Almost as soon as you were born, relatives and friends probably commented on how ‘you’ve got your mother’s eyes or ‘you’ve got your dad’s hair’. People have long recognised that children resemble their parents – they’re not identical, but similar enough to talk about a family likeness.

In fact, all living things produce offspring that resemble themselves. This is heredity. Since early times, people have known about hereditary transmission but it was not understood. Of course, we now know that the DNA molecule passes on the instructions from one generation to the next. But it’s amazing to think that the DNA in every one of our cells also links us back to the first living things that emerged on the earth.

Did You Know ...?

Genetics not only tells us how parents pass on genetic information to their offspring but it also answers questions about how all species originated from a common ancestor.

Where did it start?

In the beginning, four and a half billion years ago, the earth was impossible to live on. Yet by the end of its first billion years, a staggering variety of species roamed the land. How did it happen?

Nobody knows exactly, but organic molecules may have formed spontaneously in the chemical soup of the earth’s early atmosphere. These building blocks may have then organised themselves into single-celled organisms. And following those early bacterial colonisers came algae, worms, mollusks, crustaceans and eventually flowering plants, insects, fish, reptiles, birds and mammals, including humans.

But how did one species transform into another? That’s where Charles Darwin’s genius comes in. In 1859, Darwin published his groundbreaking book ‘On the origin of species’. In it he describes his theory of evolution by natural selection, which explains how species change and give rise to new species.

Did You Know ...?

Have you ever wondered how new species arise?

It’s only possible because we are all different. When living things reproduce, they don’t make perfect copies of themselves. There may be small variations that could give some individuals an advantage. Because they are competing for resources, some inevitably do better than others. They will increase in number while those that do less well will decrease. That is natural selection. But how does it work in practice? For instance, peacocks advertise their fitness to peahens by producing amazing feather displays. Any animal in a group that can come up with an improved version will be more successful, and have a better chance of reproducing. Its offspring will inherit that small advantage and pass it on to the next generation. While those individuals that do not perform as well fail to breed and eventually die out.

What remained a mystery, even to Darwin, was how plants and animals pass on these desirable characteristics. A monk called Gregor Mendel, who lived in the mid-19th century figured it out. In the garden of his monastery in Brno, in what is now the Czech Republic, Mendel studied pea plants to try to understand the laws of inheritance. What he found also explains how we inherit all our characteristics – everything from a big nose to a faulty thyroid gland.

Do you have the same colour eyes as your brothers and sisters? Have you ever wondered why some people don’t?

Mendel found the answer for this and every other inherited characteristic by looking at pea plants. We all carry two forms of each gene called alleles, each with a slightly different spelling. Mendel discovered that some pairs of alleles come as two types, dominant and recessive. If you receive two different alleles from each of your parents, the recessive one is swamped. So if you inherit a blue-eye allele from your mum and a brown-eye allele from your dad, your own eyes will be brown because the blue allele is recessive.

But Mendel realised that even though recessive alleles are mostly silent, if you inherit two blue-eye alleles, for example, your eyes will be blue. This can explain why in some families blue eyes can disappear in one generation and reappear in another. What’s more, Mendel worked out the frequency with which this happens. In doing so, he defined the rules of inheritance, although the importance of his discoveries was only appreciated after he died.

Mendel’s rules can also explain why an apparently healthy couple can have a child with thalassemia – a recessive genetic disease that affects red blood cells. What’s important to realise is that for every inherited characteristic, Mendel’s rules apply. So a genetic counsellor can advise parents who are carriers of the disease on the probabilities of a future child inheriting a disorder. A carrier is a person who has a faulty allele of a gene but does not suffer from the disease as their other allele functions normally.

Did You Know ...?

What about genetic diseases?

Genetic diseases are not really that common. We know of about 5000 diseases caused by defects in one gene, and most of them are quite rare. One example of a genetic disease is cystic fibrosis, an illness where fluid and mucus accumulates in the lungs. It’s caused by a spelling mistake or mutation in one gene, and 1 in 25 of us are carriers. It sounds alarming, but to actually fall ill you have to be unlucky enough to get a faulty allele of the gene both from your father and your mother. Because the chances of this happening are small, only 2500 people in the UK have cystic fibrosis.

You may not have a genetic disease, but your DNA is still full of mistakes. It’s impossible to have an error-free DNA because, as the DNA copies itself, mistakes happen all the time. And, occasionally, your DNA will be damaged. The good news is that the cell relies on powerful repair systems to fix and correct the occasional mutation.

Launch it ... Have a go at Mutation Man and see if you can identify which character is most susceptible to mutation!

Unfortunately, DNA repair systems can fail. Mutations can be sparked off by environmental influences such as nuclear radiation and excessive UV light from sunbathing, leading to cancer and other diseases. Some chemicals such as thalidomide, mustard gas, asbestos and cigarette smoke can also cause DNA damage. One mutation alone is rarely enough to cause a fully-fledged cancer, but if you accumulate mutations in different genes over time this eventually creates a cancerous cell.

Fortunately, not every person exposed to these influences develops a disease. Whether you fall ill is partly down to your genes (nature) and partly down to the environment (nurture). Many common conditions such as diabetes and heart disease are caused by a complex interaction between genes and the environment. So if you have the faulty version of a gene it may be possible to change environmental influences, such as diet or lifestyle, to avoid becoming ill. How much is ‘nature’ and how much ‘nurture’ is still poorly understood but every day scientific research is throwing up important facts.

It’s not just diseases that spark the debate over nature versus nurture. When it comes to personality and behaviour, the argument over which is more important – nature or nurture – becomes really heated. Are people born criminals? Are there genes for violence and aggression or is this a result of deprivation and bad parenting? Similar discussions rage over intelligence. Most people are reluctant to accept that genes can define how smart we are and instead prefer to think that intelligence is a result of education and upbringing. There are no simple answers. Clearly, we are a complicated mix of genes and the environment, and what scientists are trying to figure out is how much of our character comes from each.

Discuss it ... Do you think that we should try to find genes that may be associated with behavioural traits?
Teachers' Notes ... For Teachers' Notes on this subject, click here.