Genetics

In the case of a recessive disease, if one abnormal gene is inherited, the child will not show clinical disease. However, each child of these parents will stand a 50% chance of inheriting the abnormal gene. If one abnormal gene produces disease, this is called a dominant hereditary disorder. In the case of a dominant disorder, if one abnormal gene is inherited from mom or dad, the child will likely show the disease.

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A person with one abnormal gene is termed HETEROZYGOUS for that gene. If a child receives an abnormal recessive disease gene from both parents, the child will show the disease and will be HOMOZYGOUS for that gene.

If two parents are each heterozygous for a particular recessive disease gene, then each child has a 25% chance of being homozygous for that gene and therefore, of showing the disease. If one parent is homozygous and the other heterozygous, then each child has a 50% chance of being homozygous.

GENETIC DISORDERS

Almost all diseases have a genetic component, but the importance of that component varies. Disorders where genetics play an important role, so-called genetic diseases, can be classified as single gene defects, chromosomal disorders, or multifactorial.

A single gene disorder (also called Mendelian disorder) is one that is determined by a single genetic locus and the specific allele on one or both members of a chromosome pair. Single gene defects are rare, with a frequency of less than 1 in 200 births. But since there are about 6,000 known single gene disorders, their combined impact is significant.

The incidence of serious single gene disorders is estimated to be about 1 in 200 births.

Single-gene disorders are characterized by the pattern of transmission in families -- this is called a pedigree. The term "kindred" includes the relatives outside of the immediate nuclear family. The affected individual that initially comes to light (or is of immediate interest) is called the proband. The brothers and sisters of the proband are called siblings.

There are only 5 basic patterns of single gene inheritance:

The observed effect of an abnormal gene (the appearance of a disorder) is called the abnormal phenotype. A phenotype expressed in the same way (in both homozygotes and heterozygotes) is dominant. A phenotype expressed only in homozygotes (or, for X-linked traits, expressed in males but not females) is recessive.

Heterozygotes for a recessive gene are called carriers. They usually don't express the phenotype clinically, but it can frequently be identified by sensitive laboratory methods.