Mendel, the Gene and a New Age of Medicine

The discovery of genes and DNA looks sure to revolutionize society. Given this, ignorance is no longer acceptable. As with most difficult subjects, it is best to start at the very beginning.

Darwin, Evolution and the Problem of Heredity

In 1859, Charles Darwin published On the Origin of Species. This new theory of evolution was based on several observations. First, that animals and plants reproduce as often and as much as they can. Second, that life is harsh and cruel – food becomes scarce, droughts occur, new predators arrive, and so on. Individuals with some kind of advantage – longer necks to reach the higher leaves, feathers to insulate them during a snowstorm, etc – survive and thus pass on the advantageous trait to a new generation.

The feathers, claws, stripey markings, etc, give an advantage in the struggle for survival and so spread throughout the group. When the group divides, however, each is subjected to new stresses, demanding different traits. Eventually, these new traits spread and you have two separate species, unable to reproduce with one another even if re-acquainted.

Darwin’s ideas were controversial, however, and provoked much criticism. Indeed, so serious were these criticisms that by the late 19th century many considered natural selection a discredited theory.

Darwin’s central problem was variation. For example, a rabbit has 20 offspring. One of them has white fur, the other 19 have brown. In the snowy landscape, the white rabbit is difficult for predators to spot and so has an advantage. But why was the rabbit born with white fur in the first place? Where did that variation come from? And how was this white fur passed on? Darwin would go to his grave unaware that an Austrian monk named Gregor Mendel had in fact stumbled upon the answer.

Gregor Mendel and the Discovery of Genes

Of course, neither Mendel nor Darwin were the first to note that animals pass on features to their offspring. For centuries, farmers had used prize livestock for breeding. They knew that the fastest horse, or woolliest sheep, would produce the best offspring – they just didn’t understand why.

Some critics assumed that features blend together: a very tall man and a very short woman should thus produce an average size child. The more perceptive disagreed and noted how features disappear in one generation only to re-appear unchanged in the next.

Gregor Mendel was born, in what is today the Czech Republic, in 1822. In 1843, he entered the monastery of Brunn, where he pursued his interest in science and gardening. From 1856 (just three years before Darwin published his great work in England) he began using the monastery garden as a laboratory.

Mendel was intrigued by the way living things pass on information. To gain a better understanding, he experimented with garden pea plants. These were especially useful because of their clear-cut features: for example, they will be either tall or short and will have either white or purple flowers. Why were some plants tall and others short? Why were some flowers purple and others white? And why did the colour and height never blend to produce a medium-sized plant or a light purple flower?

Mendel focussed on colour. He carefully pollinated a white-flowered plant with the pollen from a purple-flowered one. He then took the seeds, planted them, and noted the flower colour of the new plants. Again, he used the pollen from these new plants, planted the seeds, and noted the result.

The seeds of the white and purple flowered plants all grew purple flowers. But when the purple flowered plants were then used to pollinate one another, a few white flowers re-appeared. Mendel argued that hidden within the plants must be units of inheritance, which he christened “factors” rather than genes. He then concluded that these factors must work in pairs.

In the case of pea plants, there must be one “factor” for the white flower and one for the purple. For some reason, in the first generation of plants the purple factor dominated. The key point is that characteristics are not blended but remain separate and distinct. This had been a major headache for Darwin. Yet when Mendel died in 1884, his work was unknown. In the 20th century it was re-discovered and his “factors” were re-named “genes.”

The Modern Understanding of Genetics

To understand the gene, you must first understand its context. Living things, whether they be trees, crocodiles, or humans, are made of cells. Within these cells you find a nucleus, which itself contains thread-like structures known as chromosomes, composed of DNA and protein. The number varies between species by the way: in a human cell, you find 46; in that of a horse, there are 64; a polar bear’s cells contain 74.

A gene is a unit of this DNA. Think of the chromosome as like a necklace and the genes as the pearls or beads strung along it. In 1903, Walter Sutton observed that the chromosomes occur in pairs, and these matching chromosomes contain the same genes at the same location. So there are 46 chromosomes in each cell, arranged in 23 pairs. In each pair, one chromosome comes from your mother, one from your father.

In essence, genes are sets of instructions for building a human body. Each set of chromosomes carries around 30,000 to 40,000 genes. Though most people’s genes are very similar, there are variations; if there weren’t we would all be identical.

The gene controlling a particular feature comes in two or more versions, known as alleles. Take eye color, for example. Imagine a child inherits the allele for blue eyes from her mother and the allele for brown eyes from her father. Should this happen, the child will develop brown eyes. Why? Because in the case of eye color, one allele will be “dominant,” the other “recessive.”

Every pair of alleles, or genes, includes one dominant and one recessive. The recessive usually fails to produce a feature in an organism because it is overwhelmed, or “dominated,” by the dominant allele. The recessive allele can produce a trait or feature, but only if the dominant allele is not present.

Genetics and the Future of Medicine

So what will the impact be on medicine? Siddhartha Mukherjee has called the gene “one of the most powerful and dangerous ideas in the history of science.” Humanity now understands what Mukherjee defines as “the irreducible unit” of biological information – of life. And that means power.

First, and most obviously, it will mean a better understanding of disease. We know, for example, that you can inherit genes that increase your risk of certain kinds of cancer, or that make you vulnerable to heart disease. Gene therapy involves replacing or fixing broken parts. Once the faulty gene is identified, it can be replaced by a normal gene.

Imagine your car splutters to a halt one night. The mechanic takes a look and explains that the spark plug has melted. He replaces it and away you go. In theory, the same thing could work for genes and cells. For example, take immunodeficiency disease, or SCID. This occurs when a gene mutation interferes with the normal functioning of the immune system. As a result, the body struggles to fight off the simplest infections.

Gene therapy is tricky and is still only in its earliest stages. However, doctors have treated SCID successfully. Cells were taken from the patient’s bone marrow, where cells known as lymphocytes are made. Lymphocytes defend the body against germs. A specially modified virus was then used to carry a normal version of the faulty gene into the bone marrow cells. The cells then multiplied to produce normal lymphocytes that fight infection!

The discovery of genes also means better drugs. Understanding the human genome will allow scientists to develop drugs that combat disease by altering the way a gene works. Your genes also influence the way you respond to a particular drug. In the near future, those drugs will be altered to match your genetic make-up.

On 28th February 1953, Francis Crick walked into The Eagle pub in Cambridge and announced that he and his colleagues had discovered DNA. Or, as he put it, “we have discovered the secret of life.” Whether we use that discovery to increase or reduce human suffering is up to us.

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