Chapter 1
Introduction to Forensic Genetics

Scott Bader

The Forensic Institute, Glasgow, UK

The Ideal Forensic Material-Individualization

Forensic genetics has been touted as the gold standard of forensic analysis. This is because DNA fulfils many of the criteria that make the perfect forensic technology to establish a person's presence at a scene of crime.

Most forensic disciplines concerned with offences against the person, and some other crimes, try to establish a link between items found at the scene and items found on or associated with a suspect. In other words, to establish whether the recovered items could have originated from the same source. This process can be summarized as

1. 1. Establishing a match
2. 2. Calculating the significance of the match

The perfect conclusion of this exercise is to unequivocally establish that the material from the crime scene could only have come from exactly the same source as that found on or associated with the suspect and no other source. The goal of most forensic matching is to reduce the potential population from which an item could have come, to one individual within the population. This extreme is the definition of identification. The process that we are more interested in, because of its more common application, is that of individualization. This is the process of individualization. Individualization is a population problem as it is necessary to be able to demonstrate how many people in a population may have the match characteristics discovered by the investigator. Therefore, modern scientific individualization techniques recognize that most, if not all, evidence is probabilistic, which is to say that we attempt to establish a probability or likelihood that two items had a common origin. The ideal forensic material must enable matching and probability calculations.

There are other qualities that a forensically useful material should have. Ideally, the material should be

1. 1. Unique
2. 2. Not change over time (i.e., during normal use)
3. 3. Likely to be left at a scene in sufficient quantity to establish a match
4. 4. Not change after being left at the scene and during subsequent examination

In this book, we shall see that DNA meets many, but not all, of these criteria and how the limitations are handled.

So what makes DNA a good material forensically?

DNA-The Molecule

DNA is sometimes called the blueprint of life and has characteristics that are appropriate to its role. Many, if not all, of these characteristics are important in Forensic Genetics, which is simply genetics in a legal context. These characteristics include its simplicity and yet complexity, both of which are incorporated within the polymeric chemical structure of the backbone molecule and the varied sequence of sidechain bases (the so-called letters of its information content), arranged in a double helix (see DNA: An Overview). The molecule is made from a relatively small number of building blocks yet contains a vast amount and range of information that can define the nature of the biological cell, and ultimately the multicellular organism, within which the DNA is located. The double helix structure is relatively stable in time yet is adaptable enough to "open up" to allow a living cell to use the contained information to go about its life functions (transcription) or to make copies of itself (replication). DNA is stable so as to enable transfer of the genetic information from generation to generation after replication (with cell division and mating where relevant), yet it can also change to varying extents. Some of the changes are important to only an individual organism and may be deleterious (e.g., mutation giving rise to a cancer), or are the basis for individual variation (e.g., mutation giving rise to a new variant, and the haploid segregation of chromosomes in gametes with the return of diploid pairing at fertilization to produce a new individual). Some changes affect a subpopulation (e.g., lineages) and even eventually an entire population (e.g., natural selection of mutations and new diploid combinations leading to evolutionary change).

The chapter on DNA describes some fundamental concepts about DNA and genetics. In summary, the genetic material of humans comprises about 3 billion nucleotides or building blocks, and is present in two copies per cell, so about 6 billion in total. This DNA is found within the nucleus of all cells other than red blood cells, in total it is called the genome and contains the genes that encode the proteins created by the cell to define the cell's type and characteristics and ultimately the entire organism of the human individual. It also contains other DNA sequences that are regulatory (i.e., affect the temporal or quantitative expression of the genes), structural (i.e., affect intracellular packaging and stability of DNA), or are as yet of unknown function or may even be foreign to a normal human cell (e.g., a viral infection). All of these elements are contained within 23 separate lengths of DNA, the chromosomes.

The concept that DNA contains the information for biological life using a genetic code encoded within the sequence of bases along the double helix molecule means that if we as forensic scientists can "read" that code we can question and determine the source of a given sample of DNA. The general DNA structure and constituents are the same so that with the right analytical toolkits, we are able to answer that question. So, we could test not only whether the DNA is from a human, horse, cannabis plant, or soil microbe, but in theory identify the individual human. Scientists are able to take advantage of the "adaptable stability" of DNA and mimic the process of replication so as to make multiple copies of a DNA sample, using the polymerase chain reaction (PCR, see method). The amplified DNA is then processed and the data interpreted accordingly.

DNA in Populations

The first main concept to elaborate upon is that of Mendelian genetics (see Mendel mentioned in DNA). For a simple biological example, I will use the ABO blood group system. Here, there is a single gene involved that defines a person's blood group. The gene controls the production of a chemical on the surface of blood cells. The gene exists within the human population in one of three forms or variants: A, B, and O, and when referring to the gene, it is written italicized. The existence of variable forms within the population is called a polymorphism, and these genetic variants are known scientifically as alleles. They control the production of a protein that exists, respectively, as either protein variant A, variant B, or is not produced (i.e., absent) and thus called O (for null).

In any individual, the genes that encode everything that eventually produces a human being are present in two copies (not including the X and Y chromosomes), one inherited from mother and one inherited from father. It is the combination of the two copies of all the genes that will determine the final characteristics of the individual. So, while there might be just the one gene for the blood cell protein described above, there will be two copies of the gene in each person. All of the possible genetic combinations seen in different individuals are therefore AA, BB, OO, AB, AO, BO, and where the variants are the same, the person is called homozygous, where they are different, the person is called heterozygous. Going back to the description of the proteins that would be produced from the genetic variants, they are as follows in the table:

The combination of gene variants possible in any human are called the genotypes (first column) and the final observed biological characteristics (in this instance the blood group) are called the phenotypes. By way of illustrating the difference-the genotypes AO and AA both have the phenotype A because only A is actually observed in blood group testing; the O is "silent." When both variants in an individual are the same, the genotype is called homozygous, and when different heterozygous.

In this example, the gene variants A and B are what is termed codominant, in that they are both observable in the phenotype. The O gene variant is termed recessive, in that when it is present with something else, the something else takes precedence and is the only characteristic observable. Here, the "recessiveness" is simply because O produces nothing, whereas A and B produce A and B proteins. Note that while we can determine the blood group phenotype of a person from his or her genotype, going in the other direction is not so simple. So, a person of blood group B, for example, may be either BB or BO genotype. Knowing the frequency of the different variants present in the...