Let the record reflect we have now been rejoined by all the members of the jury. Thank you, ladies and gentlemen. Please be seated. And Dr. Cotton, would you resume the witness stand, please. Good morning again, Dr. Cotton. You are reminded you are still under oath. Mr. Clarke, you may continue with your direct examination.
Dr. Cotton, with regard to this diagram that is on the easel at the moment, People's exhibit 237, you described how that shows DNA in a double-stranded form, correct?
All right. Could you, with the help of a new diagram, or actually, would it assist in your describing DNA in a single-stranded condition to use and draw on what I believe would be People's exhibit 238, a new chart, new diagram, rather?
KEY QUOTEAll right. Then with the Court's permission would you go ahead and use the diagram again to illustrate this single-stranded versus double-stranded DNA.
All I'm basically going to do is straighten out the helix to make a second diagram that is a little bit easier for me to draw, so if we make two strands, now all I have done is really just straightened this out, so that when you look at the bases, if you have an a on one side, you will have a t on the other. If you have a G on one side, you will have a C on the other. And there is a bonding between the a and the t and the G and the C that we are not going to bother to draw, but there is a force that is keeping these together, so you can make up any sequence here. T, A, C, C, G, for example, and if I have made up this sequence on this side of the double-stranded, then I would automatically know I have the t on this side, I must have an a on the other. I have an a on this side, I must have a t on the other and so forth. I have a C on this side, so I will have a G on the other strand, and a G on this side, so I must have a C on the other. And again, you can think about the size of the piece of the DNA that I have drawn by counting the bases. Here I have 7, so this piece of DNA that I have illustrated is 7 base pairs long. Now, if we take it apart, we haven't really left a lot of room here, but we will just put up here this is double-stranded, so I think we better--
I'm not going to be able to remember the same series of letters that I wrote down on the previous one, but if I took that piece that was double-stranded and I heated it, strands would come apart, so I would have nothing attached to this side and I would have nothing attached to the other side. However--
I'm sorry, when you say you heat it, what type of temperatures are you talking about?
You have to heat it to about 95 or a hundred degrees in order to get the two strands to come apart, and there is some--there are some temperature--some strands of DNA, if you had a lot of C's and g's, you have to heat those a little--you have to heat that a little bit more than you have a lot of A's and g's. But basically, if you think of it as heating to a hundred degrees, and when I say a hundred degrees, I'm not talking a hundred degrees centigrade.
If you wanted to put these back together, you can see that if you just move them back together, you are able to do that, they would all be correctly paired and thus they could come back together and zip up. However, if I took this strand, say, and moved it down so that I had an a or--I was trying to put a t back together with this C and an a back together with this C and a G back together with this G, it would not come back together like that. It will only come back together when the A's and T's and g's and C's are properly paired up.
So therefore DNA is at its happiest, would that be correct to say, when the pairs match up correctly?
Yes. It is happy and it is stable when they are paired up correctly. It is pretty happy when they are completely separate, but not as happy as when they are together.
As far as this ability to separate DNA and to have it return to its double-stranded form, why is that important?
It becomes important in the process of doing DNA testing and for many other reasons, because it allows you--suppose I have this DNA sequence, single-stranded, and I have this one in my hand in a test-tube and I mix them together. They will come back together. If I have some kind of a tag on here, some kind of tag and the tag could be a radioactive tag or other types of tags, if I have this piece in a test-tube and I have this piece in some other forms and I mix them together and I let them come back together, they will zip up together correctly given some time, and now I can find out where this single-stranded piece has gone to by looking at whatever kind of tag I have stuck to it.
And is the ability to do that or does that ability play a role in DNA typing itself?
Okay. We'll talk about that a little bit later. Now, as far as this overall concept of what you have described, do we in fact, to your knowledge, have a diagram that describes where DNA is found?
Okay. With the Court's permission then, I would like to have marked as People's exhibit 2--
All right. Dr. Cotton, we had marked as People's exhibit 240 a large diagram entitled "where is DNA found?" can you tell us what that diagram shows?
