HLA Typing

In 1975, Peter Doherty and Rolf Zinkernagel identified human leukocyte antigens (HLA). HLA are proteins found everywhere in the body except red blood cells. They are especially prevalent in white blood cells. Many types of HLA exist, often varying greatly from person to person. Doherty and Zinkernagel were awarded the 1996 Nobel Prize in Physiology or Medicine for this discovery.

Since many types of HLA exist, scientists can use HLA typing for genetic identification. They are able to compare the types of HLA in different people and determine if the people are related based on similarities between these proteins. HLA have become increasingly important for identifying positive matches between donors and recipients of bone marrow transplants.

The degree of variation among HLA in different people provides for fairly accurate paternity testing, with a power of exclusion from 80 to 90% when combined with blood typing and serological testing. The accuracy of HLA typing increases with the rarity of a tested person's HLA. However, this procedure is normally performed serologically, requiring a relatively large amount of fresh (no more than a few days old) blood. Additionally, the collection process can be considered uncomfortable, especially for children, and cannot be performed on infants under the age of 6 months.

RFLP Technique

In 1985, Sir Alec Jeffreys developed restriction fragment length polymorphism (RFLP), which quickly became the standard technique for DNA testing throughout the 1980s. RFLP provided the world with the first form of genetic testing based on DNA, the body's genetic material.

Each person inherits a unique combination of DNA from both biological parents, and this DNA serves as the code for all of that person's biological characteristics. By comparing the unique genetic code of one person to that of an alleged relative (for example, comparing a child to an alleged father), one can see whether or not the two people are biologically related.

Using RFLP, scientists cut specific portions of DNA into fragments for comparison. These fragments have different lengths, depending on the location of certain markers for cutting the DNA.

In a family relationship test, one person's DNA fragments are compared with those of an alleged relative. If the fragments match each other, they are considered to be from biologically related people.

In the case of a paternity test, a child's DNA fragments would be compared to the fragments of the mother and alleged father. Since the child received half of his or her DNA from the mother and the other half from the father, his or her DNA fragments should match those of both parents. Half should match the mother's, and half should match the father's. If the alleged father is not the child's biological father, the child's DNA will not match the father's.

In some RFLP cases, it will appear that the child's DNA does not match either parent's; this situation requires extra analysis. Occasionally, mutations in the DNA occur, causing a mismatch of fragments. When this happens, statistical analysis is needed to determine the likelihood of a mutation and the biological relationship between the family members.

Despite the periodic complication of genetic mutations in this procedure, RFLP is highly accurate, with a power of exclusion of 99.99% and higher.

RFLP is a highly accurate test. However, because it requires a large amount of blood and a long processing time, it is not used as frequently today as it once was.