Biology Reference
In-Depth Information
The Y chromosome
In humans the Y chromosome is approximately 60 Mb long (million base pairs) long
and contains just 78 genes [56]. The SRY gene (sex-determining region Y) located
on the Y chromosome encodes a protein that triggers the development of the testes
and through an extended hormonal pathway causes a developing fetus to become
male [57].
With the exception of two regions, PAR 1 and 2 (PAR
=
pseudoautosomal region),
located at the tips of the chromosome, no recombination occurs during meiosis.
The remaining 95% of the Y chromosome is non-recombining, male specific and
is passed from father to son unchanged, except when mutations occur. The lack of
recombination may be the reason why there are relatively few genes on the Y chro-
mosome. If there is no chromosome crossing-over, mutations within genes have little
chance to be repaired or rectified and hence will be passed on to the next generation.
Y chromosome polymorphisms
The Y chromosome contains a large number of polymorphisms including VNTRs
[58] and STRs, insertions, deletions and SNPs. As with mtDNA, the polymorphisms
can be used to classify Y chromosomes into a series of haplogroups; SNPs that have
occurred only once in the evolution of the Y chromosome are used for haplogroup
designation [59].
Most forensic work utilizes haplotypes based on STRs. The first STR locus to
be identified on the Y chromosome was DYS19 [60]. Since then hundreds of Y
chromosome STRs have been described. The development of Y STR typing has mir-
rored the development of the autosomal STRs, and multiplexes have been developed
with increasing numbers of robust and highly discriminating Y STR multiplexes
[61 - 64]. The growth in interest in Y STR loci has led to numerous population
studies to establish allele frequency databases. The Y Chromosome Haplotype Ref-
erence Database (yhrd) was established to collate STR haplotypes (www.yhrd.org).
To ensure comparability between datasets, minimal and extended haplotypes were
defined. Two commercial kits, the PowerPlex
Y (Promega Corporation) and the
STR
Yfiler
AmpF
(Applied Biosystems) incorporate all of the extended haplo-
type loci (Table 13.2).
The techniques used for profiling are the same as for autosomal STRs (see
Chapter 6). The only notable difference is that a Y chromosome profile, with
the exception of DYS385 a/b and DYS389 I, contains one allele at each locus
(Figure 13.4). The locus DYS385 is duplicated on the Y chromosome and the PCR
primers recognize both copies of the locus; the DYS389 locus is complex with two
polymorphic regions, the PCR primers amplify DYS389I, which is a polymorphic
element within the locus and DYS389II is the whole locus.
