DNA SCIENCE

DNA is the acronym for the molecule deoxyribonucleic acid. This molecule is found in the nucleus of cells of all living entities whether human, animal or plant. It is commonly described as the blueprint or building blocks of life, based on two scientifically accepted principles of genetic science:

  1. DNA is genetically inherited; and
  2. the DNA of each individual is unique, with the exception of identical twins.

A person’s nuclear DNA is inherited at conception. The fertilized egg will contain DNA inherited from the father’s sperm and the mother’s egg. The original cell will thereafter continue to divide with each cell in the body replicating from the original union. Each cell, regardless of what part of the body it comes from, will have the same DNA structure.
Approximately 99% of the chemical sequence in DNA is common to all persons resulting in, for example, common characteristics of two eyes, ears and a nose, regardless of race. It is the 1% of DNA which varies from person to person that forensic scientists are interested in for the purposes of human identification.

DNA Types

The two forms of DNA relevant to criminal investigations are nuclear and mitochondrial.

Mitochondrial DNA (mtDNA) is distinct from nuclear DNA in two significant ways: it is more numerous than nuclear DNA, cells may contain thousands of mitochondria, each of which may contain many copies of mtDNA; hence, far more copies of mtDNA are present in a cell. Of equal significance is the fact that mtDNA is inherited only from the mother or maternally. Although no two people (with the exception of identical twins) have the same nuclear DNA, all maternally related individuals will have the same mtDNA sequence (in the absence of any mutations).

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Each human cell has a single nucleus containing nuclear DNA. As stated above, each of these single cells may contain hundreds to thousands of mitochondria, each of which may contain hundreds of copies of mitochondrial DNA. Although most of the samples encountered in criminal investigations will be analyzed using nuclear DNA analysis, mitochondrial DNA analysis may be employed where samples are highly degraded such as may be the case with skeletal remains. Mitochondrial DNA analysis has enabled analysis where nuclear DNA testing has failed.

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DNA as Evidence

In addition to the established principles of genetic science, the further evidentiary value of forensic DNA analysis is determined by the following characteristics:

An individual’s DNA is the same in all cells. This means, for example, that a DNA profile derived from semen recovered from a crime scene can be compared to the DNA profile obtained from blood, the root sheath of a hair sample, buccal swab, or any other bodily substance obtained from a possible suspect. This is distinct from the traditional comparison of physical characteristics that would be made, for example, between a pubic hair found at a crime scene and the pubic hair of a suspect.

DNA Profiling

DNA fingerprinting, typing and profiling are commonly used terms. The term “DNA fingerprinting” however, is somewhat inaccurate for two reasons. First, fingerprints are truly unique to all individuals, including identical twins, whereas the DNA profile of twins is identical. Secondly, the real evidentiary value of DNA analysis lies in the statistical interpretation of the established association, whereas fingerprinting implies absolute certainty in DNA analysis methods. DNA fingerprinting is consequently a term used with some sensitivity in the legal community