Click anywhere on this panel to hide it then, move the mouse cursor over each of the yellow arrows to read the explanation of that feature of electropherograms. The numbers along the y-axis of the electropherogram represent relative fluorescent units, or RFUs, which are used to measure threshold values. The taller a peak is the stronger the fluorescence was that was created by that allele during electrophoresis. The strength of the fluorescence in turn is based on the number of copies of that allele's STR that exist in the sample.
Farthest to the left of the electropherogram are found the green peak or peaks that indicate the subject's gender. Two peaks as shown here in the top electropherogram represent an x-allele and a y-allele indicating a male. A female will display a single tall green peak representing two x alleles as shown here in the bottom electropherogram. The different colors of the peaks represent the different colors of dyes used to detect the alleles. The dyes are specially designed to attach to different primers that amplify alleles at different loci, as a result different alleles can be distinguished from each other even when they are of similar length as with this green peak, blue peak and two black peaks black peaks are used to represent yellow dyed alleles for ease of visibility if it were not for different dyes it would be impossible to tell which of these four peaks corresponds to which allele.
The extra tall red peaks on the electropherogram represent the size standard a strong mixture of DNA fragments of known lengths that is added to the sample because a certain amount of drift can occur during electrophoresis the detection equipment uses the standard as a reference against which it maintains calibration throughout the process somewhat like the synchronization track on a videotape. The red peaks therefore are disregarded when analyzing a DNA profile and sometimes an analyst will not even have the analysis software include them on the electropherogram even though the size standard is still included in the sample.
In general if a peak stands alone as in the top example it indicates a homozygous genotype at that locus both alleles are the same if two peaks of the same color appear right next to each other as in the bottom example they indicate a heterozygous genotype that locus contains two different alleles note homozygous peaks are generally noticeably taller than corresponding heterozygous peaks this is because in the homozygous case all of the genetic material at that locus is forming a single peak whereas in the heterozygous case only half of the genetic material at that locus is making up either one of the peaks it is split evenly between the two.
The numbers along the x-axis of the electropherogram represent DNA fragment length in number of base pairs the longer the DNA fragment is that forms one of the alleles in the sample the farther to the right its peak appears. The readout in areas of the electropherogram where no genetic material was detected in the sample zero RFUs is referred to as the baseline. This name however is slightly misleading the baseline is not truly aligned a certain amount of baseline noise occurs causing an erratic baseline in all four colors, which is visible when the electropherogram is viewed at large scales.
Tt is because of this that peaks must be of a particular height called the threshold value to be considered valid in order to distinguish them from meaningless irregularities in the baseline. Sometimes strong peaks will have very small peaks of the same color just to the left of them; these are called stutters they do not actually represent alleles but are essentially just echoes of the larger peaks as such they are disregarded.
