Genetics

Although humans have known about inheritance for thousands of years, the first scientific evidence for the existence of genes came in 1866, when the Austrian monk and scientist Gregor Mendel published the results of a study of hybridization of plants—the combining of two individual species with different genetic make-ups to produce a new individual. Working with pea plants with specific characteristics such as height (tall and short) and color (green and yellow), Mendel bred one type of plant for several successive generations. He found that certain characteristics appeared in the next generation in a regular pattern. From these observations, he deduced that the plants inherited a specific biological unit (which he called factors (now called alleles), genes determining different forms of a single characteristic) from each parent. Mendel also noted that when factors or alleles pair up, one is dominant (which means it determines the trait, like tallness) while the other is recessive (which means it has no bearing on the trait). It is now understood that alleles may be single genes or sets of genes working together, each contributing to the final form of a physical characteristic (multiple allelism).

The period of classical genetics, in which researchers had no knowledge of the chemical constituents in cells that determine heredity, lasted well into the Three generations of identical twins. © Gerald Davis/Phototake NYC. Reproduced with permission.
twentieth century. However, several advances made during that time contributed to the growth of genetics. In the eighteenth century, scientists used the relatively new technology of the microscope to discover the existence of cells, the basic structures in all living organisms. By the middle of the nineteenth century, they had discovered that cells reproduce by dividing.

Although Mendel laid the foundation of genetics, his work began to take on true significance in 1903 when Theodore Boveri and Walter Sutton independently proposed a chromosomal theory of inheritance. They discovered that chromosomes during gamete production behave like the so-called Mendel's particles behave. In 1910, Thomas Hunt Morgan (1866–1946) confirmed the existence of chromosomes through experiments with fruit flies. He also discovered a unique pair of chromosomes called the sex chromosomes, which determined the sex of offspring. Morgan deduced that specific genes reside on chromosomes from his observation that an X-shaped chromosome was always present in flies that had white eyes. A later discovery showed that chromosomes could mutate or change structurally, resulting in a change in characteristics which could be passed on to the next generation.

More than three decades passed before scientists began to delve into the specific molecular and chemical structures that make up chromosomes. In the 1940s, a research team led by Oswald Avery (1877–1955) discovered that deoxyribonucleic acid (DNA) was responsible for transformation of non-pathogenic bacteria into pathogenic ones. The final proof that DNA was the specific molecule that carries genetic information was made by Alfred Hershey and Martha Chase in 1952. They used radioactive label to differentiate between viral protein and DNA, proving that over 80% of viral DNA entered bacterial cell causing infection, while protein did not cause infection.

The most important discovery in genetics occurred in 1953, when James Watson and Francis Crick solved the mystery of the exact structure of DNA. The two scientists used chemical analyses and x-ray diffraction studies performed by other scientists to uncover the specific structure and chemical arrangement of DNA. X-ray diffraction is a procedure in which parallel x-ray beams are diffracted by atoms in patterns that reveal the atoms' atomic weight and spatial arrangement. A month after their double-helix model of DNA appeared in scientific journals, the two scientists showed how DNA replicated. Armed with these new discoveries, geneticists embarked on the modern era of genetics, including efforts like genetic engineering, gene therapy, and a massive project to determine the exact location and function of all of the more than 100,000 genes that make up the human genome.