DNA (deoxyribonucleic acid) is the hereditary material that contains the genetic information inherited by the offspring of an organism. It is a long molecule consisting of repetitive units of Adenine (A), Guanine (G), Cytosine (C) and Thymine (T). In human beings, it is arranged in the form of 23 pairs of chromosomes. Each pair comprises one chromosome from the father and the other from the mother.
A gene is a fragment of DNA that determines the physical appearance like height, hair, or eye color as well as dictates the various physiological processes in each human being. Sometimes it also determines what diseases a person is likely to get. A gene controls a hereditary characteristic and is responsible for the genetic transmission of most inherited traits.
A majority of the genes are the same in most people, but a small percentage of genes show differences among different people. Except for identical twins, all individuals show minor differences in the genetic information contained in their DNA (More than 99.9% of the DNA is same between any two individuals) and it is these small differences that are responsible for the uniqueness of every individual.
Genetics is the study of genes, heredity, and the variation of organisms. It determines most of the physical appearance and physiological properties of all organisms, including human beings. It accounts for both the similarities and differences between different organisms.
A genetic test examines the variations in an individual’s genes. It determines the genetic predisposition that a person has of developing a disease based on the DNA information of that person.
The Human Genome project was started in 1990 with the aim of identifying all the genes in the human DNA and determining the sequence of the approximately 3 billion nucleotides of which the human genome is comprised. After the release of the first draft sequence of the human genome in 2000, there has been an increased effort to look at this sequence information of different individuals in order to determine links between DNA sequence and disease. Studies have shown that a majority of the sequence is the same between any two human beings and only a very small fraction of the DNA shows differences in sequence. Knowledge about these differences in the DNA of different individuals can be used to obtain new methods for diagnosis, treatment and prevention of these diseases. The different variations of a single gene are called alleles. Alleles are responsible for the differences in the physiological and physical properties of each individual. Different alleles of a single gene are called gene variants or genetic variations. Basically, a genetic variation is the change in the sequence of bases in the DNA.
A genetic variant that occurs when there is a difference or change in a single base / nucleotide (A, T, G or C) within different members of a species is termed a single nucleotide polymorphism (SNP). When a single base change occurs in a population at a frequency of >1% is termed a SNP, while a base change that occurs at a frequency of <1% is considered to be a mutation. Since SNPs are inherited from one generation to the next without much change (ie. they are evolutionarily stable) and about 90% of all human genetic variations are SNPs, they extremely valuable in biomedical research. SNPs themselves do not cause diseases but they can help in determining the probability that an individual will develop a disease.
A genetic disorder is a disease caused due to variations in one’s DNA sequence.
A genetic test examines an individual’s DNA in order to look for variations in his genes. It determines the genetic predisposition that a person has of developing a disease based on the DNA information of that person.