Let us call the people involved in this case Jane Doe and John Smith. Who they are is not important to understanding the scientific issues. In the 1980s Jane Doe reported a sexual assault in a suburban town adjacent to New York City. The clinical examination she underwent at the time included taking a vaginal swab, among other items, into evidence. Doe was a type B secretor, so she would produce blood group substances B and H. When the swab was analyzed, it showed the presence of blood group substances B and H, as well as semen.
The suspect, Smith, was later apprehended as a result of the police investigation, and Doe identified him as the man who assaulted her. Smith was tested and found to be a type A secretor, so his ABO blood typing was expected to produce blood group substances A and H. No blood group substance A was found in the evidence, however.
What did this finding mean? Did it mean that Smith had to be excluded as a source of the semen? That is almost right. Smith was excluded on the face of things, but there were still a few matters the analyst had to consider.
One such matter is variation in the ratios of blood group substances. People can produce different quantities of these blood group substances in their body fluids. For instance, the relative proportions of blood group components may be different in the same person’s blood and saliva. When a person is tested to find out his or her secretor status, saliva is used. In this case the analyst had to consider the possibility that the A and H quantities could be present in different ratios in the saliva and the semen of the same type A secretor man. It was possible that an A secretor man could have more A in saliva (enough that the lab detected it when the saliva was tested) and less in semen (so that it was not detected in the semen found in the evidence specimen). Under those circumstances the man would still be included as a possible source.
But there is no way of knowing about the A and H levels in saliva versus in semen in any given case. When a person is tested for secretor status, what the lab tests is saliva because this fluid is relatively easy to obtain and getting a saliva specimen from a person is relatively nonintrusive. Forensic scientists are almost never able to test someone’s actual semen, because it is too intrusive for any court to order someone to produce such a specimen. As a result it was generally not possible to sort out these possibilities through ABO blood type testing.
A second matter affecting interpretation of lab results is what the analyst does not know. For instance, if the complainant had another sexual partner besides the person who assaulted her, the analyst would not know this unless she told the police investigators about it. Even then, the analyst would have no way of knowing whether the information was correct and true.
As a result of these uncertainties it was common, when ABO blood typing results represented the best evidence available, for forensic biologists to state conclusions about ABO blood type results conditionally. In this case the scientist might say that the man is excluded prima facie (on the face of it) by the blood group results, but that some conditions would have to be met before the exclusion could be considered absolute.
Showing posts with label Blood Typing. Show all posts
Showing posts with label Blood Typing. Show all posts
Sunday, April 10, 2011
Saturday, April 9, 2011
ABO Blood Typing and Inclusion
In the late 1970s a woman named Cathleen Crowell-Webb reported a sexual assault in a suburb of Chicago. At the time the case was “routine”; that is, it did not differ from any of the hundreds of such cases that come into forensic laboratories every year. Crowell was found to be a type B secretor, and the suspect in the assault, Gary Dotson, was also a type B secretor
The physical evidence in the case consisted of a visible drainage stain on the complainant’s underwear. This stain was tested and shown to contain semen, and ABO typing showed that it had B and H group substances. The suspect was thus included as a possible source of the semen. After a trial at which Crowell-Webb testified that he was her attacker, Dotson was convicted of sexual assault and sent to prison. The conviction was based on Crowell’s testimony, but also on the testimony of a forensic scientist about the physical evidence just described.
Six years later Crowell, by then married and called Crowell-Webb, announced at a press conference that she had lied about Dotson’s involvement: She had not been sexually assaulted at all, and the semen in evidence belonged to a consensual partner. This action caused a major furor in the press and in the courts. Eventually DNA typing proved that the semen in evidence was in fact not from Dotson but from another man who was Crowell-Webb’s boyfriend at the time of the alleged assault. Dotson was released from prison. Cathleen Crowell-Webb died in May 2008 at the age of 46.
In the scrutiny the case received after Crowell-Webb recanted her trial testimony, it came out that the forensic biologist who had done the original analysis had misinterpreted his findings. He had said that only a B secretor male could be the source of the semen. That is not true, as the following table shows.
The table shows that a male who is a B secretor could be the source of the semen found in the underwear, as the analyst said. But the source could also be a type O secretor or a nonsecretor of any ABO type. Semen from males of any of the three combinations mixed with the secretions of a B secretor female would yield the same results upon typing the mixture.
The reason this point matters is because the expert usually gives the court a percentage of the population who could be sources. Type B secretors are about 7 percent of the Caucasian population and 15 percent of the African-American population. (It is general practice to state the population percentages for different racial groups, because the blood type frequencies are different among the groups.)
But given that the source could also have been an O secretor or a nonsecretor, the sum of those three groups represents about 66 percent of the Caucasian and 85 percent of the African-American populations. Even understanding that 7 percent or 15 percent of the population is a lot of people, there is quite a difference between telling the jury that the defendant is in a population group of potential semen sources that is 7 or 15 percent versus one that is 66 or 85 percent.
This is an example of an inclusion case, the defendant was included as a potential source of the semen found in the evidence. It demonstrates two things: First, including the person as a possible source of the semen stain does not mean he is the source and, second, accurately presenting the findings to the court is important. Telling the court that only 7 percent of Caucasian or 15 percent of African-American males could have been sources was simply wrong and could well have misled the jury.
The physical evidence in the case consisted of a visible drainage stain on the complainant’s underwear. This stain was tested and shown to contain semen, and ABO typing showed that it had B and H group substances. The suspect was thus included as a possible source of the semen. After a trial at which Crowell-Webb testified that he was her attacker, Dotson was convicted of sexual assault and sent to prison. The conviction was based on Crowell’s testimony, but also on the testimony of a forensic scientist about the physical evidence just described.
Six years later Crowell, by then married and called Crowell-Webb, announced at a press conference that she had lied about Dotson’s involvement: She had not been sexually assaulted at all, and the semen in evidence belonged to a consensual partner. This action caused a major furor in the press and in the courts. Eventually DNA typing proved that the semen in evidence was in fact not from Dotson but from another man who was Crowell-Webb’s boyfriend at the time of the alleged assault. Dotson was released from prison. Cathleen Crowell-Webb died in May 2008 at the age of 46.
In the scrutiny the case received after Crowell-Webb recanted her trial testimony, it came out that the forensic biologist who had done the original analysis had misinterpreted his findings. He had said that only a B secretor male could be the source of the semen. That is not true, as the following table shows.
The table shows that a male who is a B secretor could be the source of the semen found in the underwear, as the analyst said. But the source could also be a type O secretor or a nonsecretor of any ABO type. Semen from males of any of the three combinations mixed with the secretions of a B secretor female would yield the same results upon typing the mixture.
The reason this point matters is because the expert usually gives the court a percentage of the population who could be sources. Type B secretors are about 7 percent of the Caucasian population and 15 percent of the African-American population. (It is general practice to state the population percentages for different racial groups, because the blood type frequencies are different among the groups.)
But given that the source could also have been an O secretor or a nonsecretor, the sum of those three groups represents about 66 percent of the Caucasian and 85 percent of the African-American populations. Even understanding that 7 percent or 15 percent of the population is a lot of people, there is quite a difference between telling the jury that the defendant is in a population group of potential semen sources that is 7 or 15 percent versus one that is 66 or 85 percent.
This is an example of an inclusion case, the defendant was included as a potential source of the semen found in the evidence. It demonstrates two things: First, including the person as a possible source of the semen stain does not mean he is the source and, second, accurately presenting the findings to the court is important. Telling the court that only 7 percent of Caucasian or 15 percent of African-American males could have been sources was simply wrong and could well have misled the jury.
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