Polygenic and mitochondrial disorders

Polygenic disorders come from many genes acting together, along with lifestyle and environment (diet, exercise, stress). So they do not follow simple single-gene patterns. Examples: high blood pressure, coronary heart disease and diabetes.

Diabetes mellitus means blood sugar stays too high.

• Type 1 (about 10% of cases): the body's own immune system destroys the beta cells of the pancreas, which make insulin. It needs lifelong insulin.
• Type 2 (about 90%): the pancreas makes less insulin, and the body's cells respond poorly to it (insulin resistance).

Hypertension is blood pressure of 130/80 mm Hg or more (stage 2 at 140/90 or more). It raises the risk of kidney disease, heart disease and stroke. Coronary heart disease is the narrowing of the arteries that feed the heart, by fatty deposits on their walls (atherosclerosis). The heart muscle then gets too little oxygen (ischaemia).

Mitochondrial disorders. Mitochondria have their own small circular DNA, with genes for enzymes that make energy. A fault there reduces ATP production. It harms most the organs that need the most energy: brain, heart, muscle, kidney and liver. How severe it is depends on what share of a cell's mitochondria are faulty.

The sperm gives the egg almost no mitochondria, so a mother passes her mitochondria, and any mitochondrial disease, to all her children. An affected father does not pass it on.