The analysis of short tandem repeats - An Introduction to Forensic Genetics - page 72

polymer in the capillary is replaced with fresh polymer and the next sample can
be analysed.
Interpretation of STR profiles
The spectra of the dyes used to label the PCR products overlap and the raw data
contain peaks that are composed of more than one dye colour. After data collection
the GeneScan or GeneMapper TM ID software removes spectral overlap in the pro-
file and calculates the sizes of the amplified DNA fragments. The software calculates
how much spectral overlap there is between each dye and subtracts this from the
peaks within the profile (Figure 6.5). A good matrix file, which contains information
on the amount of overlap in the spectra, will produce peaks within the profile that
are composed of only one colour. The height of the peaks is measured in relative
fluorescent units (RFU); the height is proportional to the amount of PCR product
that is detected.
To be able to size the PCR products an internal-lane size standard is used. The
internal-lane size standards contain fragments of DNA of known length that are
labelled with a fluorescent dye, and the fragments are detected along with the
amplified PCR products during CE [40]. Commonly used commercial internal-lane
size standards are the GeneScan TM -500 standards that can be labelled with either
ROX TM or LIZ TM dyes (Applied Biosystems) and the ILS600 (Promega Corporation)
(Figure 6.6).
(a)
(b)
Figure 6.5 The application of a matrix file, using the GeneScan or GeneMapperID software
removes the spectral overlap from the raw data (a) to produce peaks within the profile that are
composed of only one colour (b)