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The science of genetics

Updated Wednesday, 3rd August 2016

This extract from An Introduction to the Human Genome explores the basics of the science at the heart of genetics.

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The 21st century began with the completion of the first draft of the complete sequence of the human genome. Coupled with new techniques for intervening in genetic disorders such as mass genetic screening, gene therapy and genetic manipulation these are among the most fraught areas of debate in modern biology and medical science. Such revolutionary techniques bring with them profound social and ethical dilemmas as they open up the possibility of redirecting our future evolution.

The science of genetics

The study of genes began in 1900 when it was shown that genes govern inheritance in many different creatures. In 1907 it was shown that the same patterns of inheritance could account for the transmission of eye colour in humans. However, not until 1953 was the structure of DNA deduced by Watson and Crick. By the end of the 20th century, many thousands of genes had been discovered and their sequences determined, including many that have roles in disease. These genes are scattered throughout the genome.

Our genetic structure

Every cell of a human being has two sets of twenty-three chromosomes and every cell of a given individual has exactly the same DNA in it, with the exception of the gamete cells, that is to say the sperm cells and the egg cells. Now these specialised kinds of cells have only one set of chromosomes. So when an egg is fertilised by a sperm, one copy of chromosomes comes from the father and one copy of chromosomes comes from the mother.

 

Together they form a nucleus with two sets of chromosomes. So the fertilised egg cell, the zygote, has two sets of chromosomes. At each cell division which leads up to the formation of a fetus, and then a baby, and then an adult human being, these chromosomes are replicated and equally partitioned between each cell, so that each cell of the adult individual has the same set of chromosomes, the same set of genes - two sets of chromosomes, two copies of each gene.

Along the length of DNA in each chromosome are these units called genes, which is why DNA is referred to as the genetic material. Our characters - the structure and appearance of an individual, such as blue or brown eye colour - depend on the functions of genes. Genes also contribute to a person's behaviour and health, including susceptibility to certain diseases, such as heart disease.

Human genome sequence

When genetics is explained in the news there's usually a graphic of a whole string of lettering. This is the human genome sequence. What you see on the news is an alphabetic representation of the molecular structure of DNA.

The information is stored in the DNA as a series of letters: A, C, G, and T - that's the end product of the experiment. Each letter represents a different chemical (adenine, cytosine, guanine and thymine).

Like all codes, the one in DNA carries information or instructions; in this case ones that direct the growth and survival of each individual. To be meaningful, the letters A, C, G and T have to be in the correct order or sequence in each gene.

The Human Genome Project involves identifying each of these letters in the correct sequence for each chromosome in turn for the whole genome. The human genome comprises approximately six billion, 6,000,000,000, letters (chemicals) of A, C, G and T, joined together in pairs - 3,000,000,000 pairs - in a linear sequence along the length of the chromosomes.

The project was made possible only by a large number of personnel working in a substantial number of laboratories and by database technology of computers. Each computer is linked to the Internet on which the results are published world wide for free with no restrictions on their use or distribution.

Analysing sections of DNA

Fragments of DNA have to be propagated, i.e., large numbers of copies made of individual DNA fragments, generally grown up as sections of DNA in bacteria, or in the test tube, by all the laboratory techniques such as a polymerase chain reaction.

A polymerase chain reaction is a technique which molecular biologists use to amplify specific sections of DNA that they're interested in, in the test tube. These DNA, which have been propagated either by PCR - polymerase chain reaction - or by growing it in bacteria, are then analysed using DNA sequencing techniques to determine the sequence of basis, that is to say the letters, in the DNA. Lots of sequence determinations are assembled together using high powered computing facilities, to generate long stretches of DNA sequence such as the entire human genome sequence.

Phenotype and Genotype

Most families have collections of photographs spanning several generations. Brothers and sisters may share features that they also share with their biological parents, but in addition they have their own particular combination of characters that make them recognizable as individuals. You can detect visible features or characters such as height and colour of skin.

But what we can see is only a very small fraction of the differences between individuals. For example, we cannot see their blood group. The sum of all the characters that an individual possesses, including their morphology (the shape and structure of their body), their speed of movement and co-ordination as well as their temperament and personality is described as the phenotype.

The differences among parents and their children or among individuals in a population is called variation. Whilst we all share the same genes, variants of these genes can exist. These different forms of the same gene are commonly termed alleles and they account for whether an individual's eye colour is blue or brown. Each individual (apart from identical twins) has his or her own unique combination of alleles, i.e. their genetic make-up; the full set of genes is called the genotype.

The phenotype of each individual is the result not only the combined effect of their genes (their genotype) but is also influenced by their environment - all the physical and social factors that interact with our genotype, such as the cytoplasm surrounding the nucleus of a cell, the uterus in which a fetus develops and the size of family of diet. Some characters are influenced by the environment more than others and we can summarize the relationship between genotype and phenotype as follows:

genotype + environment = phenotype.

Inheritance of genetic disorders

Human diseases can be classified in a number of ways, from those caused by nutritional factors, to those caused by infectious agents to those with a genetic origin. In the case of the latter, individuals are ill because they are born with a defective gene. Over 3,000 genetic disorders or genetic diseases, many of them rare, have been shown to be due to defective copies of genes.

Predictive medicine

Some experts now suggest that new knowledge of human genetics is likely to transform medical practice. They propose three main possibilities:

Genetics will lead to the classification of diseases on the basis of the underlying genetics or biochemistry, rather than by symptoms;
Genetic information will identify people who are likely to respond to drugs or to be harmed by them;
Genetic variation will be a new 'susceptibility factor' or 'genetic risk factor' permitting monitoring.

Predictive medicine, if it comes, will be based on a much wider use of genetic testing - for more people, and more disorders. At the moment, there is quite a big gap between the scenarios sketched for the future of genetics-based medicine by forecasters and what the health system is geared up to deliver. As with any new technology applied to health in the context of a complex delivery system, implementation is not going to be simple.

 

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