Why don't we move--would you be able to discuss this diagram with or without referring to this particular chart that you have just made?
This exhibit is basically a summary of the things that I drew earlier on the chart, so you have an example or diagram of a cell that has a nucleus and a cytoplasm and in the nucleus there are chromosomes. Now, they don't have 46 of them drawn out here, but they have several and the real number would be 146. You take one of those chromosomes out and unwind the DNA, this is a depiction of the DNA that is packaged up into the chromosomes, and to make it larger and look at it still magnified yet further, you see again the double helix with the at and GC base pairs.
So then does this chart "where is DNA found?" then basically take step-by-step by showing with greater magnification each portion or actually each way DNA is packaged down ultimately to the individual base pairs themselves?
Okay. As far as this DNA molecule, and you have described it being the same basically in every cell that has a nucleus in our bodies, does that mean all of us are the same?
(Witness complies.) Well, it is pretty obvious that we are definitely not all the same. All you have to do is look around the room and you will pick that out right away, so within any given individual in all of my cells, the DNA will be exactly the same from one cell to the next, and that would be true for each one of us. But from one individual to the next the DNA will be different. Now, that isn't to mean that--we will use Mr. Clarke as an example--all the DNA in all of his cells is the same in each cell. That is, if you look at a liver cell and a blood cell, the DNA in the liver cell will be exactly the same as the DNA in the blood cell. His DNA will be different than anyone else's in the courtroom, but that is an overall difference. The great majority of the DNA, greater than 99 percent of the DNA, is going to be the same from person-to-person and that is because we are all human beings, our basic body structure is all the same. We all have a liver, we all have hairs, we all have a kidney and so forth. So all of that DNA information that creates those things that we have in common will be this base--basically the same from one person to the next. There is then a small portion of the DNA that is different from person-to-person and that is intuitively obvious because you can look around even in a very large room or a very--many, many people, and you will see that with the exception of identical twins, people are different. Even closely related people, brothers, two brothers are different. You can al tell them apart. And the exception is identical twins and identical twins are identical because they have identical DNA, and that is the only exception.
With regard to then these portions of DNA where we differ--and I believe you said that is something less than one percent where we differ from one another?
Why is that of interest? Why do you as a scientist care about that region or those regions where we are different?
Many scientists would not care about those regions that are different, but if you are interested in being able to distinguish one human being from the next, then it doesn't help you to look at the parts that are the same. In order to distinguish people, especially at the DNA level, you want to be looking at those sections of the DNA that are different from one person to the next, so you have to single out those sections. And for purposes of identifying or telling apart individuals, then you want to look at those sections of the DNA that are not the same from one person to the next.
You described brothers and sisters as being different and yet similar in some respects; is that right?
Well, they will be similar in some respects, because if you think about it, you have a mother and a father and they are going to--and they are going to have children. Each child will not get identically the same characteristic from the mother, so--but if you had three children, two children might get the mother's hair color and the third child might get the father's hair color and the first two children who both got the mother's hair color, one of them might get the father's stature and be big-boned and tall, and if the mother was short and small-boned, then the other child might get that characteristic. So that even though they had some characteristics in common, they both got mother's hair color, they don't have all their characteristics in common, and that is a very simplistic example, but it works.
The child will not, definitely not have the same DNA as either parent, because the child only gets half of the mother's DNA, so--and the other half comes from the father, so that child is a blend of the mother and father and the child will not be identical to either the mother or the father.
As far as, and perhaps we can go back to the chart that is currently on the easel, "where is DNA found?" these chromosomes, do they package DNA molecules or simply a portion of the entire DNA molecule in a cell?
They package the DNA molecules in the cell. One chromosome can generally be considered to be a packaged up DNA molecule.
Okay. Now, this Court has heard testimony about serology. Are you familiar with that term?
Which, as the jury has heard, involves testing various genetic markers like PGM and EAP. Are you familiar with those terms?
Is DNA different than the times of testing that go on that test proteins like PGM and EAP?
