Monogenic Factors

Mitochondrial Inheritance. Mitochondria are cytoplasmic organelles that contain extranuclear DNA (mtDNA). Mitochondria are responsible for certain oxidative metabolic functions that store and release energy. Children (both males and females) inherit mtDNA from their mothers only. Since the father's mtDNA is located in the sperm midpiece (the mitochon-drial sheath), which is lost at fertilization, all children of the same mother are hemizygous for maternal mtDNA and are thus identical to each other and to their mother. Because of its cytoplasmic location in eukaryotes, mtDNA does not undergo meiosis and there is normally no crossing-over, hence there is no opportunity for intro-gression of the father's mtDNA. All mtDNA is thus inherited maternally; mtDNA has been used to infer the pedigree of the well-known "mitochondrial Eve." In mammals, the mtDNA ring contains some 16,000 base pairs and codes for 13 proteins, 22 tRNAs, and two rRNAs. Mutations in mtDNA can result in maternally-transmitted conditions that compromise cell respiration, such as mitochondrial encephalopathy lactic acidosis syndrome (MELAS) and other mitochondrial myopathies.

Penetrance and Expressivity. Penetrance is the frequency of expression of a certain phenotype; some alleles, even when present, are expressed less than 100% of the time, and thus are said to have a lowered pene-trance. Examples of variable penetrance conditions include asthma, diabetes, dyslexia, and PTC tasting. Expressivity is the range of phenotypes exhibited by a certain specific genotype as a variant from the normal. Examples of variable expression include familial adenomatous polyposis and long-QT syndrome.

These ambiguous and perhaps outdated concepts may soon be supplanted by other terminology as newer epigenetic factors, such as genomic imprinting and anticipation, are discovered and described.

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