Up until the beginning of the last century it was generally believed that in humans sex was determined by environmental factors such as maternal nutrition. With the discovery of the human X and Y chromosomes in the early 1920s (Painter, 1923) it was first assumed that the number of X chromosomes determined the sex of a human individual. It took an additional 36 years to establish that sex determination in humans and other mammals is independent from the number of X chromosomes and that the presence of the Y chromosome is responsible for the male sex (Jacobs and Strong, 1959). During more than three decades of continued, intensive research into the molecular basis of human male sex determination, a series of putative candidate sequences on the human Y chromosome have been established, e.g. simple repetitive sequences (Epplen et al., 1983; Kiel-Metzger et al., 1985). Finally in the early 1990s the so-called 'sex determining region Y' or SRY gene (Sinclair et al., 1990) was identified and it was subsequently shown that the transfer and expression of SRY in female mouse embryos led to the development of testicles (Koopman et al., 1991). Today it is generally accepted that the SRY gene expresses the testis-determining factor and is the key gene responsible for male sex formation in humans and other mammals (Berta et al., 1990).
These (and other) discoveries in human genetics opened the door for forensic DNA-based human sex identification. In the 1970s luminescence microscopy was used for detection of the human Y chromosome in forensic material (Pearson and Bobrow, 1970; Radam and Strauch, 1973). Later, advances in molecular genetics allowed more sensitive detection of the Y chromosome using, for example, Y alphoid DNA (Stalvey and Erickson, 1987). The breakthrough for DNA-based sex identification in forensics came with the introduction of the polymerase chain reaction (PCR) for the sensitive detection of various regions on the human Y chromosome (Witt and Erickson, 1989), including the amelo-genin gene (Akane et al., 1991).
Although the detection of DNA from the non-recombining region of the Y chromosome identifies the presence of male material, not detecting Y-specific DNA does not mean that a sample contains only female material. This is because a negative result in a Y chromosome DNA test can have other reasons than there being no Y chromosome present in the sample being investigated, e.g. technical failures, no amplifiable DNA, etc. For this reason, combined tests to detect both Y-chromosomal and X-chromosomal DNA were developed. Of these, the amelogenin gene test has become the most established in forensic laboratories (Akane et al., 1991). The amelogenin gene is present on both the X and the Y chromosome and the test is based on a length polymorphism within the gene itself differentiating the Y-chromosomal from the X-chromosomal copy. Nowadays this test is included in many commercial kits for the DNA-based identification of human individuals. However, it should be noted that the reliability of the test has been criticized due to the occurrence of Y-chromosomal deletions that can include the amelogenin gene (Santos et al., 1998). Although the frequency of such deletions is generally low, their incidence can be increased in certain populations due to events in the population history (Santos et al., 1998; Steinlechner et al., 2002; Thangaraj et al., 2002). Furthermore, due to the highly repetitive molecular structure of the Y chromosome, deletions are known from many Y-chromosomal regions. Therefore, the reliability of DNA-based sex tests can be improved by increasing the number of Y loci tested (Santos et al., 1998). Naturally, the detection of Y-chromosomal DNA polymorphisms as used to identify male lineages because of their property to carry genetic variation between male lineages (see next chapter) is informative for male sex identification.
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