Single-gene defects

[Marquetta]

Genetic disorders determined by a single gene (Mendelian disorders) are easiest to analyze and the most well understood. If expression of a trait requires only one copy of a gene (one allele), that trait is considered dominant. If expression of a trait requires 2 copies of a gene (2 alleles), that trait is considered recessive. One exception is X-linked disorders. Because males usually have no paired allele to offset the effects of most alleles on the X chromosome, the X chromosome allele is expressed in males even if the trait is recessive. Other exceptions, such as mitochondrial disorders, exist as well. Mitochondrial genes are typically inherited only from the maternal oocyte. Proteins active in mitochondrial structure and function originate both from mitochondrial genes and also from nuclear genes.
Many specific single-gene disorders have been described (see Table: Examples of Genetic Disorders With Mendelian Inheritance).
(See also Overview of Genetics.)