Biology Reference
In-Depth Information
D18S51
13
15
60% of peak height
16
20
15% of peak height
Figure 7.9
A mixture of two individuals will lead to up to four peaks at each locus. The area
between the dotted lines represents the zone where the minor component of the mixture can be
interpreted. The dotted lines represent 15% and 60% of the major peaks: below the 15% line is the
zone where stutter peaks from the major alleles can occur; peaks below the 60% cannot be easily
explained by peak imbalance. At this locus the major component can be interpreted as 13-15 and
the minor component's genotype is 16-20
by several people: there are many circumstances when mixtures of material can be
collected. A mixture in a DNA profile can be recognized by the presence of more
than two alleles at any locus within the profile, normally there will be several loci that
have three or four alleles present and a loss of peak balance within individual loci.
Having determined that the profile is mixed, the first task is to assess how many
contributors are represented in the profile. Two-person mixtures are most commonly
seen in forensic casework; with a two-person mixture a maximum of four alleles will
be present at any locus, whereas three-person mixtures will contain up to six alleles
at a locus. When four alleles are present at a given locus and there is a major and
minor component, the interpretation is relatively simple (Figure 7.9). The ratio of
peak areas within a locus generally corresponds with the ratio of template molecules
[17 - 19], peak areas that consider the morphology of the peak as well as the height
[16], are commonly used as a guide to interpret mixed profiles [20]. Even in a two-
person mixture when there are shared alleles between the major and minor profiles,
the interpretation becomes more difficult - especially in mixtures where the minor
profile is less than one-third of the level of the major profile [16].
In mixtures where the major component is in large excess, it is often possible to
deduce the major profile; however, in such cases it is difficult to get much information
from the minor component where the interpretation is complicated by artefacts in the
profile, such as stutter peaks, and also by the major profile masking the minor profile
[21]. Software has been developed that helps with the interpretation of complex
mixtures [22].
