The ABO blood typing procedure can be utilized to exclude paternity. However, it can not definitively state that a particular man is the father of a particular child. DNA testing, on the flipside, can definitively determine paternity. To confirm paternity using the DNA testing procedure, forensic scientists make profiles of the child's and mother's DNA. Then these profiles are compared to the suspected father's profile. The child's DNA band pattern must be a hybrid of the profiles of the father and mother. The child's profile can not include a DNA band that was not contributed by either parent. However, the child's profile does not need to have each and every band that each parent has in his / her own band pattern. Because both parents must contribute DNA to the child, he or she can not have a DNA band that neither parent has.
Through the union of the sperm and egg , each parent contributes half of his or her genetic material to the zygote, the fertilized egg of the new child. Neither parent contributes all his or her DNA. Chromosomes are arranged into twenty-three (23) pairs, and each parent contributes only one (1) chromosome of each pair to the zygote (child). Thus, the zygote contains a mixture of genes contributed by each parent. Parents may have DNA that the zygote does not have. However, the zygote definitively can not have DNA that was not contributed by a minimum of one of the parents. Therefore, when a child's DNA is broken up and separated into a unique band pattern through a process of capillary electrophoresis, each of the resultant bands must also be located in the DNA profile of at least one parent. Electrophoresis is a method of applying an electrical current through a transparent gel such that fragmented DNA will travel to varying distances on the gel based on the fragments' individual size and weight.
In forensic paternity testing, if the child has a DNA fragment that is not found in either the mom or the suspected dad, it can be concluded that the suspected dad is not the father of the child. This DNA fragment must have been contributed by somebody else (the true biological father), and the suspected dad can be excluded as the true father.
The next time you watch your favorite CSI program, you will have a better understanding of what DNA testing is all about when determining paternity of a child. If you have seen one particular episode of CSI: Miami called "Man Down," forensic DNA testing was used in the story to exclude the character, General Cruz, as the biological father of another character / villain, Clavo Cruz. It was determined that Joseph Trevi is the true father of Clavo.