Genetic diseases are those that involve changes in genetic material, that is, in DNA. 
Some of them may have a hereditary nature, being passed from parents to children.
However, not all genetic diseases are hereditary. An example is cancer; it is caused by changes in genetic material but is not transmitted to descendants.
There are three types of genetic diseases:
• Monogenic or Mendelian: When only one 
gene is modified.
• Multifactorial or polygenic: When more than one gene is affected and there is also interference from environmental factors.
• Chromosomal: When chromosomes undergo modifications in their structure and number.
For example, we know that the human species has 23 pairs of chromosomes; if there is a lack or excess of one of them, a genetic disease arises.
What are the most common genetic diseases?
The main genetic diseases in humans are:
• Down Syndrome: Down syndrome is a genetic alteration caused by the presence of an extra chromosome in pair 21.
• Sickle Cell Anemia: Sickle cell anemia is a genetic alteration with hereditary characteristics, in which red blood cells lose their normal shape and take on a sickle shape.
• Diabetes: Diabetes is a disease that compromises the production of the hormone insulin, responsible for controlling blood glucose levels.
• Cancer: Some types of cancer are caused by genetic factors. The term represents a set of diseases characterized by uncontrolled cell growth that can spread throughout the body.
• Color Blindness: Color blindness is a sex-linked inheritance, specifically on the X chromosome. It is characterized by the inability to distinguish colors.
Some genetic diseases are extremely rare, meaning they have a low incidence in the population. Some of the most well-known include:
• Duchenne Muscular Dystrophy
• Cystic Fibrosis
• Patau Syndrome
• Turner Syndrome
• Albinism
• Phenylketonuria
• Progeria
• Hypertrichosis
Source: www.todamateria.com.br