dna and dna evidence 105only minor recognition for his discovery, his successors became much
more famous. James Watson, Francis Crick, and Maurice Wilkins received
the Nobel Prize for Physiology and Medicine in 1962 for their elucidation
of the structure of DNA. Their revelation that nucleotide base pairing was
the functional essence of the DNA molecule, and that those pairings were
faithfully consistent throughout biology, was the basis of the award. The
nucleotide base adenine (A) always pairs with thymine (T), and guanine (G)
always pairs with cytosine (C); the end result is a long molecule composed
of two antiparallel strands in a twisted ladder shape that is called a double
helix.^4 With the understanding of the molecular structure came a series of
techniques in which DNA could be manipulated, including splicing, cloning,
sequencing, and even replicating the molecule. These steps led to a new phase
of molecular biology called recombinant DNA technology.3,5–8 Now, scien-
tists could mimic and exploit in the laboratory some of the same changes in
the DNA molecule that occur routinely in the natural environment.
The impact of Astbury’s discovery on forensic science was the realization
that we inherit the code to produce our characteristics but, strictly speaking ,
not the characteristics themselves. Thus, by uncovering the code that exists
within a biological sample, we have a quantifiable and unique basis for indi-
vidualization. And by focusing on the fundamental code, we remove the
subjectivity that arises from analyzing the characteristics, which are the
end product of the code and may be significantly impacted by unpredictable
environmental forces.
We receive half of our genetic material from our mothers and half from
our fathers, but because of the shuffling around of genes (called indepen-
dent assortment) prior to the creation of the egg and sperm, our parents con-
tribute a different allotment of their genetic material to each offspring. The
exception to the rule regarding a unique chromosomal constitution for every
person is the occurrence of identical twins. Children that are born from a
single fertilized egg that subsequently splits and forms one or more embryos
will have the exact same complement of DNA.
The DNA that is passed from one generation to another in this manner
is found in twenty-three pairs of chromosomes. Twenty-two pairs of chromo-
somes are autosomes and the remaining pair consists of two sex-determining
chromosomes, which are grouped as X,X (female) or X,Y (male). Both
autosomes and sex chromosomes are located inside the cell nucleus and are
sometimes referred to as nuclear DNA (nucDNA). Forensic scientists may
choose to analyze autosomes for individualization and sex chromosomes for
g e n d e r d e t e r m i n a t i o n. S p e c i fi c a n a l y s i s o f t h e Y c h r o m o s o m e i s a n i n c r e a s i n g l y
common practice in sexual assault cases. This allows the analyst to separate
the DNA of the male perpetrator from the DNA of the female victim.
The human contains a second, less well-known genome consisting of
mitochondrial DNA (mtDNA). As the name implies, this small, circular