Inherited Traits

An inherited trait is a feature or characteristic of an organism that has been passed on to it in its genes. This transmission of parental traits to their offspring always follows certain principles or laws. The study of how inherited traits are passed on is called genetics.

The study of genetics or heredity began in the early 1800s when scientists first began trying to explain the existence of different species and variations within the same species. The French naturalist, Jean Baptiste de Lamarck (1744–1829), was one of the first to seriously consider the idea that present-day life forms descended from common ancestors. This is the principal idea of biological evolution. However, Lamarck also put forth some incorrect notions about biological development and heredity. One of these was that new organs and capabilities could be developed out of need and would also grow and improve when routinely used over time.

GREGOR JOHANN MENDEL

Austrian botanist (a person specializing in the study of plants) Gregor Mendel (1822–1884) is considered to be the father of genetics. After years of breeding peas and studying their characteristics, he discovered the basic laws of heredity that apply to all plants and animals. His work not only explained English naturalist Charles Darwin's theory of evolution (the process by which living things change over generations) by natural selection, but laid the foundation of modern genetics.

Gregor Mendel was named Johann when he was born in Heinzendorf, Austria (now part of the Czech Republic). The son of a peasant who took care of the fruit trees on a rich man's estate, the young Mendel took the name Gregor when he became a priest. Although very poor, he had been helped by the church to obtain a basic education and eventually received some higher training in mathematics and science. However, when his financial situation got very bad, he entered a monastery in 1843, mainly as a means of trying to continue his education. Although he had not intended on becoming a priest when he entered the monastery, four years later he decided to become a priest and was ordained that year. Eventually he was sent to the University of Vienna to study zoology, botany, chemistry, and physics. After becoming a science teacher, he repeatedly failed the examination that would have enabled him to teach at a higher level, so he finally just gave up and decided to pursue his own interests while remaining a priest at the monastery. Since he was particularly interested in both mathematics and botany, he decided to combine his two loves and to see if it were possible to predict the kinds of fruits and flowers a plant would produce.

Until Mendel, no one had ever done any real statistical analysis of breeding experiments. So starting in 1868, Mendel began a long-term project—almost a hobby—to see if he could conduct a range of breeding experiments and keep accurate records of his results. Mendel wanted to see if he could begin to understand how traits pass from one generation to another, so after taking over the monastery's garden plot, he chose to breed pea plants (today's "sweet peas"). Peas were an especially good experimental plant because they had characteristics, or traits, that could be easily observed (tall or short, wrinkled seeds or smooth seeds, yellow or green seeds). Mendel was very careful in all of his experiments, transferring pollen by hand from the male to the female part of the flowers to produce seeds. He even wrapped his plants so they would not be accidentally pollinated by insects. He would save the seeds from each self-pollinated plant, plant them separately, and study the new generation. He also crossbred plants with different characteristics.

After eight years of work and careful recording, Mendel found that, indeed, he was able to predict what a certain plant would produce as long as he knew which plants were the parents. In fact, he was so certain of what he found that his conclusions are now called Mendel's "laws" of inheritance. What he discovered after years of breeding more than 30,000 plants was that there are powerful traits, called dominant, and weaker traits, called recessive. He also found that when mixed together they do not blend. For example, although he at first expected to breed a medium-size plant when he crossed a tall plant (dominant trait) with a dwarf plant (recessive trait), what he found was that he eventually wound up with a mixture of tall and short plants according to a given ratio. He concluded, therefore, that in every instance, mixing traits did not result in a blend but instead sorted themselves out according to a fixed ratio. Mendel also concluded that each parent plant contributed a factor, later found to be a gene, that determines what a certain trait will be. Unfortunately for Mendel, he published his results in a journal not read by many in Europe. When he wrote directly to a prominent botanist of his time, the heavily statistical arguments he offered confused a man who was unused to seeing mathematical data in a botany paper. Discouraged, Mendel later put away his work and died totally unnoticed. It was not until 1900 that his work was discovered and made public. Upon close consideration, a new generation of life scientists realized that Mendel's laws of inheritance supported and even explained Darwin's theory of evolution by natural selection. Thus, the quiet priest who worked with the humble garden pea is now recognized as the giant who laid the groundwork for the modern science of genetics.

In other words, Lamarck said that a characteristic could be acquired (the long neck of a giraffe was acquired by continuously stretching its neck to reach for leaves) and then could be passed on to its offspring. He also said the opposite, that those characteristics that were not used would eventually disappear. Lamarck also argued that when an organism acquired a new skill it passed on that ability to its offspring. Of course, we know today that if one person learns a foreign language, there is no genetic way he or she can pass that talent on to its children. However, Lamarck was on the right track since he did suggest that traits can be inherited from generation to generation and that species do undergo long-term evolutionary changes.

In 1859, the English naturalist Charles Robert Darwin (1809–1882) published his landmark work, On the Origin of Species, in which he outlined his theory of evolution through natural selection. Darwin argued that members of a particular species always have slightly different traits or characteristics, and that in the competition for food, space, and shelter, some of these differences would make one member more suited to survive and produce offspring than others of its species. He continued this line of reasoning and said that those traits that were an advantage would be passed on to later generations, while those that were not would eventually disappear as their carriers died out. This meant that after centuries upon centuries of competition, or natural selection, recent members of a species could be quite different from their ancestors. Despite its eventual acceptance by the scientific community, Darwin's theory lacked an explanation for the mechanism or manner in which these random variations were inherited.

It was not until 1900 that the means of transmission of inherited traits was understood. Some forty-five years earlier, the Austrian monk, Gregor Johann Mendel (1822–1884), had begun experimenting with pea plants at about the same time that Darwin set forth his ideas on natural selection. Through his careful experiments, Mendel demonstrated that what he called "hereditary factors," now called genes, are transmitted to offspring. He also discovered that traits are inherited in pairs and that usually only one trait in each pair is actually expressed in the offspring. Although Mendel had established the laws of heredity by 1865, his work remained unnoticed until it was independently rediscovered by three scientists in 1900. With their discovery, it became apparent that Mendel had formulated the fixed rules of inheritance that applied to the entire plant and animal kingdoms.

After 1900, science began its search for the key part in all living things that contained the actual information that determined every detail of what an organism was. This search soon led to an understanding of chromosomes (a coiled structure in the nucleus of a cell that carries the cell's heredity information), and then to the realization that they in turn were made up of other, smaller things later named "genes." By the early 1950s, science knew that the chemical deoxyribonucleic acid (DNA) was somehow at the center of heredity, and in 1953 the American biochemist, James Dewey Watson, and the English biochemist, Francis Harry Compton Crick, explained exactly how. That year they discovered the "double helix" structure of the DNA molecule, demonstrating exactly how DNA carries the genetic code for all living things.

By the end of the twentieth century, our knowledge of the mechanism of inherited traits had nearly reached the point where the actual location of every human gene on every chromosome was identified and every letter in the 3,000,000,000-base code deciphered. Finally, on June 26, 2000, scientists announced that they had completed a rough draft of the human genome—the complete set of chromosomes that determines humans inherited traits. When completed, this human genome will lead to an understanding of each gene's precise chemical structure and its function in health and disease. This information will be invaluable since it may lead to cures, or possibly preventions, of certain genetic disorders.

[See alsoDominant and Recessive Traits; Genes; Genetics; Mendelian Laws of Inheritance ]