Y-STR is a relatively new term for geneticists, becoming common in DNA literature in the second half of the 1990s. The Y stands for the male chromosome, which, as a reproducing cell, does not recombine. STR is an abbreviation for short tandem repeat. Compared to other polymorphisms (different forms of the same trait), STR refers to faster-mutating polymorphisms such as microsatellites.
For an unknown reason, short sequences of DNA repeat themselves, with the number of repeats varying. The analysis of diverse polymorphisms with different rates of mutation and different numbers of repeats allows analysis of nonrecombining parts of the Y chromosome. This has helped provide more knowledge of human evolution of males just as mtDNA analysis provides knowledge about female evolution.
Analysis of the Y chromosome has grown dramatically in recent years, along with a large increase in the number of polymorphic markers (an identifiable piece of DNA) available for analysis. There are now about 150 Y-STR loci that may provide analytical information, although only a small number is usually used because of economic reasons.
The use of polymorphisms for genetic analysis became available after Kary Mullis, in 1983, discovered the way to get a very small section of DNA containing a gene or a fragment from a genome to reproduce itself exponentially, thus being able to provide a sufficient amount of DNA for analysis. Mullis received a Nobel Prize in Chemistry in 1993 for developing this method, the polymerase chain reaction (PCR).
From an academic perspective, geneticists and anthropologists have applied this new knowledge to study the origin, dispersal, and distribution of modern humans, the sequential settlement of Polynesia, and numerous other similar topics. From an applied perspective, forensics and genealogy have become major areas of research. The technique for forensic DNA fingerprinting was introduced in 1984 by Alex Jeffreys. For both forensic and genealogical research, a number of positions (markers) on the DNA chain are tested, and a number is provided for each marker on the basis of the number of STRs. A list of numbers is referred to as a haplotype, regardless of the number of markers used. The more markers two men share, usually the closer they relate genetically.
Forensics frequently has used only six markers, which generally means that only one man out of every one million men randomly chosen will have a specific haplotype. The selection is similar in principle to most lottery drawings. This technique is especially good for excluding some suspects. However, most geographical areas do not have a random distribution of haplotypes, so there has been a push to increase the number of markers used. In genealogical research, a much larger number of markers is used. There is a nine-marker Y-STR database, maintained by the Institute of Legal Medicine in Berlin that, near the end of 2004, had more than 26,000 male DNA samples from more than two hundred sites in different parts of the world. Commercial companies now test men for their haplotypes, for genealogical purposes. In the United States, the largest firm is Family Tree DNA in Houston, Texas, which tests 37 markers.
References:
- Bosch, E., et al. (1999). Variation in short tandem repeats is deeply structured by genetic background on the human Y chromosome. American Journal of Human Genetics, 65, 1623-1638.
- Institute of Legal Medicine. (2004). Y chromosome haplotype reference database. Retrieved November 21,2004, from the Berlin, Germany Institute of Legal Medicine, Charite, University of Medicine, Berlin
- Kleiman, Y. (2004). DNA and tradition: The genetic link to the ancient Hebrews. New York: Devora.
