Y-STR
Y- STRs are Short Tandem Repeats (STRs, refer to the STR page for
more details) found on the male-specific Y Chromosome. The coding genes,
mostly found on the short arm of the Y Chromosome, are vital to male sex
determination, spermatogenesis and other male related functions. The Y-STRs
are polymorphic among unrelated males and are inherited through the
paternal line with little change through generations.
Y-STRs have been used by forensic laboratories to examine
sexual assault evidence. In a sexual assault case, evidence such as vaginal
swabs will contain both female and male DNA. Differential extraction is
often used to separate the male component from the female component. More
often, however, the male and female components cannot be separated
completely. As a result, the female component could exist prominently even
in the male component after separation. When the "male DNA
sample" undergoes the PCR amplification process, the female DNA
component is amplified as well, sometimes masking the male DNA, which makes
analysis difficult.
Masking does not occur when Y-STRs are examined. Since there
is no Y-STR in the female evidence, the only contribution of Y-STR can only
come from the assailant(s) in a sexual assault case. The male component
will be easily detected, since only this part of DNA will be amplified. The
Y-STR system is especially helpful when there are
more than one assailant. The mixed pattern in the evidence can help to
identify those males responsible for the assault.
Y-STR is also used for non-sexual assault cases where mixed
samples are collected from evidence. Sometimes, regular STR will cause the
masking effect if there is a very small quantity of male DNA in the mixed
sample. Performing Y-STR testing can help to identify all males who have
contributed to the evidence.
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