Gene theory is the idea that genes are the basic units in which characteristics are passed from one generation to the next. Genes themselves are the basic units of heredity. The gene theory provides the basis for understanding how genes enable parents to transmit traits to their offspring. It is also a key element in the study of genetics.
Genes are the central objects studied by the science of genetics. The theory of genes (or gene theory) enables the science of genetics to be able to explain how information that is needed to make a new organism is passed from one generation to the next. Today we know that genes are made up of deoxyribonucleic acid (DNA), and we are able to state clearly what are known as the rules or laws of inheritance. However, less than 150 years ago scientists knew nothing about what went on at the cellular level that affected heredity. Since then, the science of heredity, or genetics (taken from the Greek word genes meaning "born") has been making regular and spectacular advances, so that at the beginning of the twenty-first century, scientists are close to learning the entire set of genetic instructions that go to form a single human being. Today scientists know that the 80,000 or so genes that make up what might be called the human blueprint are so individual that no two people (in a world of billions) are exactly alike—except for identical twins. Gene theory shows us how this extreme individuality can actually occur.
GREGOR MENDEL DISCOVERS DOMINANT AND RECESSIVE TRAITS
The existence of something like genes was recognized by the Austrian monk Gregor Johann Mendel (1822–1884), whose experiments with breeding different types of pea plants led him to describe what he called "hereditary factors," or genes. The first thing that Mendel discovered was that in crossing plants with different pure traits, such as all-tall plants with all-short ones, only a single trait was expressed. He therefore considered this expressed trait "dominant." Traits were therefore not blended, resulting in a medium-height plant, but were "expressed" as individual traits. He also found that a regular ratio of 3 to 1 existed for the number of dominant (tall) versus recessive (short) traits. This led him to decide that plants must contain what he called "factors" and "particles of inheritance," or what is now called genes. Mendel's other contribution was his correct assumption that both male and female parent contributed one "factor" per trait to an offspring. By 1900 it was realized that Mendel had given biology the basis for a new science of heredity, and the search began for the single "factor," or key, in all living things that contained the crucial information that dictated every detail of what an organism would be.
WATSON AND CRICK DISCOVER DNA STRUCTURE
By 1900, the existence of chromosomes was also known, and three years later, the American geneticist Thomas Hunt Morgan (1866–1945) announced the findings of his fruit-fly experiments, stating that chromosomes (the coiled structure in a cell that carries the cell's DNA) were made up of other, smaller things—later called genes. It was not until 1953 that the American biochemist, James Dewey Watson (1928– ), and his colleague, the English biochemist, Francis Harry Compton Crick (1916– ), were able to explain the molecular structure of DNA. With this new understanding, life scientists could formulate a fuller and more satisfying gene theory. Put simply, chromosomes are found in nearly every cell of our bodies. Chromosomes are made of DNA, and DNA stores genes. It is genes that carry the vital codes and information that not only tell a cell what to do, but which get passed on to the next generation by sexual reproduction.
The final part of gene theory explains how traits are passed on, and how no two individuals are exactly alike. During sexual reproduction, when a single human sperm fertilizes a single human egg, each contains only half the full set of forty-six chromosomes. Unlike other cells in the human body that have a complete set of forty-six chromosomes, sex cells contain only twenty-three. Consequently, when egg and sperm unite, the first new cell created gets twenty-three chromosomes from the mother and twenty-three chromosomes from the father to form a complete set of forty-six. This process, along with other "shuffling" of genes that occurs, guarantees that the new organism created is a unique individual.
Gene theory is the key to the genetics of the twenty-first century. Understanding how genes work and the knowledge that genes can change, or mutate, will lead to the prevention and cure of genetic diseases, as well as to the use of genetic engineering (the deliberate alteration of a living thing's genetic material to change its characteristics) to improve certain animal and plant species.
[See alsoChromosome; DNA; Gene Therapy; Genetic Code; Genetic Disorders; Genetic Engineering; Genetics; Inherited Traits; Nucleic Acid ]