Genetic diseases are diseases caused by changes in the genetic material, which are entirely or partially determined by genetic factors and are usually transmitted vertically between the upper and lower generations in a certain way. Although the genetic material is present in the cells of the human body, they can be transmitted to the offspring in a certain way only through the union of germ cells of both sexes. It is important to note that it is the genetic material (also called genetic information) that is passed from parent to offspring, and not the genetic disease itself. The genetic information is passed on to the offspring through the germ cells, and the offspring will grow and develop according to this information before expressing the trait or clinical symptoms of the genetic information, which is the process of expression of the genes related to the genetic disease. Therefore, genetic diseases have the following characteristics: 1. A certain percentage of incidence is found among individuals who are related by blood. 2. There is generally no incidence among individuals who are not related to each other although they belong to the same family, such as spouses, aunts and uncles. 3. Each disease has a specific age of onset and course. 4. The incidence of disease is higher in monozygotic than in heterozygotic individuals because monozygotic individuals have more of the same genetic material. Etiologically, genetic diseases can be classified as chromosomal, monogenic, polygenic, mitochondrial genetic and somatic genetic diseases. In fact, however, numerous studies in genetics and molecular biology have shown that human diseases are almost always associated with environmental factors and genes, except for trauma. Many disease-specific genetic abnormalities have now been identified in humans, distinguishing pathogenic genes, susceptibility genes, and protective genes according to their relationship to clinical disease pathogenesis. Pathogenic genes are rarely allelic, have strong epistasis and strong effects (deletion of genes or abnormal nucleotide repeat expansions, etc.). In contrast, disease susceptibility genes usually have alleles, with weak effects of the genes themselves but with cumulative and epistatic effects (polymorphism of genes). The risk of disease in an individual determined by the genetic basis is called susceptibility, which can also be interpreted as the risk of disease in different individuals under the same environment, determined exclusively by genes; whereas in polygenic genetic diseases, the combination of genetic basis and environmental factors determines the magnitude of the likelihood of disease in an individual is called susceptibility, and under certain The susceptibility represents the number of disease-causing genes accumulated by an individual under certain environmental conditions.