THE GENETIC REVOLUTION

From the most ancient times a rule of life has been seen to be true: insects breed insects, birds breed birds, cows breed cows and humans breed humans. If you take acorns from an oak tree and plant them the result is more oak trees. Creatures and plants remain true to type, faithfully passing on their characteristics from generation to generation. Where thereare slight variations, for example in skin pigmentation or in the colouring of flowers, then these too mcan usually be traced down the generations. The basis of life has been remarkably stable considering its complexity.

The basis of this inheritance was not understood however. An understanding of how organisms are built out of cells only emerged with the invention of the first light microscope by Robert Boyle in the eighteenth century (check date). It was many decades later before we began to understand how the cell works. Most of the structures in a cell could only be seen with the high power of the electron microscope.

However, for many centuries experiments were already taking place with cross-breeding - the earliest technique of genetic engineer­ing.

In order to understand the mechanism of inheritance, we need to start in an Austrian monastery around 1760, in the potting sheds of a gardener called George Mendel.This monk was curious to know what would happen if he took pollen from one type of plant and used it to fertilise another.Would the pollen be accepted?Would it succeed in fertilising the plant?If it did, would seed result which would germinate?Finally, when it germinated, what kind of plant would grow?

For thousands of years previously such attempts had been made with animals.For instance, in the time of Jesus, it was common to allow a horse to mate with a donkey:the result was a cross-fertilised egg which went on to develop into a rather strange-looking creature at birth.The creature had some of the best characteristics of both parents and was known as a mule. This new species had one important drawback: you could not breed from it because it was always sterile.

Hundreds of others examples could be given over previous centuries of selective breeding - indeed Jacob in the Old Testament seemed to know what he was doing in selectively breeding white and black sheep to produce a herd entirely coloured as he wanted, at a time when sheep ownership was being determinedsolely by colouring of their woolen coats.

The process of inheritance has been well understood by families who observe - say -grandpa's orange hair through to a grand­child or other family likenesses.However, the mechanism has only relatively recently been fully understood.Why was it that dark haired parents could occasionally produce a fair-haired child ?

Mendel was interested in all this. Moreoverd the monastery stood to gain from improved strains of cereal plants. Mendel found that when he cross-fertilised closely related plants with obvious differences, he got neither a mix nor equal numbers of aech type. Instead he found a curious pattern. After a while he found he could predict in advance not only what variations he would see, but also how many of them.He realised that in each seed there was a lot more information stored than would ever be used to form the new plant.

Some of this information it seemed was hidden away in many plants and only expressed when cross-fertilisation took place. It seemed like each plant had its own strong and weak features. Weak features only came to the surface under certain circumstances. These strong features have become known as "dominant" while those which tend to be hidden away are called "recessive".

This same information and understanding is used daily in dozens of Genetic engineering laboratories all over the world every day. When he cross-ferti­lised tall and short varieties of the same plants he found he always landed up with seeds that produced plants in a fixed ratio of three tall to one short (check which way round). From this he prposed a theory which was to revolutionise our thinking about breeding.

He came to the conclusion that each plant must have two sets of instruc­tions for each part of its structure. Therefore each plant had two set of instructions for height. However if the plant had a mixture, then the tall one was always dominant.

You can see how this works in Fig 1. When sperms or eggs are made - or their equivalent in plants - the original cells divide into two, with only half the full set of instructions needed for life in each half. So parents with a mixture of tall and short instructions in their cells will produce sperm or eggs with either one or the other.

Fertilisation happens when pollen and ova meet, (or sperm and eggs in animals). When this happens, the new composite cell ahs a complete set of instructions and is able to start forming a new plant. Clearly four types of plants could result: one type where both pollen and ova have provided tall instructions, another where both are short, and two where there is a mix. Three of these out of four will be taall. The only plant type that will turn out short will be the one where both sets of instructions are short, because both parent plants passed on the recessive gene.

Fig 1

"Mother" "Father"

T S T S

Both these plants have a tall gene in the pair so both are tall.

Fig 2

1. Mother Father

T T A tall plant

2. Mother Father

T S A tall plant

3. Mother Father

S T A tall plant

4. Mother Father

S S A short plant

So in his classic experiment:two tall plants cross-fertilised produced short plants one time in four.Interestingly,if short plants are only fertilised by other short plants then you can see that no more tall plants will ever be produced.A new strain will have been created.

Simple methods like this have been widely used by gardeners and horticul­turists for over a hundred years: selective breeding from plants showing the characteristics you want to encourage. The development of pedigree dogs is an ancient art which has worked on the same principle: only allowing dogs to mate that have the right characteristics.

Incidentally you can see straight away a major problem: if you go on inter-breeding from just one small group, then more and more recessive genes may emerge. Some may have hidden dangers for the animal. Take dogs again as an example: in the wild they breed widely producing a group of fairly even appearance. If bad traits emerge , they tend to be eliminated because they dogs do not survive long enough to breed or because the recessive traits are covered up by dominant genes from others in the group. However in domestic breeding, the dominant genes are being deliberately trimmed out. The result is a beautiful breed but one which may be susceptible to a high rate of blindness, tumours or hip problems for example. Ther are many inherited disorders in humans that can arise in a similar way.