When you find irregular familial occurrence of a qualitative trait, the reason usually a combination of several single gene variations and environmental factors such as properties, for example. Congenital pylorus-stenosis (narrowing of the lower abdomen mouth), club foot and cleft lips with or without cleft palate, shows like a large variation in severity. They are also more frequent in the population than either single ticket diseases. In each family, there will not be so many with the disease as it is in families with single ticket disease.
Finally, one can prove the method of family how much of the variation of a quantitative trait, for example. Body height or intelligence, which is due to inheritance. Eugen Fischer showed in 1918 that the sum output of many individual genes, which we must assume is the specific allele in polymer systems, provides a quantitative normal distribution when each allele contributes moderately to the inheritance (polygenic or multifactorial inheritance).
Family studies can thus give us help when we want to estimate how much of the variation between individuals is due to polygenic inheritance, and how much is due to environmental factors. An example is that we are differently predisposed to ulcers. Some family data suggest that 2/3 of the variation due to environmental factors, and the third is due to polygenic genetic variation. For this polygenic disposition may be the height of 5% is attributed to the two known one-way systems: secretor vileness and AB0-blood group system.
People of blood type 0 (zero) has approx. 1.4 times as likely to develop ulcers in the duodenum that people with blood type A, B or AB. A person with blood type 0 that cannot secrete blood group substance in saliva, and gastric juice (nonsecretor) have a risk of duodenal ulcer, which is 2.5 times the risk of other types of people. Which gene's predisposing seems you can find on the determination of whether there is association (see below) between a single ticket system, and polygen affected property. When it comes to the disposal of the disease, such as increased disposition to develop cancer, it made great progress. When breast cancer is mutations in one of two genes that can cause an inherited disposition. Proven carriers will be followed up prevention. The same applies to family history of colon cancer.
The twin method was used already in the 1880s by Francis Galton. The method can be used for a relative assessment of inheritance versus environment causal factors. The twin method can, however, little or no extent, tell us something about the genetic mechanisms underlying the phenomena.