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By Professor Valerie Corfield, US/MRC Centre for Molecular and Cellular Biology, Faculty of Health Sciences, University of Stellenbosch

Genes come in pairs because they are carried in paired chromosomes. Only one gene of each pair goes into the sperm or egg that fuse together (at conception) to make a baby.

New technology shows us that very small differences in the DNA code in our genes result in different versions of genes. These genetic differences make us look different from each other, for example whether we have blue or brown eyes. How does this work?

• One of every pair of genes that your mom has came from her dad (your grandpa)
• The other pair of genes came from her mom (your grandma).
• When these genes were separated into the egg that made you, you inherited either the version of your grandpa’s or your grandma’s gene.
• The same is true for the genes you inherited from your dad. You have either the version that came from his mom or his dad (your other grandma or grandpa).
• If you have brothers or sisters, chance will determine whether they got the same version of each gene as you, or whether they got the other version. That is why you look different from each other.

What happens if you inherit two different versions of a gene?

What happens if you inherit two slightly different instructions, for example, the one to make blue eyes and the one to make brown eyes? Inheritance follows its own laws and often one gene version 'wins' over the other one. The feature (trait) controlled by that particular gene is called dominant. The one that 'loses' out is called recessive.

Brown eye colour is dominant over blue eye colour, so if you have one gene version instructing your body to make blue eyes and the other telling it to make brown eyes, the brown-eye gene will 'win'. Recessive traits are only seen if you inherit two copies of the gene that codes for it, for example, if you get the blue-eye gene from both your mom and your dad.

Following the patterns and laws of inheritance

The laws that govern inheritance were first studied by an Austrian monk called Gregor Mendel in the 1800's. He worked with peas but his discoveries apply to humans and animals too. They have helped people who study genetics to understand how individual traits are inherited and the patterns seen are called Mendelian inheritance. Mendel's laws are applied in plant and animal breeding programmes and are used in genetic counselling in families who suffer from inherited diseases.

An experiment in genetics

1. Make a list of what features you want to study in your family. Look at some ideas on the note.

   Any other feature characteristic of your family. What about other body parts, e.g. hand and foot shapes? You can look at photographs or ask your parents about their grandparents and even their great grandparents. Don't forget your aunts and uncles and your cousins.
   
   

2. Draw a pedigree showing all the relatives that you can investigate. Here is an example of how geneticists draw a pedigree. You can change this to fit your family.
   
   
3. Write the version of each chosen trait (such as curly or straight hair) under each relative on the pedigree. If you have studied a lot of different traits, you might want to use abbreviations so that you can list them under each person on your pedigree.
   
   
4. What features 'run' in your family? Can you see examples of dominant traits (e.g. dark eye colour, dark hair colour)? Can you see examples of recessive traits (e.g. red hair, chin dimple)?

Extract DNA from wheatgerm

• A cup of wheatgerm (from health shops or some grocery stores)
• Table salt (about 8 heaped teaspoons full)
• Clear alcohol (cane spirit, gin or rubbing alcohol from the chemist)
• Green dishwashing liquid (not the gel type)
• Lemon juice (fresh or bottled)
• Two glass bottles or large glasses
• A sieve or strainer
• Clean water

This experiment will allow you to extract one of the building blocks of life – isolated DNA – from plant cells. Although each DNA molecule is too small to see, if you follow the instructions, you will end up with visible DNA.

Break down the cell walls of the wheatgerm

In a large glass, dissolve one level tablespoon of salt in 300 ml of tap water. Add four squirts of lemon juice. Now add half a cup of wheatgerm to the solution and stir gently for 15 minutes. The lemon juice will break down the cell walls of the wheatgerm. Press this mixture through the sieve and discard the liquid. You will be left with a soggy pulp. Do the same for the other half a cup of wheatgerm. The pulp you now have contains the cell contents without the cell walls.

Dissolve the DNA

Put one level tablespoon of salt in 300 ml of water, stir the mixture until the salt is dissolved and add six teaspoons of alcohol. Add nine large drops of the washing- up liquid and stir gently. Add the soggy pulp from step one and stir it gently for about 20 minutes. During this period, the detergent in the washing-up liquid will dissolve the DNA into the mixture. Now add about 10 level teaspoons of salt and stir gently for 10 minutes.

Separate the DNA solution from the mixture. This step is easy. Just let the mixture stand and allow the solids to settle out. Then gently pour the liquid into another glass, until it is about a quarter full. Take care that the solids do not mix with the solution. The solution in the new glass now contains the DNA in a dissolved form.

Extract the dissolved DNA from the solution

Take the quarter-filled glass, fill it up with alcohol and stir very gently. As you stir, you will notice that the DNA precipitates out as very fine white threads. You can leave this mixture to further allow the DNA to settle. Gently pour the liquid off and there ... you have DNA!

View a short history of DNA

When DNA is detective ...

Just like fingerprints, every human has unique DNA. Scientists have found ways to tell one person's DNA from another person's; but unlike fingerprints, which can be changed using surgery, you can't change your DNA. Also, unlike fingerprints, which are only left at a crime scene if a person touches a suitable surface with bare fingers, DNA is tucked away in the centre of every cell in your body. DNA can be extracted from hairs, skin cells, blood, skeletons, bits of bone, teeth and body fluids left after a crime. So when traditional fingerprints are fuzzy and not much help, DNA fingerprints can speak out loud and clear.

DNA can last for a long time, especially when it is protected inside bones and teeth. Scientists have developed ways to extract DNA and to do DNA fingerprinting tests from very small amounts of material, like a dried blood spot or even from cells in saliva left over from a person licking a stamp.

DNA fingerprinting has provided evidence used to convict thousands of criminals. It also enables scientists to look at old cases using stored samples and evidence. This has allowed many prisoners who were found 'guilty' to be set free when DNA tests showed that they did not commit the crime. DNA fingerprinting was also indispensable in identifying victims of the September 11, 2001 bombing of the World Trade Centre in the United States, when scientists only had scraps of tissue or shards of bone or teeth to work with.

DNA fingerprinting has also been used to solve long-standing mysteries and identify people who pretended to be someone else (imposter). It can also be used to identify how people are related (parentage), such as in the case of Happy Sindane. In addition, mummies and skeletons that are hundreds and thousands of years old can now "tell us" if they are male or female, healthy or sick, related, even what they had for dinner, helping scientists to reconstruct the details of how these people lived. If only they'd tell us where they hid the treasure...

But what exactly is a DNA fingerprint?

A DNA fingerprint looks very different from an inky thumbprint on a page. So what does it look like and how are these DNA fingerprints made?

When police have a suspect, they take a blood sample from that person and take the DNA from the blood cells. The forensic scientists then focus in on specific areas of the DNA that show small differences between two people. The differences between these different parts of the DNA generate a pattern, like a supermarket barcode, that is unique to the person the scientists are investigating. This 'barcode' is called a DNA fingerprint. Sometimes at crimes scenes, only a very small amount of DNA, such as one hair, is left behind. In cases like these, the target areas of the DNA can be 'copied' so scientists then have enough to make a DNA fingerprint.

Fact file: Scientists solving crimes

Forensic science is the study of objects that relate to a crime. This evidence is analysed by the forensic scientists, who observe, classify, compare, count, measure, predict, and interpret data.

Fact file: How to become a forensic scientist

Forensic scientists work in the laboratory, in the field and in the courtroom. To become a forensic scientist you will need a bachelor’s degree in science (chemistry and biology); good speaking skills; good note-taking and writing skills; curiosity and personal integrity.

From a poster by Rapid Phase (Pty) Ltd for the Public Understanding of Biotechnology programme.
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