Mendelian or monogenic inheritance, is caused by changes or mutations that occur in the DNA sequence of a single gene. There are more than 6,000 known single-gene
disorders, which occur in about 1 out of every 200 births. Some examples are cystic fibrosis, sickle cell anemia, Marfan’s syndrome, Huntington’s disease, and hemochromatosis.
Single-gene disorders are inherited in recognizable patterns: autosomal dominant, autosomal recessive, and X-linked. Multifactorial inheritance, aka complex or
polygenic inheritance, is caused by a combination of environmental factors and mutations in multiple genes. For example, different genes that influence breast cancer
susceptibility have been found on chromosomes 6, 11, 13, 14, 15, 17, and 22. Some common chronic diseases are multifactorial disorders. Examples include heart disease, high
blood pressure, Alzheimer disease, arthritis, diabetes, cancer, and obesity. Multifactorial inheritance also is associated with heritable traits such as fingerprint patterns, height, eye
color, and skin color. Chromosome abnormalities, can cause disease, for example, Down syndrome or trisomy 21 is a common disorder that occurs when a person has three
copies of chromosome 21. There are many other chromosome abnormalities including Turner syndrome (45,X), Klinefelter syndrome (47, XXY), the cat cry syndrome (46, XX or XY,
5p-), and so on. Mitochondrial inheritance is caused by mutations in the nonchromosomal DNA of mitochondria. Mitochondria are small round or rod-like organelles that are
involved in cellular respiration and found in the cytoplasm of plant and animal cells. Each mitochondrion may contain 5 to 10 circular pieces of DNA. Examples of mitochondrial
disease include an eye disease called Leber’s hereditary optic atrophy; a type of epilepsy called MERRF which stands for Myoclonus Epilepsy with Ragged Red Fibers; and a form
of dementia called MELAS for Mitochondrial Encephalopathy, Lactic Acidosis and Stroke-like episodes. Mitochondrial ineritance comes exclusively from the mother because only the
mother’s mitochondria are passed on to the child.