Diseases that affect a much smaller number of people than the general population are called “rare diseases” and, for example, in Europe, a disease is considered “rare” if it affects 1 person in 2000. Diseases may be more common or rare depending on the region/geography where they occur. To give an example, Thalassemia (Mediterranean anemia), which is a common single gene disease in our country, is among the diseases that are common in the Mediterranean region and rare in Northern Europe.

There are thousands of types of rare diseases, over 7000 have been identified so far, and this number is increasing rapidly every day thanks to new diagnostic methods.

Almost all genetic diseases are grouped under the title of “rare disease”, but not every rare disease is a genetic disease. Rare diseases are often serious, chronic and progressive diseases, and the etiological origin of many of them is not yet known. While some of them can be diagnosed at birth, the findings of some of them may appear at later ages. For example, single gene diseases such as spinal muscular atrophy (SMA), metabolic diseases (such as osteogenesis imperfecta, enzyme deficiencies), and diseases with severe malformations (such as chromosome diseases, skeletal dysplasias) show symptoms in infancy and early childhood. Polygenic multifactorial diseases (such as Crohn’s disease, amyotrophic lateral sclerosis), Huntington’s disease, and rare cancers are examples of rare diseases diagnosed in the adult age group.

Screening programs have been created that show differences and similarities between countries depending on the frequency of rare diseases. The rare diseases for which screening is performed in the newborn period in our country are: phenylketonuria, congenital hypothyroidism, biotidinase deficiency, cystic fibrosis, congenital adrenal hyperplasia. These diseases are enzyme/metabolism/hormone diseases that occur due to a single gene defect. The earlier these diseases are diagnosed, the better the child’s development will go. In addition, thanks to genetic diagnosis, preimplantation genetic diagnosis (PGT) or prenatal diagnosis opportunities may arise in couples who have a family history of this group of diseases and are planning a child, and healthy child / healthy embryo selection can be made. Expanded newborn screening panels aim to screen for more rare diseases (especially metabolism/enzyme diseases). The rare diseases that are well-known and frequently encountered in our country are summarized in Table 2, and the majority of them are single gene diseases.

Candidate couples are also screened for certain diseases before marriage. Although these are rare diseases all over the world, they are single gene diseases that are common in our country and have high carrier rates. Couples who are carriers of these diseases are referred to Medical Genetics Polyclinics and PGT or prenatal diagnosis options are evaluated (thalassemias, spinal muscular atrophy).