The Genetic Basis of Identity

DNA technology has many practical uses. Because every individual has a unique DNA sequence, DNA samples can be used for identification. The legal system is now using DNA evidence to determine guilt or innocence. The application of DNA technology began in Britain in the mid-1980s and appeared in America shortly afterward. Today many societies have reached the point of compiling DNA databases of known criminals—especially serious offenders. However, the most frequent use of DNA evidence is actually in cases of unknown or disputed paternity.

Identity can also be established in other ways. Just a casual glance reveals major differences among people. Geneticists refer to this outward appearance as the phenotype . Most physical differences between people are due to complex interactions of several genes during development. Some are obvious at a glance; others require close observation. Fingerprints are the classic example of a phenotype used in law enforcement. They are due to variations in the pattern of dermal ridges, small skin elevations on our fingers. Fingerprint patterns depend on more than one gene (i.e., they are multigenic ). This creates the huge genetic diversity underlying this phenotype. Although you might expect the fingerprints of identical twins to be the same, they are not identical. Minor variations in fingerprint patterns occur as a result of environmental factors affecting development. Fingerprints were being used for identification by the late 1800s.

Retinal scans provide a more high-tech form of unique identification. These take advantage of the unique pattern of blood vessels on the retina at the back of the eyes. Scanning typically takes about a minute, because several scans are needed. The subject must place the eye close to the scanner, keep the head still, and focus on a rotating green light. Infrared light is used for scanning because blood vessels on the retina absorb this better than the surrounding tissues. A computer algorithm is then used to convert the scan into digital data. There is about 10-fold more information in a retinal scan than in a fingerprint.

Moving to the molecular level, another set of identifying features are the proteins and polysaccharides made by all cells. Good examples of individual differences that involve proteins are the various blood types found in human populations. But for ultimate identification at the molecular level, we must examine the genes themselves to determine the genotype. This is what is meant by DNA typing or DNA fingerprinting , a technique that is described later.


All kinds of body tissues and fluids may be used to establish identity. Although DNA technology is relatively new, it is a logical outgrowth of the work on blood typing that has been used in the courtroom for more than 50 years. Although blood analysis is most common, other body fluids such as sweat, tears, urine, saliva, and semen also have cells with surface proteins that can be analyzed.

The membranes of red blood cells contain several proteins and lipids with attached carbohydrate portions that are exposed on the outside of the cell. These are highly antigenic, and in blood typing, they are referred to as blood antigens . Binding of an antibody to the corresponding antigen is highly specific. Consequently, two related blood antigens with only relatively small shape differences can be told apart because different antibodies will bind them.

Several groups of blood antigens are routinely used in identification. The best known is the ABO blood group system. Three different glycolipids, A, B, and O, are involved. These consist of different carbohydrate structures attached to the same lipid. The A antigen is made by adding N -acetylgalactosamine to the end of the O antigen, and the B antigen by adding galactose. Antibodies are made against the A and B antigens, but the shorter O “antigen” is poorly antigenic and invokes little antibody production. The closely related enzymes that make the A and B antigens are encoded by different alleles of the same gene. Absence of this enzyme gives the O allele.

Thus there are three alleles—A, B, and O—present in the population. Because we all have two copies of each gene, we all have two alleles for the ABO system. These may be identical or different in any given person. The alleles for the A and B antigens are both dominant; therefore, if at least one allele for either A or B is present, that antigen will be expressed. A person with one A and one B allele will express both antigens on the surfaces of his or her blood cells (AB blood group).

People do not make antibodies against those antigens present on their own red blood cells. Each individual makes antibodies against foreign antigens, that is, those present only in other individuals. Consequently, people with type A blood will express antibodies against


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