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Genetics is the scientific study of the structure, function, and transmission of genes in living things. The field of genetics includes many disciplines and uses many different techniques. Historically, genetic scientists (geneticists) investigated patterns of inheritance in whole organisms by observing the distribution and segregation of physical characteristics across several generations of breeding. This type of genetic research, called classical or Mendelian genetics, is still conducted today and remains invaluable for identifying and describing inheritance patterns and traits. The development of modern molecular biology and biochemistry has facilitated the growth of a different but complementary branch of genetic research, known as molecular genetics. This branch focuses on understanding the detailed molecular mechanisms that govern the transmission of genetic material from one generation to the next.

The observable characteristics that describe any organism (for example, height or eye color in humans, or flower size and color in plants) can be broadly grouped into two categories: those that are acquired because of environmental effects, and those that are inherited. Genetics is concerned with this second category—that is, inherited characteristics. Gregor Mendel, an Augustinian monk of the mid-nineteenth century, observed that characteristics such as shape and color in peas were passed from parent to offspring regardless of the environment the plants grew in. He meticulously counted and documented thousands of crosses between different pea varieties to deduce the principles that governed this inheritance. In the end, his work was so influential and vital to the development of genetics that the term "Mendelian" genetics is now a synonym for classical genetics.

Several key concepts put forward by Mendel have been expanded, as the science of genetics has grown. It is now known that genetic information is passed on as a series of discrete units known as genes, each of which is associated with specific traits. Furthermore, most organisms (including humans) get two copies of their genetic information, one from each parent. This means that most living things have two copies of each gene, and that these two copies are not necessarily the same, since they came from different parents. When an organism reproduces, it passes only one of its two copies to an offspring. Importantly, copies of different genes separate (segregate) randomly into the next generation, which means that an offspring can receive either of the two copies that the parent has, and that the set of copies that is passed is different from offspring to offspring.

A growing interest in the biochemical basis of life led geneticists of the twentieth century to attempt to identify the substance that carries genetic information from one generation to the next. Intense research led to the discovery of DNA (deoxyribonucleic acid), the molecule that encodes the genetic information of almost all organisms (a few viruses use RNA—ribonucleic acid—to encode their genes). Understanding the relationship between observable, inherited traits and the structure and organization of the genetic material (DNA) is the primary focus of molecular genetic research. The powerful combination of classical genetics and molecular genetics has led to many important advances in biological and health-care research and continues to be a major force in the advancement of science in the twenty-first century.

Daniel J. Tomso

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Medicine EncyclopediaGenetics in Medicine - Part 2