
require a blood test for PKU before a newborn leaves the hospital.
If an infant is kept on the special diet, development can be normal.
The diet must be low in all high-protein foods, including meat,
eggs, cheese, milk, and legumes. Proteins that do not contain PA are
a critical part of the diet. Women who have PKU must be careful to stay on
the special diet during any pregnancies, or the fetus's brain
development will be abnormal, even if the fetus does not inherit a
defective gene from the father. The mother's blood courses through
the fetus, and high PA in her blood will produce the same bad
effects as if the fetus produced the high level. On the other hand,
a mother without PKU can produce a child who is normal at birth,
even though the child has two defective genes, because her normal
enzymes keep PA levels normal in the fetus's bloodstream. Thus early
detection of PKU in infants, plus knowing when a pregnant woman has
PKU and keeping her on a special diet, can eliminate the
bad effects of PKU. An infant with PKU born to a mother without PKU
will not show signs of PKU until several weeks or months after
birth. One interesting characteristic of children with PKU is that
their hair is blonde and their eyes blue because PA interferes with
the production of melanin, which colors eyes and hair. Thus if a
child born to brown-eyed, brown-haired parents has blonde hair and
blue eyes, and shows signs of slow development, it is a good
idea to test for PKU. It used to be thought that the special PKU diet
could be abandoned once brain development was complete, but experts
now recommend that people with PKU stay on the special diet for
life. If PKU is not detected at birth, and causes abnormal
development, it is still helpful to start the special diet as soon
as the condition is diagnosed. Further damage can be prevented, and
some improvement is usually seen, even in adults with PKU, if they
are placed on the special diet. PKU is caused by a "recessive" gene.
Genes come in pairs, so a person with a "good" dominant
gene and a "bad" recessive PKU gene will not have PKU;
both genes must be defective for the condition to appear. About 1
person in 50 in the population has one PKU gene. Therefore there is
1 chance in 2500 (1/50 x 1/50) that each of two randomly chosen
adults will have one recessive PKU gene. If these two adults are
husband and wife, their child might inherit a PKU gene from both of
them and have PKU, even though neither parent had it. However, there
is only 1 chance in 4 that the child would have the bad luck to get
the defective gene from both parents. Thus the probability that a child
of randomly chosen parents would have PKU is only 1 in 10,000
(1/2500 x 1/4), and that is approximately the percentage of people
with PKU in the population Of course the probabilities change radically if
one or both of a child's parents have PKU. If one parent has PKU and
the other parent is chosen randomly, there is a 1 in 50 chance that
the second parent will have a recessive gene for PKU. If the second
parent does have the recessive gene, the chances that their child
will have both genes for PKU is 1 in 2. It is certain that they will
inherit a PKU gene from one parent, and a 1 in 2 chance that they
will get one from the other parent. Thus the overall chance that the
child of one parent with PKU and one without will have PKU is 1 in
100 (1/2 x 1/50). If the second parent lacks the recessive PKU gene,
the child cannot inherit two PKU genes and therefore cannot have PKU,
but will certainly carry a copy of the recessive gene, inherited
from the parent with PKU. If both parents have PKU, then the child
necessarily inherits two defective genes and will have PKU. It is possible to test for the presence of the
recessive PKU gene in people who do not have PKU. People with PKU
who want to have children may, therefore, wish to seek genetic
testing for their partners before proceeding to have
children--perhaps before marrying. Although PKU can be managed, a
lifetime of dieting is expensive and troublesome, to say the least,
and not something that parents would wish for their children.
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