Blood Factors and the DNA Fingerprint

CODIS is used in the national, state, and local index system networks to link typing results from unsolved crimes with cases in multiple jurisdictions or persons convicted of offenses specified in the data banking laws passed by the jurisdictions. Blood’s individual qualities were recognized years ago when doctors realized some transfusions were successful while others were immediately fatal. From observation came the ABO typing system and the first understanding of the Rhesus factor.

While there are extremely rare or exotic blood types, most people can be classified into the A, B, O, or AB blood types. The fact that an AB type exists at all told early investigators that every individual actually carries two alleles, or traits that determine blood type-one inherited from each parent. Further studies proved that if each parent contributed an O allele, the child would be type O, but if one parent contributed an A and the other an O, the A dominated. Likewise, if one parent contributed a B and one an O, the B dominated.

A and B do not dominate one another, though, so if one parent contributes an A and the other a B, the child displays both traits and is AB. Obviously, having no A or B factors, two O parents can only have OO children (usually just called O). Two AB parents can never have an O child, as neither of them has an O allele to pass along. Two parents of types AO and BO could combine their O’s to produce an O child, their A and B to produce an AB child, their A and O to produce an AO child(called A), or their B and O to produce a BO(called B).

In paternity cases, studies of family history can often reveal if an individual is AA-receiving an A type from both parents, or an AO-receiving an A from one and an O from the other. But for the practical purpose of receiving or donating blood, it doesn’t make a difference and it isn’t possible to separate AO and AA from one another in any sort of testing. Another factor present in some blood is the Rhesus factor. It is called rhesus because it was first identified in rhesus monkeys (Microsoft Encarta Encyclopedia, 2003). A person that is A+ has type A blood with the rhesus factor. A person with A- has type A blood but doesn’t have the rhesus factor. There’s a common misconception that there are two strains of the rhesus factor, one positive and one negative. That’s not true. You either have it or you don’t, but you can’t have a negative version.

This factor can be important during pregnancy if the mother is Rh negative; that is, if the mother does not have the rhesus factor. Children conceived with an Rh positive husband could receive the rhesus factor from him. The mother’s body, which doesn’t have it, perceives the new factor as an invader, like a cold virus, and starts building defenses against it. This usually happens during a first pregnancy with an Rh positive child. If a second pregnancy with an Rh positive child occurs, the defenses developed in the first pregnancy swing into action and serious complications can result, which is why physicians urge blood testing for both parents when a pregnancy is discovered. Now that the rhesus factor is understood, treatments can allow second and subsequent pregnancies to proceed normally.

With just ABO typing and rhesus testing, there were eight possible blood types: O-, O+, AB-, AB+, A-, A+, B-, and B+. For excluding suspects, this was occasionally sufficient. If the blood under a victims nails was B- and the suspect’s type was O-, there was no match and the suspect was cleared Problems arose when the blood typing did match. Obviously, law enforcement couldn’t arrest everyone with B- blood.

Statisticians knew that some blood types were more common, or more common in certain ethnic groups than others, and tried to bring mathematics to blood typing (see charts one, two and three, page 16 and 17) ( and ). Blood typing remained circumstantial evidence, but it wasn’t going to convict anyone on its own. Unlike fingerprints, which were unique to an individual, blood types could be shared by millions of people. Other biological clues like the Barr bodies, a piece of chromosomal material in the nuclear membrane of female cells which is the remnant of one of the two female chromosomes, found in the cells of females might help narrow the pool of suspects, but even half a million people is still a pretty big pool.

For that kind of individualization, law enforcement had to wait until DNA analysis, the study of deoxyribonucleic acid, advanced. DNA, the physical material that we inherit from our parents when that one sperm finds that one egg, is absolutely individual-with one exception. Identical twins form from one fertilized egg, so their DNA is identical (Microsoft Encarta Encyclopedia, 2003). While fingerprinting is still the only surefire way to separate identical twins, DNA testing provides its own advantages. Every cell in an individual’s body contains identical DNA. Fingerprints come only from fingers, but DNA can be found in blood, in urine, in feces, in saliva, in some hair, in the shed skin cells found in a facecloth or toothbrush, even the sweatband of a hat ( A suspect doesn’t have to bleed at the scene to leave DNA. Semen at rape scenes, saliva on the envelope of a ransom note, skin cells scraped onto a rope while tying up a victim, all provides the opportunity for collection and analysis.

DNA can survive much longer than a fingerprint. While some few prints have been collected years after being made, DNA analysis has been done on Egyptian mummies. DNA can provide closure to a victim’s family years latter by making it possible to identify bodies that might have been buried a decade ago as John or Jane Doe. DNA can indicate familial relationships. Though it’s been theorized that prints in family members might have similarities, it’s unproven. But because DNA is inherited from two parents, a significant number of matches in a sample can point to a “first-degree” relative-a mother, father, or sibling. In cases where groups are involved in crimes, this is important evidence.

DNA evidence doesn’t combine. Blood evidence at a scene frequently comes from more than one individual; either the attacker and the victim or a number of victims. If one hypothetical victim is type A and another is type B, a combined sample of their blood might suggest an AB individual. With DNA, the traits of both victims would be found in the sample. But with samples from the victims themselves available for comparison, it would be possible to prove that it’s a combined sample possible from only these two individuals (Evett, I.W. & B.S. Weir., 1998). It makes determining who was at a specific point in the scene possible.

With so much at stake, collecting DNA evidence at the scene, as well as from both victims and suspects, is imperative. But it’s not always easy. Collecting and preserving quality DNA samples requires that investigators first determine where DNA samples might be found, and then to ensure that it is collected without any contamination occurring to what may well be a tiny sample. Before it can be collected, it must be found.

And if one of DNA’s advantages to the investigator is that small samples are still useful, then one of its disadvantages is how hard to spot such tiny deposits can be. Two of the aids investigators may use are chemicals like luminal and alternative light sources like UV. Many biological samples fluoresce naturally in certain wavelengths of light. Semen, blood, and amniotic fluid are amenable to this method. If it’s not possible to use an alternative light source, a spray of luminal (which reacts with the iron in blood and any other blood containing biological specimens) proves valuable.

The ABO blood group system is a standard method of determining the blood types among individuals. This system is based on the presence of specific antigens that are presented on the cell surface of the red blood cells or erythrocytes. …

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