Birth Defects

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  • Topic: Chromosome, Congenital disorders, Sex linkage
  • Pages : 7 (860 words )
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  • Published : October 8, 1999
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Birth defects, or congenital

malformations, are the faulty formation of

structures or body parts present at birth.

Sporadic, hereditary, or acquired defects may be

immediately observed or may become manifest

later in life; they may be visible on the body

surface or present internally. Birth defects may be

life threatening and require surgical correction, or

they may interfere with function or appearance. It

is estimated that about 3% of all children are born

with major defects; minor defects or variations are

estimated to occur in 10% to 15% of births.

Malformations may be single or multiple. Multiple

malformations that occur in a regular recognizable

pattern are referred to as syndromes--for

example, the FETAL ALCOHOL SYNDROME

sometimes observed in infants of mothers who

drank heavily when pregnant. Birth defects may

result from the action of genes, chromosomes, or

the environment on the developing fetus, but often

the cause cannot be determined. Inherited Defects

Abnormal genes cause a significant number of

different birth defects. Some can be identified as a

single-gene disorder that is inherited in a simple

Mendelian mode, that is, either a dominant or a

recessive pattern. For example, lobster claw

deformity of the hands and feet (split hands or

feet) is inherited and results from the effect of a

single dominant gene. A person who has this

deformity runs a 50% risk (1 in 2) of bearing

offspring who will inherit the gene and will

therefore also be affected. Autosomal recessive

inheritance and X-linked recessive inheritance

account for the other forms of single-gene

inheritance that cause birth defects. In cases of

autosomal recessive inheritance, both parents are

normal but each carries a silent, or recessive, gene

that, if matched in an offspring, causes the birth

defect. Because both parents are so-called

carriers (heterozygotes) of the same abnormal

gene, they run a 25% risk (1 in 4) of having a child

with the birth defect caused by that particular

gene. Examples of birth defects inherited in this

autosomal recessive manner are TAY-SACHS

DISEASE and SICKLE-CELL ANEMIA. In

cases of X-linked recessive inheritance the

abnormal gene is located on the X chromosome.

The normal mother has two X chromosomes, one

of which carries the gene for the abnormal

condition; but if her son inherits her X

chromosome with the abnormal gene, he will be

affected with the condition. HEMOPHILIA is

inherited in this matter. Multifactorial Defects

Many common birth defects do not occur in a

pattern that indicates simple Mendelian

inheritance. They seem to result from an

interaction of genes and the environment, including

the intrauterine environment, and each factor

includes a number of different hereditary and

environmental influences; hence, these defects are

called multifactorial. Among them are congenital

heart disease; neural tube defects, including

SPINA BIFIDA, myelomeningocele, and

anencephaly; and CLUBFOOT, CLEFT LIP

AND PALATE, and dislocated hips.

Chromosome Number An increase or decrease in

the total chromosome material can cause birth

defects. For example, the additional chromosome

material in DOWN's SYNDROME (mongolism)

caused by an extra chromosome, number 21, is

responsible for the characteristic mental

retardation, short stature, and facial appearance.

The gain or loss of chromosome material may

involve a partial or entire chromosome. The

specific birth defects that occur depend on the

chromosome involved as well as on the amount of

loss or gain of chromosome material.

Environmental Factors Environmental causes of

birth defects include teratogenic (literally,

"monster-making") agents and physical

abnormalities in the mother's uterus. Certain

medications and chemicals as well as alcohol...
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