DNA Fingerprinting Uses

What are some DNA fingerprinting uses? We’ve seen how DNA fingerprinting works, but what is it good for? There are a number of interesting examples of DNA fingerprinting uses. Alec Jeffreys invented DNA fingerprinting in 1985, and his method immediately was used to solve some serious issues. The first case that came to his attention was involving immigration. A family from Ghana had immigrated to the United Kingdom. When one of the four sons of the parents went for a visit to Ghana, he tried to come back and was detained at the airport by British immigration authorities, because his passport just wasn’t right. The authorities claimed that he wasn’t the son at all, but a cousin from Ghana who was trying to sneak into the country.

Jeffreys did DNA analysis of the mother and the three undisputed sons, as well as the son in dispute. The result was that he was definitely the son, and they allowed him to the country. Here we have an interesting DNA fingerprinting use, solving an immigration issue.

Criminal Cases

Soon, Jeffreys was called to a criminal case in Leicester. Two girls had been raped and murdered in the same area under similar circumstances two years apart. A man in jail confessed the second crime, but claimed innocence of the first one. Jeffreys did DNA fingerprinting of the victims, the suspect and semen that was found on the victims. As a result, Jeffreys concluded the same man had committed both crimes. It wasn’t the man who confessed, though. Why this guy confessed no one knows.

The police then asked all men in the area to give a blood sample. Five thousand men did so. When DNA analysis was done, there was still no match. Coincidentally, a woman overheard a man saying: “I gave two blood samples, one for me and one for a friend who didn’t want to give it”. The police found the man and asked him to give a sample. When they did the analysis, he turned out to be the killer.

DNA fingerprinting, as a result of this case, is now widely used in forensic cases.

Identifying People

Abhilasha Jeyarajah was a baby who was torn from his mother’s arms when, in 2004, a Tsunami hit Sri Lanka. Amazingly, Abhilasha survived. While his parents franticly looked for him, the baby was picked up by a local teacher and brought to the regional hospital. It was the worst day imaginable for the hospital staff. Hundreds of dead and dying children and adults were everywhere. When Abhilasha was brought in, the nurses were surprised to see that he was alive and healthy. It was a true miracle. He quickly became a celebrity in the hospital. Because he was the 81st infant brought in that day, they called him “baby 81”.

A few days later, his frantic parents came to the hospital, where they heard there were unclaimed babies. Joyfully, they were reunited with Abhilasha. “Not so fast”, said the hospital staff. In the previous two days, other couples had come to the hospital searching for their missing babies. Eight couples had claimed baby 81 was theirs. The question ended up with a judge.

There is a story in the Bible where King Solomon faced a dilemma of ruling which of two women was the true mother of an infant. Wise Solomon said: “I’m going to cut the infant in two so that each of you can have a half”. Of course the real mother was immediately revealed. In Sri Lanka, the judge had a worst case. He had nine couples claiming to be the parents of a six-month old boy.

Unlike Solomon, who just had wisdom, the judge had DNA for identification. Testing by molecular biologists soon found the real parents.

DNA in History

In 1918, with the Russian Communist Revolution raging; Czar Nicholas II, his wife and three of their children were killed in a town and buried in a shallow unmarked grave. In 1991, in a not Communist Russia, two amateur historians found what they thought was the grave of Nicholas II. There were two older people and three younger people. The sizes of the skeletons were consistent with the family. There were golden dental fillings. At that time not everyone in Russia could afford gold fillings.

The skeletons were too damaged for further identification, but fortunately, the bones had DNA. The short tandem repeats where compared in the bone with those of survivors of their family. There are surviving members of the Romanov family (the emperor’s family). For example, a great granddaughter of the Czar’s sister is still alive. The great grandson of the Czar’s aunt is still alive. Also, the body of the Czar’s brother was exhumed and some DNA was there.

The result: the same repeats were present in the dead family. That proved that they were from the Romanov family. There was a huge military funeral with full honors.

