Introduction to Genetics

Genetics is the study of the control codes (DNA) that create and order our body. With regard to diseases, genetics refers to how much of a disease is built into our bodies. It also refers to how much of a disease is inherited from our parents. Any diseases in the genes in our DNA are with us from birth. However, there are several different types of diseases with genetic aspects:

• Genetic diseases: These diseases are caused by an error in a single gene in the DNA. These are "pure" genetic diseases. They are strongly inherited from parents, and usually have very clear patterns of inheritance. Some examples of well-known serious genetic conditions include cystic fibrosis, muscular dystrophy, and dwarfism (achondroplasia). See Introduction to Genetic Diseases.
• Chromosome diseases: These diseases are caused by a major error in the DNA, with an entire chromosome having a problem. Chromosomes have hundreds and thousands of genes, so these diseases have major errors in the DNA code. The most common example is Down syndrome. See Introduction to Chromosome Diseases.
• Polygenic diseases: This simply means "multiple genes". A polygenetic disease is under the influence of multiple genes, but not a single gene. Usually this means that a disease does not have a high level of genetic causes, and is not strongly inherited down families, but there may be a slight familial inheritance pattern. In fact, saying a disease is polygenic is almost like saying it is "mostly non-genetic". Many of the big name diseases are in this class of diseases including cancers, heart disease, autoimmune diseases, and many others. With most of these conditions, they are not regarded as being caused by genetics, nor are they directly inherited from parents. However, a family history of disease is a risk factor for the disease, indicating that there is some inherited risk in the genes. The genetics of this type of disease is an area of current research for all of the major diseases. See Introduction to Polygenic Diseases.
• Non-genetic diseases: Many diseases have no genetic basis at all. Usually a physical injury such as a bone fracture is not caused by genetics when it is caused by something else. (Of course, there could still be an underlying genetic cause of weak bones or osteoporosis that really caused the fracture.) Similarly, a virus or bacterial infection is caused by an external microbe and not by some internal error in your DNA.

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