The protein markers that are mainly used in serology are coded up, that is in some--you can think of them as a product of the information that is in the DNA, so when you look at the DNA you are looking at the most basic level of information that you can look at. When you look at a protein, that is a product of the information that is in the DNA, so you are sort of looking at a secondary level of information.
So if I understand correctly, by typing a protein you are really looking a little bit indirectly at the particular genetic marker; is that right?
As opposed to DNA typing where you are looking directly at basically the blueprint itself?
All right. Dr. Cotton, I would like to take you back and shift topics a little bit and go back to Cellmark Diagnostics itself.
I have been at Cellmark about 7 and a half years. I began to work there in January of 1988.
We do DNA testing and we really--we basically do DNA testing for two purposes: One is to answer questions in criminal cases, and I will just give you an example. Umm, if you had a bloodstain and you had a known individual, could be a suspect or a victim, it doesn't matter, you would attempt to say could the known individual have been the donor of the DNA that was taken from the bloodstain? It is exactly the same question that serology is used for in a crime lab. The other type of testing that Cellmark does is paternity testing and for that purpose we are usually receiving a blood sample from a mother, from a child and from an alleged father and the question is, is--could this alleged father be the father of this child or is it impossible for this man to be the father of this child? And that kind of testing can be done using DNA.
Going back to your education, and you briefly mentioned the fact that you had a Master's as well as a Ph.D., could you describe a little bit in more detail when you received those degrees and at what schools.
I received both a Bachelor of Science and a Master of Science degree from Southern Methodist University in Dallas, Texas, and for the both of those degrees I was biology major. Later on I moved to California and attended graduate school at UC Irvine and finished a Ph.D. in molecular biology and biochemistry.
You touched a little bit on DNA played a role in your higher identification. Can you describe basically and briefly, from your undergraduate work through your graduate work, what contact, what work did you do in the area of DNA itself?
Well, if there is a way of briefly summarizing it, perhaps you can package all three different degrees.
I did--for a bachelor of science degree you take a series of biology courses and you learn about DNA and you learn some biochemistry and you learn some cell biology and you do laboratory experiments. And I don't actually recall that any of my laboratory experiments specifically had anything to do with DNA, but actually it has been a while ago, so I'm not really positive. And for my master's degree I did do work in a laboratory, but that work involved electron microscopy which is a method of achieving extreme magnifications and that didn't have anything to do with DNA. And in graduate school at UC Irvine my research focused on DNA and how it is packaged into chromosomes and how the information in DNA is translated for the cell, and so for the four to five years that I was in graduate school, umm, all of my research work had to do with DNA, and then I took the required graduate classes and some of those had something to do with DNA and some of them did not.
As far as your graduate work, did it include actual hands-on work with DNA testing itself?
Well, it included a lot of hands-on laboratory work, because the whole point of being in graduate school is to learn how to do research. You just asked me whether it had to do with DNA testing. The kind of--if you are talking about the kind of testing that Cellmark does, no, it didn't, because that wasn't being done at that time. It had to do with other aspects of DNA.
All right. As far as your employment, and then once you obtained your degrees, did you then seek employment?
I worked for about three and a half years in the Department of Biochemistry at the University of Iowa and I did research there, which was also related to basically how DNA is packaged up into chromosomes. And then I went to work at the national institutes of health in Bethesda, Maryland, and I did research there for about five years, and that also involved DNA and that--at that point now did involve doing basically the same types of procedures that are currently used in DNA testing.
It is basically sort of looks like, if you were to go visit there, it looks like a big college campus and--but not--there isn't very much in the way of teaching done there or not certainly at the undergraduate level. There is some, but not a lot. And it is groups of researchers and it is divided into different institutes. I used to know all the various institutes. There is a heart, lung and blood disease institute. I happened to work this institute for alcoholism and alcohol abuse. There is the nci, National Cancer Institute. So there are--i don't remember how many institutes there are in all, probably more than 20, but these various institutes altogether comprise the national institute of health and the main campus for the national institute of health is in Bethesda, Maryland, and then there are other areas where researchers are located also besides that main campus.
