Biology Reference
In-Depth Information
13
Lineage markers
Genetic lineage markers comprise polymorphisms that are present on the maternally
inherited mitochondrial genome and the paternally inherited Y chromosome. The
analysis of lineage markers is limited in most forensic casework because they do
not possess the power of discrimination of autosomal markers. Even so, there are
some features of both mtDNA and the Y chromosome that make them valuable
forensic tools.
Mitochondria
The mitochondria are organelles that exist in the cytoplasm of eukaryotic cells. They
carry out the vital job of producing approximately 90% of the energy required by
the cell through the process of oxidative phosphorylation.
Inheritance of the mitochondrial genome
Mitochondria contain their own genome (mtDNA), which is maternally inherited
[1, 2]. This was discovered in the 1950s after unusual patterns of inheritance of
certain phenotypes were explained by the existence of extra-nuclear genomes that
did not obey Mendel's laws of inheritance.
During fertilization of an ovum, the sperm penetrates the ovum and the sperm mid-
piece, which contains between 50 and 75 mitochondria, enters the ovum along with
the head [3]. The ovum has around 1000 times more mitochondria than the sperm
[3]. Although some paternal mtDNA enters the ovum it is actively removed [4]. The
process is not always completely effective and very rare cases of paternal mtDNA
inheritance have been documented [5].
Copy number
The mtDNA genome is present in multiple copies: individual cells can contain hun-
dreds of mitochondria and a single human mitochondrion can contain several copies
of the genome [6 - 8]. Somatic cells, therefore, have thousands of copies of the mito-
chondrial genome and approximately 1% of total cellular DNA comprises mtDNA
[9, 10]. This compares with only two copies per cell of the nuclear genome.
