Except for identical twins, variations in just a small fraction of our DNA account for the major ways in which one individual is different from another. These small variations in DNA are called SNPs, which stands for ‘single nucleotide polymorphisms’.
Simply, we can think of DNA as a string of 3 billion gene letters, in which the sequence of gene letters follows a precise order. If we compare the DNA sequence of any two people, we sometimes see variation in a letter. For example, at a specific position along the string of DNA, one person may have the letter “A“ whereas another person may have the letter “T”. It's those very few differences in our DNA that create the diversity we see around us: different skin color, hair color, eye color and so on.
So if we would run along the chromosome, every thousand or so bases we would find a site with a gene letter difference. Scientists have identified about 3 million of those locations. They are just random variations that occurred by a mutation some time ago.
Most of these variants are not of much consequence because only a small part of the hereditary material contains the actual blueprint for an organism. The rest—about 98%—is often referred to as “junk”. This is a bit unfair, as the other areas of the DNA might have other functions which are not yet known.
If, however, the variants are located in the genes, then the information content of these genes is almost always altered. Figuratively speaking, the genetic material could be compared to the brochure for a piece of furniture in which the assembly manual and many pages of advertising are contained. If there is a misprint in the pages with the advertisements, the assembly of the piece of furniture is not affected. However, a misprint in the actual assembly manual can cause parts of the piece to be assembled in another way. Thus a variant of the piece of furniture is produced. For genes the same holds true – the protein may be either not produced at all or it may be assembled defectively, so that it no longer functions properly.
The lasting changes in genes can cause diseases, or at least they can lead to an altered susceptibility to diseases. For example, certain genetic variants in the blueprint for the blood pigment hemoglobin cause the disease sickle cell anemia. At the same time, however, the affected people are also more resistant towards the malaria pathogen. (
"Malaria: Gene Variants - Out of Africa").
Furthermore, these letter variations seem to explain why people react differently to different types or amounts of medicines. For example, patients can react differently to the same heart medication, such as a “beta-blocker”. Since SNPs can affect the structure and function of proteins and enzymes, they can influence how efficiently a medicine is absorbed and metabolized.
A major goal of the National Genome Research Network is to use the science of the genetic variants to determine which drugs are most suitable for any given patient. This way it might be possible to develop tailor-made therapies.
The information given by the genetic variants also promises to revolutionize the process of finding the position of disease genes in our hereditary material, since genetic variants serve as “mapping criteria” along the DNA strand. By means of these recognition sites, a specialist can find out relatively easily which area of the DNA was inherited from which parent. This is important in the quest for disease genes: Testing for genetic variants can identify areas of an individual’s genetic make-up that may be functioning less than optimally and thus may predispose to diseases.
