A genetic disorder is a genetic problem caused by one or more abnormalities formed in the genome. Most genetic disorders are quite rare and affect one person in every several thousands or millions. The earliest known genetic condition in a hominid was in the fossil species Paranthropus robustus, with over a third of individuals displaying Amelogenesis imperfecta.
Genetic disorders may be hereditary, meaning that they are passed down from the parents' genes. In other genetic disorders, defects may be caused by new mutations or changes to the DNA. In such cases, the defect will only be passed down if it occurs in the germline.
Some types of recessive gene disorders confer an advantage in certain environments when only one copy of the gene is present.
In the USA, 50% of the heart disease cases come from 5% of the families. In a study of 25,000 families in Texas, only 3,000 (12%) of them were seriously at risk of heart disease, strokes or cancer.