Genetic testing is mainly used for the diagnosis of genetic diseases. The main genetic diseases diagnosed by genetic testing technology are monogenic genetic diseases and mitochondrial genetic diseases. To clarify this issue, it is necessary to understand the basic concepts of genes, genetic testing and genetic diseases. Genes are DNA and RNA sequences that carry genetic information, i.e. DNA and RNA fragments with genetic effects, also called genetic factors. Genetic testing is a technique for testing DNA from biological samples such as blood, other body fluids, and tissue cells. Genetic diseases are diseases caused by alterations in the genetic material of a person. Human genetic diseases can be classified as chromosomal diseases, monogenic genetic diseases, polygenic genetic diseases and mitochondrial genetic diseases. Different types of genetic diseases are diagnosed using different methods, such as cellular karyotyping to diagnose chromosomal diseases and genetic testing for genetic diagnosis of monogenic genetic diseases. There are more than 8,000 single-gene genetic diseases, of which more than 5,000 have been found to have causative genes. Genetic testing allows genetic diagnosis and prenatal diagnosis of these genetic diseases, as well as identification of carriers of disease-causing genes in the family and analysis of the risk of recurrence of genetic diseases. Common monogenic genetic disorders such as phenylketonuria, methylmalonic acidemia, hereditary leukoencephalopathy, epilepsy, spinal muscular dystrophy and progressive muscular dystrophy can be genetically diagnosed and prenatally diagnosed through genetic testing.