Biology Reference
In-Depth Information
Table 13.1
An example where an mtDNA profile has been generated from the HV-I of five bones
that were found in close proximity. The mtDNA profiles of three women who were maternal
relatives of three missing individuals are also shown. Maternal reference 1 clearly matches the
bone, while maternal reference 2 and 3 can be excluded as potential maternal relatives as they
have different mtDNA types. In this particular case the mtDNA profiling helped to establish the
identification of the human remains, and that the bones all came from the same person [39]
Sample
HV-I sequence
Right femur
16 189C
16 223T
16 271C
16 278T
Left femur
16 189C
16 223T
16 271C
16 278T
Right pelvis
16 189C
16 223T
16 271C
16 278T
Left ulna
16 189C
16 223T
16 271C
16 278T
Left tibia
16 189C
16 223T
16 271C
16 278T
Maternal reference 1
16 189C
16 223T
16 271C
16 278T
Maternal reference 2
Same as Cambridge Reference Sequence
Maternal reference 3
16 278T
16 293G
16 311C
-
16 189
Figure 13.3
The above sequence shows the presence of heteroplasmy at position 16 189, two
bases, a C and an A, are present in approximately equal amounts
It is, however, possible to find more than one type of mtDNA within a cell: this
is known as heteroplasmy and it arises when a mother passes on a normal version
of her mtDNA genome (wild type) and also a version of the genome that contains
a mutation. An individual will therefore possess two versions of mtDNA, usually
only differing by one base [22, 23] but have been observed to differ at two and
even three positions [43]. Two factors, the severity of the bottleneck and subsequent
genetic drift, determine the relative levels of wild to mutated mtDNA [16, 17].
Heteroplasmy can be stable through several generations before one of the mtDNA
versions becomes fixed [16, 17, 32, 41, 44].
Haplotypes and haplogroups
The sequence of a particular mtDNA genome is its haplotype (see Table 13.1).
mtDNA genomes that are not identical, but closely related are said to be within
