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Inheritance of Single-Gene Disorders

By David N. Finegold, MD

The traits produced by a gene can be characterized as dominant or recessive. Dominant traits can be expressed when only one copy of the gene for that trait is present. Recessive traits carried on autosomal chromosomes can be expressed only when two copies of the gene for that trait are present because the corresponding gene that is not for that trait on the paired chromosome is usually expressed instead. People with one copy of an abnormal gene for a recessive trait (and who thus do not have the disorder) are called carriers. With codominant traits, both copies of a gene are expressed to some extent. An example of a codominant trait is blood type. If a person has one gene coding for blood type A and one gene coding for blood type B, the person has both blood types (blood type AB).

Whether a gene is X-linked (sex-linked) also determines expression. Among males, almost all genes on the X chromosome, whether the trait is dominant or recessive, are expressed because there is no paired gene to offset their expression.

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