My main duty is that I am ultimately responsible for the scientific work that is done at Cellmark, so that is my sort of umbrella overall responsibility. My duties include, along with two other Ph.D. staff, supervising the research that is done at Cellmark and reviewing cases that are done at Cellmark. When we receive samples in the lab and we analyze those samples and write a report, the person who has done the work at the bench, which is not me, signs the report and I--and myself or one of the other two Ph.D. staff would review the report and sign it also. And I testify in court when required.
All right. We will return to both of those a little bit later. As far as your position as laboratory director, how long have you been in that role?
This jury has heard testimony previously about an organization known as the American Academy of Forensic Sciences. Are you a member of that group?
I'm a member of the American Society of Cell Biology, the American Society of Human Genetics and the American Association of Blood Banks.
The American Academy of Forensic Science, I assume the title describes it, involves forensic science; is that right?
When I use the term "forensic science," I am generally referring to the work that we would do in the analysis of samples where a criminal case is involved.
The other organizations you described then, I think one of them was, for instance, the American Society of Human Genetics that you are a member of?
It is an organization or a group of scientists that are interested in all--that is the scientists that are members of that organization would be interested in some aspect of human genetics. They might be interested in disease diagnosis, they might be interested in gene mapping. They might be interested in forensic science, but it is sort of a generic group of scientists whose overall interest would encompass human genetics.
I think you also mentioned was it the American Association of Blood Banks that you are a member of?
Mostly it is people who are knowledgeable about the storage of blood and blood typing and the use of blood in--for medical purposes.
Such as transfusions and certainly that is not my area of expertise. The reason that I am a member of that organization is that that organization has a program for accreditation of laboratories that do paternity testing and we are a participant in that program and they usually at their national meeting will get together scientists who are interested in paternity testing and have a specific forum for that small group, so that is a very small group within the American Association of Blood Banks.
Do you attend meetings, as you are able to and as your case work permits, of these organizations?
Generally it is related to perhaps a new technique that could be used in criminal cases or a case study that was done in a very unusual case, something like that.
All right. As far as publications--and there is scientific literature in the area of DNA; is that correct?
As far as that literature is concerned, do you attempt to stay current or do you stay current on that portion of the literature that directly impacts your job at Cellmark Diagnostics?
I mainly am now able to stay current with the scientific literature on DNA as it impacts on human identification issues. I do stay current with some of the other literature, but it is in a relatively narrow area.
Basically I mean being able to distinguish one human being from another, and as the ability of science gets more and more sophisticated in this area, given enough information, you can distinguish one human being from basically all others.
Incidentally, as far as the literature is concerned, do you act as a peer reviewer for that literature?
I am sometimes asked to review articles that have been submitted to journals and give my opinion on the quality of the work that is in that paper.
All right. I believe you described that one of your duties as laboratory director includes testifying in courts across the United States?
And I am referring now to the area of human identification as a result of DNA typing?
Could you describe for us approximately how many times that has occurred before today.
Umm, I don't keep a running total, but I've probably testified in about ninety or so cases.
Well, again, I don't keep a running total of that, but I'm sure it is at least twenty.
Would this be your first time in the County of Los Angeles, or not, testifying as an expert in DNA typing?
Dr. Cotton, I would like to shift gears a little bit and talk with you about DNA typing itself. Is there more than one or just one approach to typing DNA for purposes of human identification?
All right. Are there two primary approaches that are used commonly in forensic science?
The two primary approaches are generally referred to by their acronym or their initials. One is called RFLP analysis and that is the acronym standing for restriction fragment length polymorphism, and obviously you don't have an explanation of that. The word polymorphism just is referring to differences meaning it means many forms, so it is a technique that is allowing you to look at many forms of the DNA, and that is one major type of DNA testing that is done in this country today and actually many other countries as well. The other type is referred to as PCR. Now, there are many different kind of PCR tests, and PCR is the basic procedure that allows you to do the test and PCR--the initials stand--and again this won't make any sense yet--the initials stand for the polymerase chain reaction. It is a procedure and that procedure can be applied to different types of genetic markers and different types of tests, but any test that uses that procedure is generally referred to as a PCR type test.
