READ: Blood Stain Patterns

Principles of Heredity and Paternity

Blood from all humans may look similar, but it actually can be quite different in terms of "type". Although there are actually many different systems to classify blood types, there are eight basic blood types that are used to classify blood the majority of the time. Typically, blood is classified first by the ABO group and next by the Rh Factor. The eight basic blood types are: A+, A-, B+, B-, AB+, AB-, O+ and O-.

To develop an understanding of blood types, it is important to understand how traits are inherited. Deoxyribonucleic acid (DNA) is the blueprint of life. Chromosomes are composed of DNA, and sections of DNA are composed of genes. Genes are the basic unit of heredity. Each person inherits 2 sets of genes; one from each parent. Different forms of genes are called alleles and influence a person's characteristics. The pair of alleles that are inherited are the person's genotype. The phenotype is the observable physical or biochemical characteristics of an organism, as determined by both genetic makeup and environmental influences.

In humans, there are 3 alleles for blood: A, B, and O. Alleles A and B are both dominant over O. A person's genotype lists two blood groups, such as "AO", because everyone inherits one blood type allele from their mother and one blood type allele from their father. The phenotype in that example would be blood group A because both blood types A and B are dominant over type O. In order for a person to have a type O blood phenotype, he or she must inherit the recessive O type allele from both parents. While this is how blood type is inherited the majority of the time, there are some exceptions which make blood type inheritance less straight-forward. One of these exceptions is a rare third type of Antigen, known as antigen H, which can be present on red blood cells and prevent the normal inheritance of A and B alleles. A phenotype involving the H antigen is known as the Bombay Phenotype because it was first found in Bombay, India. Some examples of blood type inheritance can be found in the interactivity below:

When considering blood evidence; it is important to understand the limitations of it. Blood evidence can be both individual and class evidence in certain scenarios. Blood type is class evidence because everyone essentially has one of 8 blood types. However, DNA extracted from white blood cells from the same blood sample is considered individual evidence! Prior to DNA Analysis, investigators often relied heavily upon blood type to include or exclude suspects in cases! The advent of DNA Analysis has resulted in more than a few exonerations due to the use of the less reliable blood and serology evidence in the past.

Georgia Virtual, Forensic Serology, CC BY-NC-SA 3.0