Any complex structure, from an office block to a jet airliner, is built from a set of plans. The human body is no exception. The body grows, develops and functions according to a master plan. The basic plan for a human being is passed from parents to offspring, together with minor variations that make each of us an individual within the species. The plan consists of molecules of DNA (deoxyribonucleic acid) which contain, in a chemically coded language, all the instructions required to make a human. The DNA molecules are enormously long and are packaged, together with protective proteins, as the chromosomes in the nucleus of every human cell.
Heredity is the process by which the instructions are passed on (inherited) from one generation to the next. Its study is called genetics. The basic unit of inheritance is the gene; and ourdetermine whether we have blue eyes or brown, dark skin or fair, straight hair or curly hair and so on.
Genetics is a complicated subject with its own language. One thing that often causes confusion is the determination of the exact nature of a ‘gene’. First, a gene is known to be a functional unit – that is,are named by what they do rather than after the substance of which they are composed. In the early days of molecular genetics it was thought that genes were single units, sections of DNA strung out in linear fashion like beads on a necklace, each one representing a different trait; eye colour, hair colour, height, resistance to infection, and so on. It has now been revealed that different bits of DNA from various parts of a chromosome, and even from different chromosomes, cooperate to produce one result.
Second, the system is certainly not as simple as one in which one gene equates with one body structure or function. In many cases numerous genes work together and interact to produce a specific characteristic such as body height. And of course in addition to all this are the effects of environment. A person may have genes to give him great musical ability, but if he or she is not given the opportunity to develop this talent during childhood, it may never come to light. That person will then never fulfil his or her true genetic potential.
The laws of
The principles ofhave been used for many years by breeders who have crossed animals or plants with different characteristics in order to produce new breeds which possessed certain desirable characteristics. New breeds of dog had been produced for many years before the laws governing breeding were understood. The principles of the laws of heredity were in fact discovered not by a scientist but by an Austrian monk, Gregor Mendel (1822-1884). His work involved experimenting with garden peas, the results of which were published in 1866. It was not until nearly a hundred years later that the biochemical puzzle behind Mendel’s work was unravelled by the American biochemist J.D. Watson (1928) and the British biochemist F.H.C. Crick (1916), when they published their findings on the nature of DNA and the genetic code. These and subsequent findings made biochemical sense out of all the discoveries of the previous hundred years. They also laid down the ground rules of genetic engineering.
From the humble vegetable garden, Mendel was able to deduce the principles of heredity which are best summarized in three laws. The first law states that for each possible plant or animal characteristic, there is a pair of factors (now called ‘genes’) that govern it. The second law states that when gametes (sex cells such as sperm and eggs) are formed these two factors separate and pass into different gametes. This law is called the law of segregation.
The third law, known as the law of independent assortment, states that genes segregate randomly from other genes. If the genes of a mother cause the child to have fair hair, this does not mean the child will also inherit the shape of her nose. The genes of hair colour are transmitted independently from the genes of nose-shape.