I would like to start with the RFLP process first, if we could. And first of all, could you describe for this jury, taking weeks, what the RFLP process is and how it actually is conducted?
To your knowledge are we going to in fact limit it to something substantially shorter than that?
We are going to--based on my discussions with you, we are going to try to make it much shorter than that and pick out just those sections that are really important to have an understanding of what the final result looks like and what it means.
It wasn't developed on a special day. It is a series of procedures that all tied together are called RFLP. Some of those procedures were developed in the mid-seventies and some of the information that scientists needed to actually finally put all this together was not developed until the early eighties. So you can't give it a date, but you can give it a time frame from about 1975 to about 1985. The information that is used in RFLP testing for identification was discovered during that time frame.
Was there a time when locations on the DNA molecule that you have already described were found to be different amongst people, as you earlier testified to?
That has been known for a long time. Early on of course it was just known sort of intuitively that there had to be sections that were different since people are very different. The information about heredity and how it relates to DNA was described back in the late 1800's and early 1900's, but the discovery of the specific types of differences that this test looks at was basically about 1983, `84, and `85.
Was that as a result of in particular one or two scientists in this country and outside this country?
Umm, the two major contributors in this area, and again it is very hard to say this, because without previous information they wouldn't have been able to make their major contributions, so you have a whole series of scientists that are kind of contributing here, but the two contributors to the kind of information we are looking at in this testing would be Dr. Ray white from Utah and Dr. Alec Jeffries from the University of Lester in England.
Incidentally, was Dr. Jeffries in England honored for any of his work in this area?
Yes, he has received several honors, including being knighted for his discoveries in this area.
I know it is Sir and then he is also a Professor and some other things, so there is sort of a series of Sir, Professor things that go in front of his name, but I don't know that I could get it right.
Did Dr. Jeffries play a role in any manner in the formation of Cellmark Diagnostics?
Basically he did. The University of Lester where Dr. Jeffries works owns the patents to his discoveries, so the series of events was he made these discoveries, certain of this information was patented, and the parent company that owns Cellmark Diagnostics purchased the rights to those patents, and they, along with Dr. Jeffries' help, did the developmental work to take it from--excuse me--umm, being a procedure that he did daily in his lab to having a protocol that could be followed in what's more like a production lab, where you are doing the same type of test on each sample that comes in the lab.
Is there any relationship between your laboratory in this country, as well as Cellmark in the United Kingdom?
Was Cellmark in the United States formed before or after Cellmark in the United Kingdom?
I think it was just about a half a year after the formation of Cellmark in the United Kingdom.
As far as the procedures that were used at Cellmark in this country at its formation, did you have anything to do with what had been used before or even at that time in England?
At the time that Cellmark began doing case work in the United States we were using the identical procedures to those that were used in England. Umm, now the procedures are probably not identical. They don't do very much testing for criminal cases in that lab any more and so our procedures would no longer be identical, but they would still be very similar.
There is a very large lab in London and they do almost all of the forensic testing for the United Kingdom.
Now, with regard to--and you have used this term RFLP typing process that we will get into a little more detail later. Is this RFLP testing approach used exclusively in human identification?
The--the--let me give you an answer in sort of two phases. A very similar procedure can be used to identify animals and in fact there is a lab in Oregon and another lab in Canada that uses the same types of procedures to look at poaching. Somebody has, you know, some meat in their freezer, and they say, well, this is just a deer, you know, and the--I don't know what the organization would be that would be checking on this, but whatever organization it is, they would say, well, no, this is some endangered prong-horn antelope--I am sort of making this up as you can see--then they can use these same kind of procedures to determine whether or not the meat in the person's freezer is in fact from a deer or from the endangered species, so that is a very analogous type of thing to what is being done in a criminal case. The general procedures that are used in this testing are used in genetic diagnosis and in gene mapping and in a lot of other research applications other than human identification.
Let me stop you and ask you about when you use the term "genetic diagnosis" that this technique is used for, is that determining whether or not an individual suffers from a genetic disease?
