Biology Reference
In-Depth Information
The Y chromosome has a non-random distribution among global populations,
largely due to the widespread practice of patrilocality [70, 71] (where the female
moves to the male's birth place/residence after marriage). This makes it a useful
tool for inferring the geographical origins of biological material recovered from a
crime scene and human remains [72]. As with mtDNA, the inferences regarding
geographical origins have to be treated with caution. Finally, in cultures where the
male name is passed on to male children, there is also the potential of attributing
surnames to Y profiles, which has the potential to be useful [73, 74].
Evaluation of Y STR profiles
When the Y chromosome profiles from a reference and an unknown sample match,
the significance of the match has to be assessed. The first step is to assess the
frequencies of the Y STR haplotypes in the population of interest. The simplest
method is to report the frequency of the Y STR haplotype in the population, known
as the counting method. The figure quoted is entirely dependent upon the size of
the database and is normally based on frequency databases that are constructed for
the major ethnic groups represented within individual countries, although compar-
isons can also be made with the combined data in the yhrd databases with over
80 000 haplotypes (representing at least the minimal haplotype). So, for example a
match can be reported as 'the haplotype has been seen twice in 400 UK Caucasian
individuals'. Corrections can be made to allow for sampling error in the reference
databases [75, 76].
The Y chromosome will accumulate mutations as it is passed through the patrilineal
line and direct comparison between males on the same lineage may result in a
false exclusion if mutations are not considered. The mutation rate in Y STR loci is
similar to autosomal STRs, at approximately 2.8
×
10
−
3
[75, 77 - 79]. Although most
mutation events affect only one locus, there is at least one observed instance where
three Y chromosome STRs differed between a father and son [80].
Further difficulties arise in the interpretation of the Y chromosome because of
the patrilineal inheritance and clustering of male family members in relatively small
geographic areas. This geographical clustering of male relatives coupled with the
limited size of the haplotype frequency databases (many haplotypes are seen only
once) makes the estimation of profile frequencies hazardous [81]. An alternative
method for assessing the significance of a match is to use a likelihood ratio and
to incorporate population subdivisions with the increased potential for common co-
ancestry [52]. Regardless of the method used to calculate the matching frequency,
when presenting the results of Y chromosome analysis, as with mtDNA, there is a
need to state how the use of Y STR typing varies from that of autosomal mark-
ers and that there will be other males in the population with the same Y STR
haplotype.
