genetics

Branch of biology concerned with the study of heredity and variation – inheritance. It aims to explain how characteristics of living organisms are passed on from one generation to the next. The science of genetics is based on the work of Austrian biologist Gregor Mendel whose experiments with the cross-breeding (hybridization) of peas showed that the inheritance of characteristics and traits takes place by means of discrete ‘particles’, now known as genes. These are present in the cells of all organisms and are the basic units of heredity. All organisms possess genotypes (sets of variable genes) and phenotypes (characteristics produced by certain genes). Modern geneticists investigate the structure, function, and transmission of genes.

Before the publication of Mendel's work in 1865, it had been assumed that the characteristics of both parents were blended during inheritance, but Mendel showed that the genes remain intact, although their combinations change. As a result of his experiments with the cultivation of the common garden pea, Mendel introduced the concept of hybridization (see monohybrid inheritance). Since Mendel, the study of genetics has advanced greatly, first through breeding experiments and light-microscope observations (classical genetics), later by means of biochemical and electron microscope studies (molecular genetics).

In 1909, Danish botanist Wilhelm Johannsen coined the term ‘gene’, from the Greek word genos meaning ‘birth’, to describe these particles. In 1911, he went on to make the distinction between ‘genotype’ and ‘phenotype’. Genotype refers to the sets of genes carried by an organism that are capable of being passed on to the next generation, whereas phenotype refers to the physical traits or characteristics that the genes produce in an organism.

A major discovery in genetics came in 1944, when Canadian-born bacteriologist Oswald Avery, together with his colleagues at the Rockefeller Institute, Colin McLeod (1909–1972) and Maclyn McCarty (1911–2005), showed that the carrier of hereditary information was deoxyribonucleic acid (DNA), and not protein or any other material as was previously thought. A further breakthrough was made in 1953 when James Watson and Francis Crick published their molecular model for the structure of DNA, the double helix, based on X-ray diffraction photographs. The following decade saw the cracking of the genetic code. The genetic code is said to be universal since the same code applies to all organisms from bacteria and viruses to higher plants and animals, including humans, with only a few exceptions (such as viruses). Today the deliberate manipulation of genes by biochemical techniques, or genetic engineering, takes place.