A few patients have been described who, despite high blood levels of phenylalanine, have normal brain function. Using magnetic resonance spectroscopy, it has been shown that their brain concentrations of phenylalanine are normal, in contrast to the vast majority of patients with PKU, in whom brain concentrations are high. Some special features of the mitochondrion in biochemical genetics are of note. Understanding of the biology and genetics of the human mitochondrion has in part aided by newly discovered mutations in the mitochondrial genome which cause human disease. The mitochondrial genome relates to the function of oxidative phosphorylation carried out in the mitochondrion.
Several distinctions make the mitochondria of interest in the study of human genetics.
Mitochondrial DNA is very compact. All have been shown to code for components of the oxidative phosphorylation pathways within the mitochondria. There are little or no intervening sequences and no 3' o5 5' untranslated regions. Mitochondrial genes are maternally inherited, since they are transmitted from parent to child via the cytoplasm of the oocyte. The conditions thus caused demonstrate novel patterns of inheritance which are exceptions to the Mendelian patterns.