What is a genetic disease?
A genetic disease is due to an error in the DNA sequence. This error (called a mutation) may lead to a lack of building blocks of the body or a deficiency in important reactions of the metabolism. In each cell there is a double copy of DNA (one coming from the mother, one from the father). Sometimes an error in one copy is sufficient to give a disease, it is a DOMINANT disease. More often the disease only appears if the two copies (paternal and maternal) have the same mutation, it is a RECESSIVE disease. The mutations are transmitted to sperm and egg cell and consequently to the descendance.
What is a DNA sequence and a mutation?
The DNA is a large molecule comprised of four basic units linked together in a long strand. If we assign each unit a letter, then we can envision the DNA as a long string of letters. Genes can be thought of as long words scattered along . These words have special meaning to the worker molecules in the cell, and encode a set of instructions as to what they are to build. Any mistake in a word scrambles the instructions, and either the workers in the cell make no product or a defective one is made. These mistakes in genes are known as mutations.
How does a mutation appear?
Mutations are a natural consequence of life. Every time that a cell divides it has to make a complete copy of its DNA so that the two progeny cells each have a full DNA complement. This copying involves the faithful reproduction of a sequence containing three thousand million bases. The machinery that cells have evolved is incredibly efficient, containing systems that attempt to identify and correct any mistakes that might occur, but it is not absolutely perfect and copying errors do occur. In each sperm or egg there are 100 neomutations with 3 or 4 very harmful if in both DNA copy.