DNA fingerprinting has been used in many more instances. It was used to identify victims in the World Trade Center terrorist attacks in the United States in 2001. Many of the victims were beyond identification by any other means.

DNA databases are being built all around the world. For example, for some time in the United Kingdom, every one arrested for serious crimes has been DNA typed. There are now 3.5 million people in their database. In the United States there had been a number of propositions dealing with DNA typing for arrested people. The FBI has 3 million DNA stored and typed.

How Does DNA Fingerprinting Work

How does DNA fingerprinting works? DNA fingerprinting is a reliable method for identifying people. How DNA fingerprinting works as to do with something called the HLA system (human leukocyte antigen system). The HLA is a system that codes for proteins on the surfaces of many cells, including white blood cells. HLA genotyping is used in transplants. With this system, people could match transplant genetically, so that a transplant from one person to another wouldn’t be rejected by the recipient.

There are four different HLA genes, labeled nicely A, B, C and D. HLA A has 23 alleles. HLA B has 47. HLA C has 8 and D has 23. A person could be, for example, A11, B16, C5 and D11. With more alleles is more likely that people would be different from one another, and that parents and children can be matched better.

There are a couple of problems with the HLA system, however. First, you need well preserved tissue or blood. That’s a little difficult sometimes. Second, HLA proteins are not always present in our cells. Third, a lot of mixtures of these genes go on when gametes are produced. Fourth, there are pretty common HLA alleles, and some extremely rare ones.

The Short Tandem Repeats

Human genome sequencing has revealed that the genome contains short sequences that are repeated many times in tandem. These are appropriately called Short Tandem Repeats (STR). For example, let’s consider the DNA sequence TCAT. Looking through the whole genome, there are different Short Tandem Repeats, and the repeat numbers are inherited. You might inherit one chromosome that has TCAT repeated five times, and the chromosome from the other parent might have TCAT repeated seven times.

You might ask how this block repeat happens. Molecular biologists, believe it or not, say that it is not clear how it happens. They have some ideas, though, but it is not clear. There are 10000 of STR’s scattered throughout the genome, but for DNA fingerprinting purposes, we use typically 13 of them.

These 13 repeated sequences are polymorphic. This means that there is more than one type of repeats. I might inherit five repeats of TCAT at a certain location, and seven from my other parent. If we were all the same for this repeat number, they wouldn’t do any good. These different numbers of repeats are what set us apart.

To analyze DNA this way, however, we need first to make a population survey. We need to know the frequencies of the alleles. For example, five TCAT are present 50% of the time in the population, and seven is present 50% also. If somebody comes across and has ten would be a totally different individual.

Supposing we are dealing with two of these 13 short tandem repeat chains, and that they have three alleles: A, B and C. Let’s say that the A allele is frequent in one person in a hundred. The B allele is one in five. The C allele is the more common, four in five. For the second short tandem repeat: A allele one in ten, B one in two, C two in five. A, B and C are just the number of repeats. A for example might be five, and B seven repeats.

So, let’s review:

Short Tandem Repeat 1:
A: one in a hundred
B: one in five
C: four in five

Short Tandem Repeat 2:

A: one in ten
B: one in two
C: two in five

Here is the key argument for doing DNA identification in this way. It comes from Mendel and probability. For a person to be carrying both the A and B alleles of STR number 1, the combined probability is the product of the two probabilities. The combined probability of A(1/100) and B(1/5) is:
The probability for STR number 2 A(one in ten) and B(one in two) is:

So, what is the probability of having both of them at once? Yes! It is the product of their probabilities:

We’re now getting a pretty low probability. One in ten thousand! This is the probability of carrying the four alleles: A and B of STR 1, A and B of STR 2. Think about it. We have 13 different systems for identifying people. If we’ve got these different alleles, the probability of two people having identical alleles is vanishingly low. It turns out that we’re all virtually unique in these sequences. That’s why DNA fingerprinting is so useful in identifying people.