special subjects
Single Gene Defects
Introduction
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 DNA Abnormalities , 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. (See also Many specific single-gene disorders have been described. Gene Gene Dominant Dominant Recessive Recessive Non–X-linked Marfan Syndrome None Huntington Disease None Cystic Fibrosis None X-linked Hypophosphatemic Rickets None Alport Syndrome None Red–green color blindness Hemophilia None 