Police obtained a warrant to search the suspect’s residence for clothes that might have some blood on them from the struggle with the assault victim. They seized a pair of trousers that appeared to be bloodstained. The trousers, along with blood standards from the suspect and the assault victim, were submitted to the forensic laboratory for examination. The stains on the trousers were of human blood, and the genetic typing results were as shown in the following table.
The first column of results is the ABO type of the people and the evidence. The rest of the columns are isoenzyme name abbreviations. Each abbreviation has a specific meaning and a set of characteristics associated with it. But it is not necessary for one to understand all the numbers and letters to understand this case. One can look through the table and quickly see what matches and what does not. The genetic systems are independent, so a nonmatch in any column would be an exclusion.
What the table shows is that the stains on the trousers are not of the suspect’s blood. This fact is clear from the ABO type: He is O, and the stain is A. The stains came from someone else. They could be from the assault victim, because all the system types match. But that does not mean the stains came from her to the exclusion of all other people; this kind of blood analysis is not powerful enough to individualize. So, what can be said or concluded by the analyst?
Here is where population genetics comes into this picture. Population genetics is a big subject, but briefly, scientists can find out how often each different type in each of the systems is seen by typing people in the population for all the genetic systems. These numbers give pretty good estimates of the actual distributions. They are estimates because it is not practical to type everyone; a sample of people is typed. The other important factor is that all these systems are inherited independently; that is, the ABO type that someone inherits from his or her parents does not influence nor is it influenced by the PGM type he or she inherits, and so on. This is like successive flips of a coin, or successive throws of dice. What one gets on the first toss is independent of what one might get on the second toss.
What it all means is that one can multiply the separate occurrences (or probabilities) together to get the combined figure. For instance, the chances of a head on the first coin toss are ½ (or 50 percent). The chances of getting two heads in a row are ½ times ½ (or 25 percent). The chances of getting three heads in a row are ½ times ½ times ½ (or 12.5 percent), and so forth.
The same thing can be done with the data in the table above. It is possible to figure out about how many people in the population would have all the types found on the trouser stains. This figure comes out to about 2 percent for Caucasians (the number would be different if the calculation were done for African Americans, Hispanics, Asians, or another group). So the analyst could tell the court that, if the blood came from a Caucasian, it could have come from about two in every 100 people. In other words, it could have come from the victim in the case, but it could also have come from many other people. In a city of 1 million people, 20,000 would be expected to have the set of types seen in this evidence.
The case shows how several genetic systems can be used together to decrease the size of the population segment that a specimen might have come from. It also shows that even if one gets results that yield a fairly low percentage of the population as possible depositors, there are still quite a few people who share the same types.
