Genetic Predispositions to Diseases

Happily, most children arrive in this world normal and healthy. By adulthood, children have had their share of colds, coughs, stomachaches, and other minor illnesses. Compared with the usual childhood ailments, hereditary diseases are rare, but they do occur, and all parents should be informed about the more common ones. Genetic counseling is advisable for parents who are in certain high-risk groups.

Central to any discussion of hereditary diseases is an understanding of the basis of heredity -- the gene. Genes are bits of chemical information that determine all of our inborn characteristics. They are carried in structures called chromosomes within the nucleus of all cells.

Genes are composed of varying arrangements of molecules of deoxyribonucleic acid (DNA), which makes up the chromosomes. Each gene, by virtue of its unique DNA sequence, holds the code for a specific trait.

Except for the sex cells (eggs and sperm), each cell in the body contains 46 chromosomes. Forty-four of the chromosomes are called autosomes. The two remaining chromosomes are the sex chromosomes.

The autosomal chromosomes are paired. For each pair, one chromosome comes from an individual's mother, the other from the father. Each gene on one chromosome is matched to a corresponding gene on the other chromosome. Thus, for every genetic trait, there are two genes.

The two genes that provide the code for a trait may not be identical. For example, if a gene pair governs eye color, one gene may code for blue eyes, the other for brown eyes. Because the gene for brown eyes is dominant, the eyes will be brown. The gene for blue eyes is recessive. A person with brown eyes may also have two genes for brown eyes. For a dominant trait to be expressed, however, only one dominant gene is necessary. A person with blue eyes must have two genes for blue eyes. To express any recessive trait, a double dose of recessive genes is necessary. A person with only one copy of a recessive gene exhibits no evidence of the trait.

Like eye color, certain illnesses are genetically determined. In some instances, the defective gene has been passed from generation to generation within a family by carriers who show no symptoms of the illness. Parents may be aware of a family history of hereditary disease. Whether their baby develops that illness depends upon the inheritance of dominant or recessive genes.

The better known genetic disorders include sickle cell anemia, Tay-Sachs disease, and cystic fibrosis, which are more prevalent among certain demographic groups. Other diseases include Phenylketonuria, autosomal dominant disease, hemophilia, muscular dystrophy and may include conditions like cleft lip, spina bifida and Down syndrome