DNA (deoxyribonucleic acid) is a molecule that contains all of the genetic instructions needed to develop and maintain an organism. It resides in the nucleus of each cell, and carries the complete set of instructions for making all of the proteins a cell needs to function properly. Each type of cell uses only a part of the genetic information stored in DNA. Genetic material in DNA is responsible for all of the traits that are passed on from one generation to the next.
DNA exists as two long, paired strands forming the shape of a double helix. The information in DNA is written using four letters (nucleotides): A (adenine), T (thymine), G (guanine) and C (cytosine). In one DNA molecule there are millions of nucleotides arranged in a specific order. The order in which the nucleotides occur determines the information that is conveyed.
A gene is a region of DNA that contains a particular set of instructions that enables a cell to produce a protein. While the DNA provides instructions, proteins are the molecules that are responsible for nearly all aspects of cellular activity. It is estimated that humans have approximately 20,000 – 25,000 genes and the complete set of genes is called a genome. Every person has two copies of each gene: one inherited from the mother and one from the father.
Alleles are different versions of the same gene. Alleles can be present in one of two states: homozygous (alleles inherited from the father and from the mother are similar) or heterozygous (alleles inherited from the father and from the mother are different). An allele can be dominant (encodes a protein in the heterozygous state) or recessive (encodes a protein only in the homozygous state).
Different versions of the genes (alleles) arise from mutations or SNPs (single nucleotide polymorphisms). Mutations are rare (frequency < 1% in a population) changes in the DNA sequence, whereas SNPs are more frequent. Both mutations and SNPs can be harmful if they destroy the DNA sequence needed for encoding a functional protein. The functioning of a cell (and also the entire body) depends on a continuous interplay of thousands of proteins acting together in just the right amounts and in just the right places. If no protein or a non-functional protein is encoded due to a mutation or SNP, the activity of a cell can be disturbed, which in turn can lead to the development of a disease or result in an inability to metabolize or break down drugs normally.
Recently, more and more evidence regarding the associations of certain changes in DNA and different disorders have been found. Many diseases have their roots in our genes. Common disorders such as diabetes, heart disease, and most cancers are caused by the complex interaction between multiple genes and environmental factors.
Genetic testing means examining a patient’s DNA for mutations or SNPs linked to a disease or disorder. By detecting various DNA markers, genetic testing can help to diagnose or predict a disease.