Because of genetic testing, hereditary or genetic disease patients often know early on that they will be terminally ill. A test can predict the onset of a hereditary disease. Marriage among blood relatives is the cause of hereditary diseases in several societies. Every human has about 6 defective genes. But he/she also has two versions of each genetic material. So usually there is still a healthy gene that makes up for the defects in its sick double. Only when the genetic material on both copies in a gene, i.e., in a similar place, affected, a certain function cannot be performed, and people are suffering from a genetic or hereditary disease. For example, in patients with a recessive hereditary disease such as cystic fibrosis, both parents each carry a sick gene but are healthy because they still have a healthy gene. The children have a high probability (25 percent) to get the disease. People who are related have a similar genetic substance. Siblings, for example, always have 50 percent of the same genes. Cousins have over 10 percent similar genes. The consequence: the likelihood of the same defective genes occurring in children with related parents is greatly increased. People from societies that traditionally marry within the extended family experience genetic diseases much more frequently than in other populations.