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Genetics: Introduction

 
Genetics - Introduction
Cellular processes
Reading the instructions
Copying genes - Mitosis
Copying genes - Meiosis
Meiosis and Fertilisation
The sex chromosomes
Is there a gene for it?
Inheritance
Genes and personality
Non-Mendelian Inheritance
Epigenesis
The Maternal Effect

 
For many thousands of years, humans have known that, if one saved the best seed from a particular crop for sowing the following year, the resulting crop was likely to be more successful. Moreover, they had discovered that stock could be progressively altered to better suit their requirements by selective breeding. In the late eighteenth century both plant and animal breeding were being approached in an increasingly methodical way. In particular, Kolreuter set out to understand hybridisation and the difference between species and varieties, though, like most people, he believed that both parents produced a uniform 'fluid' that blended in the offspring, so that they were a mixture of the two. Meanwhile, there were the working breeders, such as Robert Bakewell in England.

In a monastery at Brno in Moravia, which was then part of the Austro-Hungarian empire, Gregor Mendel began to study the mechanisms involved, using the garden pea as his subject. Far from being an "an obscure corner of Europe" as some writers have it, Brno was one of the epicentres of biological and horticultural research, and Mendel was a respected member of the scientific community. As a physicist he was "one of the first biologists to show how maths can be used (and must at times be use) to solve the problems of life, just as it does of the physical universe."(1) He was among the first to apply what might be considered the modern scientific method, taking a large sample population and carefully analysing his results, foreshadowing the later development of statistical analysis by Pearson and others.

The pea was not a random choice. Firstly, the anthers are enclosed, since the flower normally self-pollinates, so the likelihood of unwanted cross-pollinations was low. Secondly, he was able to select certain features which, after a few seasons breeding, would be stable and clearly identifiable - that is, they would breed true.

For example, he identified strains of peas in which the seed cases were either green or yellow. He then used pollen from yellow-seeded plants to fertilise green-seed plants. The seeds of the offspring were all yellow - not a mixture of both. Nor were they greenish-yellow.

When the plants of this yellow-seeded generation were self-pollinated, green coloured seeds reappeared in the next generation. Overall, the ratio was always the same - three yellow-seed plants to one with green. Moreover the same effect occurred with the other characteristics, but the appearance of yellow and green seeds did not coincide with, say, their appearance as smooth or wrinkled. In other words, although the characteristics appeared individually in a certain pattern, they did so independently of each other.

Characteristics that pass down through inheritance do not do so as the mixing of some kind of essence, or in the blood. That is, there were no greenish yellow, or yellowish green peas. Inheritance is transmitted as discrete bits of information that might or might not be expressed.

But why was the ratio 1:3? Mendel deduced that there was a unitary "factor", as he referred to it, passed on by both parents. If the factor from both was yellow then the offspring's seeds were were yellow. Similarly if the factor from both parents was green. If however, the factor from one parent was yellow, while that from the other was green, the offspring's seeds were yellow. In other words the yellow version of the factor was dominant.

We nowadays call these factors "genes" Any gene for a given characteristic may have variants, called alleles of each other, which may be dominant or recessive with respect to each other.

Mendel's work created little stir when he published his findings. It took twenty or more years for the realisation that this was to answer to a problem which had bothered Darwin - the fundamental mechanism underlying evolution.

Exactly what constituted the gene remained elusive until improvements in biochemistry occurred. In 1928 Griffith confirmed that there was something in bacterial cells which determined their structure, and in 1946, Avery, McLeod and McCarty showed that it was DNA - deoxyribose nucleic acid. Their work was not generally accepted, however, until, in 1953, Watson and Crick showed how the elements of DNA were arranged in pairs, and succeeded in building a model of the DNA molecule.

The representation of the long double spiral molecule of DNA has become a familiar sight in today's media. Watson and Crick showed how the individual elements, called base pairs, could form an elaborate code, that elements could be altered to change the code and, most importantly, that the two spirals could unwind, take up fresh material and become two molecules where there was one before.

In this scheme, genes for individual characteristics may consist of a few, or many hundreds of base pairs. The complete DNA molecule of a species is often referred to as the genome, while the specification for an individual is its genotype, a term which may also be used for the general genotype of a population. Each base pair codes for the production of a particular protein and it is important to keep in mind that is the aggregation of proteins produced by DNA that is important. What these proteins produce, in the end, is the phenotype, the sum of the physical characteristics of the individual organism that develops.

The genome is not a blueprint, with a one to one mapping to individual features, nor is it simply a linear code that is read off like a string of beads. It is better described as a programme that specifies the course of development of an organism, a process called ontogeny

Think of a town or city. Wherever we are, our requirements are the same, places of shelter, administrative buildings and so on. Yet individual cities are quite different, depending on where they are situated, the weather conditions and many other factors, not the least being their history.(1)

While DNA provides the code to produce the combinations of proteins within the cell, it can only do so within the environment of the cell, which in turn can only operate within the environment within which it is situated.

Every human being that is conceived has a slightly different programme, which may be read in different ways. In a very real sense, then each of us is a new design.

There has been much in the news about cloning. What if Saddam Hussein cloned himself? As someone pointed out "So what? All you'd get is two babies." Who we are, individually, is our own particular phenotype, the interaction of our personal genotype and our personal environment. The interaction of nature and nurture has a whole new meaning.

Endnote:There is now a computer programme called SimCity, where you set the initial conditions and the city emerges from them.

Bibliography and Good Reading

  1. Tudge, C., (2000) In Mendel's Footnotes London: Jonathan Cape
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Bland, J., (2001, 2004) Genetics Introduction http://www.gender.org.uk/about/03gene/30_intro.htm
 
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Web page copyright Derby TV/TS Group. Text copyright Jed Bland.
08.04.98 Last amended 30.04.02, 24.12.03, 07.03.04