The Y chromosome is only carried by males and is passed along paternal lines. It is also the third smallest chromosome, which is why it is more likely to remain intact in heavily degraded samples. Since all Y-STR loci are located on the single Y chromosome, these alleles do not undergo independent assortment during gametogenesis (production of sperm or ova) and are instead passed to the next generation in the same whole unit (haplotype). Consequently, all paternally linked males have the same Y-STR profile. Random mutations may occur along the Y chromosome, potentially resulting in a minor change in the haplotype passed to the next generation; however, these mutational events occur at about the same frequency as in autosomal STRs, which is only approximately 0.1 to 0.4 percent. With Y-STRs, all of the genetic markers are, by definition, located on the Y chromosome and are therefore linked. The rarity of a Y-STR profile is determined by the counting method, so the statistical power in Y-STR analysis relies on the size of the reference population database. Because females do not have Y chromosomes as part of their genetic material, all female contributions in an evidence stain are virtually ignored with Y-STR analysis. This is advantageous when analyzing evidence samples that contain high levels of female DNA and only a small amount of male DNA, which is common in vaginal swabs of a rape victim. Other applications of Y-STR's are also noted. 3 figures