Yes, or you might--a parent might want to know if they were the carrier of a specific genetic disease or you might, in the process of diagnosing a disease, want to know if this was a genetically--a specific genetically inherited disease, and there may be a test for that that uses these kind of procedures. Another use, for example, is in bone marrow transplantation. You have two--you have a donor and you have a recipient of the bone marrow from the donor, and after the transplant is done, you can use these procedures to follow whether or not the transplant has been effective by looking at the blood cells in the recipient and saying is the original genetic source of those cells those of the recipient, which you are hoping it won't be, or those of the blood marrow donor, and you are hoping that it will be that and you can track the progress of the transplant using the very same procedures that are used for paternity or identification testing.
As far as disease diagnosis, if I can return to this, is this RFLP procedure then used to determine, for instance, whether an individual suffers from cystic fibrosis?
It can be. There are now easier tests than the RFLP procedure, but that was certainly early on the procedure in the research stages that that would have been used.
You also brought up the use of this technique, for instance, in the poaching scenario. Is this RFLP technique also used to help save endangered animals?
What about this technique, this RFLP technique, in for instance, plants? Does it have any role there?
There are--the main role that I am aware of, and I am not a plant biologist, so there may be many other aspects that it is useful for that I am not aware of, but in plant breeding where a company has a corn hybrid that has specific characteristics, those characteristics can be identified and attracted in that hybrid using RFLP analysis with exactly the same types of procedures that are used in testing for paternity.
Does this RFLP testing technique play any role in, for instance, identifying the remains of American war dead?
Now, you have described a number of general areas, including human identification, that this RFLP technique is used. Would the human identification use of this technique be the majority or most or a small portion or what?
Overall it would be a small portion--if you look at anyone in science who might use this procedure or this set of procedures that makes up RFLP analysis, human identification would be a very, very small fraction of its total application.
Is this a technique that was basically born out of forensic science or did forensic science borrow it from the greater scientific community?
What about in terms of how wide this technique is used and I'm talking about geographically? Is it used in the United States, in England alone, or is it used elsewhere?
As far as the RFLP typing approach, is it used strictly in English-speaking countries, for instance, or is it used in some wider area than that?
Now, if I can return to the RFLP method, first of all, would it assist, if I asked you to describe briefly this method, to use again the drawing pad for purposes of demonstrating what happens?
In fact, Dr. Cotton, if we could, before we do that, could we go back and perhaps if you could step down, I'm going to ask you to put just very brief titles on the previous exhibits, the previous drawings.
And perhaps we could start with the 46 chromosomes chart that I believe is People's exhibit 236. No, 235, excuse me. That is labeled "46 chromosomes." would that be an appropriate title for that chart?
Okay. Perhaps you could just underline it and then we will underline the titles on each one.
All right. If you could go to the next drawing that you made, which is exhibit 236. Does that drawing depict on the left the single pair of chromosomes and then the two strands of DNA to the right?
If you have a different color, I think you could put it in the middle will be fine, as long as you underline it.
And then as far as the two strands of DNA to the right, would it be appropriate to give that a title?
That is just the two strands from these two chromosomes, so it is still a chromosome pair.
Okay. Then if you would go to the next diagram, which I think has already been titled "double-stranded." would that be an appropriate title for what I believe to be People's 238?
And again, your Honor, that would be People's exhibit 239. Now, Dr. Cotton, let's provide you with a new page, which will be People's 241, I believe. First of all, why don't we label it at the top "RFLP method."
And if you would now use that particular piece of paper to help describe this RFLP testing process.
Do you want me to just--okay. What I want to do is go back to the idea that you have two chromosomes or you have a chromosome pair and that we are going to look at the DNA from that specific chromosome pair and I want to explain to you what the--we keep talking about looking at differences in the DNA. I want to show you an example of the kind of difference that we are looking at, because I think that is the only way it will really make any sense, and then once we have done that we can sort of walk through the procedure.
