Biology Reference
In-Depth Information
BOX 9.1
USING DNA PROFILES TO ASSIGN PARENTAGE
In 1985, Alec Jeffreys and colleagues discovered that there was enormous
genetic variability which could be used to assign paternity and maternity with
great precision. Since then, methods for DNA profiling have been refined and
they have revolutionized field studies by linking behaviour to individual
reproductive success. An individual's DNA is isolated from a tissue sample (e.g.
blood) or non-invasively, for example by analysis of epithelial cells on dung
(these cells are sloughed off as faeces pass through the intestine).
Microsatellites, or Simple Sequence Repeats, are stretches of DNA that contain
short nucleotide repeats of two to six base pairs. The microsatellite below
contains a two base pair (dinucleotide) repeat of the bases G and T.
This particular allele has 19 repeats of the bases GT. Microsatellites have several
important features which make them ideal for parentage assignment and
kinship analysis.
(1)
They are highly polymorphic. Microsatellite polymorphism occurs when
homologous microsatellite loci differ in the number of repeats between
individuals (often ten or more alleles per locus). The repetitive nature of
microsatellites is thought to lead to an increased rate of slipped strand
mispairing (slippage) during DNA replication in comparison to other neutral
regions of DNA, leading to a change in length of the microsatellite (e.g. from
19 to 20 dinucleotide repeats).
(2)
Microsatellites are inherited in a Mendelian fashion; every individual contains
two copies of each microsatellite locus, one allele inherited from the individual's
mother and the other from its father. Comparing the alleles present in offspring
to those present in putative parents allows us to assign the most likely parents.
Microsatellites are amplified using PCR (polymerase chain reaction) and then
either labelled with fluorescent dye and visualized using a DNA sequencer or
visualized by gel electrophoresis as dark bands, as shown in the example below.
This example is from a study by Hazel Nichols
et al
. (2010) of banded
mongooses
Mungus mungo
in Queen Elizabeth National Park, Uganda. In the
microsatellite locus shown in Fig. B9.1.1, male A is heterozygous (two bands,
representing two different 'lengths', or alleles, of the microsatellite), male B is
homozygous for a third allele and the female is homozygous for one of male A's
alleles (homozygous means two copies of the same allele, hence just one band
on the gel). There are seven pups. Pup five matches the mother and male B. The
rest of the litter match the mother and male A. By using a number of different
microsatellite loci (fourteen in this study), parenthood can be assigned with
great precision. In the banded mongoose study, the oldest three males in each
