DNA as legal evidence

DNA is being used increasingly as legal evidence, such as in cases of paternity suits or to identify suspects in a crime. These applications exploit the fact that, with the exception of identical twins, each person is genetically unique. 99.9% of the 3 billion bases of the human genome are the same from one person to the next, giving us all the characteristic structures and behaviors that make us human. The remaining 0.1%, or 1 in 1000 bases, can vary from person to person.

• Polymorphism - The prefix 'poly' means 'many' and the root 'morph' means 'forms' or 'shapes'. A polymorphism is a place in the human genome where a base can take on multiple forms. For example, a certain base may be A in one person and C in another. The length of some sections of DNA are also polymorphic. For example a particular segment may be 12 bases long in some people and 21 bases in others. The multiple versions of some genes, called alleles, are an example of polymorphisms.



• locus - A 'locus' is a unique location on the DNA strand; a way of specifying a base or section of the genome. For example, a gene, which is a section of DNA encoding information for a particular function, can be called a locus. A locus can also refer to a section of DNA which does not have a known function.



• genotype - Refers to the distinctive allele or combination of alleles in an individual.

Identifying suspects

Many polymorphic loci have been identified, and scientists have tried to characterize each locus by determining what the possible variations are at that site, and how frequent each variation is in the population. For example, for a hypothetical polymorphic base, 70%(.7) of people may have an A, 20%(.2) may have a T, and 10%(.1) may have a G. Each possible variation is called an allele. Everyone has two alleles for each locus, which may be the same or different. If the two alleles are the same, the individual is said to be homozygous at that locus; if they are different, he or she is heterozygous. The chances of having a combination of alleles is calculated by multiplying the probability of having each individually. For example, in the case of the hypothetical allele described above, the chance of having an A on one copy of the chromosome and a G on the other copy is (.7) x(.1)=.07, or 7%.

In attempting to identify an individual for legal evidence, many polymorphic loci are examined, which leads to greater specificity. The genotype from the crime scene and from the suspect must be the same at all the loci studied to make a match a possibility. If this is the case, the scientists can then proceed to calculate the probability of this occurence. In the example below, three loci are examined:

The probability calculated in this manner, suggests that approximately one in a thousand people have this genotype. While this narrows down the possibilities, it does not conclusively prove that the DNA from the crime scene came from the suspect. In real cases, a larger number of loci are studied, to give a more precise match.

Paternity tests

When there is a question of whether an individual is the parent of a child, it is possible to use genotyping technology to determine the likelihood of the relationship. Usually, the parent in question is the father, and so the term 'paternity test' is used here, but the same principles would apply to determining maternity.

As described in the section on How genes are inherited, each human has two copies of every chromosome, one from the mother and one from the father. Therefore, for any polymorphic locus, one of the alleles is from the mother and one is from the father. In a paternity test, the child and potential father are genotyped at a number of polymorphic loci. They must have at least one matching allele at all the sites for paternity to be possible. When this is the case, the probability of non-paternity can be calculated by multiplying the allele frequency for the shared allele. For a certain locus, both alleles may match, and it may not be possible to tell which was inherited from the potential parent (although sometimes information from a second, known, parent, may help). If this is the case, that locus cannot be used in the calculation. The example below illustrates a test involving 5 loci:

The probability of these two genotypes matching by chance is 1/208. In reality a larger number of loci are sampled, so that the probability of having a match by chance is very low.

Limitations of the technology

One challenge in determining a genotype or DNA profile is the often small amounts of DNA found at a crime scene. DNA is quite fragile, so the samples obtained for analysis in criminal cases are sometimes damaged, making it more difficult to determine the genotype. In addition, there is the risk that the DNA samples obtained are not from a single person, but have been contaminated with the DNA of other people or even other organisms. These cautions do not apply to paternity testing, in which a blood sample is drawn, providing a large and uncontaminated sample of DNA.

Another major limitation of genotyping technology is that it depends very much on accurate knowledge of allele frequencies in the population. This is complicated by the definition of 'population,' as allele frequencies can vary significantly between geographical regions and ethnic groups. In a population with complex and dynamic composition, such as exists in many parts of the world, it is almost impossible to determine a precise figure for allele frequencies.

These techniques of identifying people by their genotype at various loci is powerful, yet limited. The general public, because they are not familiar with the technology involved, often assume that DNA and other science-based evidence is infallible. In fact, while DNA evidence is an impressive new tool, it is only one part of the full body of evidence which should be used in evaluating a case.

More detailed and technical explanations of DNA forensics are available through The Biology Project at the University or Arizona.

Note: Human genetics is an extremely complex topic. This website is meant only as an introduction and overview. If you are concerned about how genetics may affect your health, always consult your physician.