So let's go back and say we have our two chromosome's worth of DNA here and they have, you know, the a and t and GC base pairs all along, and so if we are looking around on these chromosomes, you will come to a section that contains what is referred to as repeated sequences. I'm going to make a very short and simple--"short" meaning short in physical length--simple example of this, but this is not an unrealistic example. Suppose we come along and we are looking down the sequences of bases and we see that we have a C followed by an a followed by a t with another C followed by another a followed by another t and another C followed by another a followed by another T. Now, what I'm going to do is only draw one side, so if you will just assume with me that the other side has the appropriate matching base. So we have this sequence, C, A, T, repeated three times. Now, if we went and looked at another chromosome, this is now you know from one person and you are going down the chromosome and you are looking at the base pairs and you come to sort of a matching section and you find that it has a C followed by an a followed by a T. And you see that that sequence of three base pairs is repeated quite a number of times, but on this chromosome it is repeated more times. Here we have--on the one on the left we had three repeats and here, (Indicating), we have on the one on the right, we have five repeats. So we have this short section of something that is repeated over and over again. Now you have to sort of think about if we made this same drawing for yet another individual and another and another, what you would find is that another individual might have a section--they would have this--you would go along the chromosome and they would have this same repeat, C, A, T, but it might be repeated ten times on one of their chromosomes and 18 times on the other. So that everybody has the C, A, t sequence on their chromosome, but not everybody has the same number of repeats. That is the difference that RFLP testing looks at. So if you imagine in your head that we had a method to cut out this section and then you add to that that we had a method to measure how long this section was that we cut out, for this person we--if we could do that measurement, we would have a section that was nine base pairs long and for their other chromosome we would have a section that was 15 base pairs long, so this one becomes nine. And I'm going to use the abbreviation "BP" to stand for base pairs, base pairs being the GC or the at pairs, and over here, (Indicating), are five repeats is equal to 15 base pairs. So--and then if we had another person, we could make another diagram just like this. If they had a chromosome that had ten repeats, that would be a section that was 30 base pairs. So it is those lengths that are different from person-to-person. And in this abbreviation, the "p" standing for polymorphism, the "l" stands for length. This test looks at length differences in the DNA and the length differences are created by the number of repeats that happen to be end to end at a specific location on a chromosome pair.
So is it then the case that this RFLP method is simply a tool to determine how we vary from one another as human beings at this particular region of DNA where these repeats vary?
As far as the method itself, to your knowledge do we have a prepared chart that illustrates how this difference is actually determined using the RFLP test?
We have or you have, and I have looked at, a prepared chart that doesn't illustrate this, but illustrates the set of procedures that allows you to ultimately look at these length differences.
And is it these length differences that are simply what you are looking for as a DNA scientist when you are using this RFLP method for identification?
All right. Your Honor, with the Court's permission I ask to be marked as People's exhibit I believe it is 242--
Now, Dr. Cotton, first of all, as you have described on your drawing, People's exhibit 241, where you have described these differences among us as human beings and the number of times we repeat a particular sequence, how are these particular regions of DNA picked out or selected to use for identification purposes?
Okay. All right. Then why don't we go ahead and then if you would describe this RFLP method in the basic steps that are delineated and perhaps I can ask you a question first. Have you had an opportunity to look at this large chart entitled "the RFLP method" that was previously prepared before coming to court today?
And in that review does this diagram basically describe the individual steps that are part of this RFLP DNA typing method?
Could you, with the help of a new diagram, or actually, would it assist in your describing DNA in a single-stranded condition...describe this RFLP testing process... Could you describe it in taking weeks?
You can think of them as a product of the information that is in the DNA, so when you look at the DNA you are looking at the most basic level of information that you can look at. When you look at a protein, that is a product of the information that is in the DNA, so you are sort of looking at a secondary level of information.
The great majority of the DNA, greater than 99 percent of the DNA, is going to be the same from person-to-person... There is then a small portion of the DNA that is different from person-to-person and that is intuitively obvious because you can look around even in a very large room... and you will see that with the exception of identical twins, people are different.
It is those lengths that are different from person-to-person. And in this abbreviation, the 'p' standing for polymorphism, the 'l' stands for length. This test looks at length differences in the DNA and the length differences are created by the number of repeats that happen to be end to end at a specific location on a chromosome pair.