Genetics

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Genetics

Genetics is the branch of biology that is concerned with the study of heredity or the passing on of characteristics from one generation to the next. It is also concerned with variation or what makes one living thing different from another. Geneticists are people who study genes in an attempt to understand how the inherited information genes contain is stored and passed on.

Genetics is a fairly young science and was started by the landmark work of the Austrian monk and botanist Gregor Johann Mendel (1822–1884), who first put forward his theory of heredity in 1865. Until Mendel, most everyone knew that noticeable traits were usually passed on from one generation to the next, but no one knew where to begin to find out what controlled and influenced the passing on of these traits. It was Mendel who carried out the first scientific study of how traits pass from one generation to the next by conducting a series of experiments with pea plants. The experiments took him eight years to complete. Mendel decided to use ordinary green peas, like the ones commonly eaten today, since they are easy to breed for what are called "pure traits." This means that a purebred plant that produces yellow pods will always produce yellow pods. Mendel selected pea varieties that differed in a single trait (such as height or color or wrinkled seeds) and then crossed them with plants of a different trait (short with tall, yellow with green). For example, he would cross a pure tall with a pure short and then accurately record the number of each type he harvested for each generation. Luckily for Mendel, he had concentrated on only a few traits, and each of these happened to be governed by a single gene (which he did not yet even know existed). The first thing that Mendel discovered was that crossing a tall plant with a short one did not result in a blend of medium-height plants, but rather in plants that were all tall. After allowing these offspring plants to pollinate (the transfer of pollen containing male sex cells to the pistil containing female sex cells) themselves, he found that the next generation produced plants three-quarters of which were tall (which he then called a dominant factor) and one-quarter of which were short (which he called a recessive factor). After crossing hundreds of plants and keeping careful records, Mendel discovered that a regular 3 to 1 ratio or pattern existed for the number of dominant versus recessive traits. This led him to decide that plants must contain what he called "factors" and "particles of inheritance," or what we now call genes. This also led him to believe that there must be laws or rules that determined how these "particles" were passed on. After much study, Mendel formulated what are now called the "Mendelian laws of inheritance." He stated correctly that traits did not blend but remained distinct; that they combined and sorted themselves out according to fixed rules; and that both male and female contributed equally. Mendel was also correct when he said that each parent contributed one "factor" for a particular trait to its offspring. Although Mendel's laws had laid the foundation of the new science of genetics, his work was unknown until 1900 when it was separately discovered by three different botanists (in three different countries) who realized that Mendel had discovered the laws of genetics long before they did. Although each published his own version of these laws, each cited Mendel as having been the real discoverer, and all said that their work was merely a confirmation of what Mendel had accomplished thirty-five years before.

MORGAN INTRODUCES CHROMOSOME THEORY OF INHERITANCE

By 1900, when biologists realized that Mendel had given them the basis of a new science of heredity, they began to search for the key part in all living things that contained the crucial information that determined every detail of what an organism would look like. This search soon led to an understanding of chromosomes and then to the discovery of a single molecule called deoxyribonucleic acid or DNA. By 1900, science already knew that chromosomes (the coiled structure in a cell that carries the cell's DNA) existed in the nucleus of every cell, and by 1903, some biologists were making a connection between chromosomes and the process of heredity. One scientist, the American geneticist Thomas Hunt Morgan (1866–1945), conducted experiments on the rapidly bred fruit fly and arrived at the idea that chromosomes are themselves made up of other, smaller things (later named "genes") that were linked together and arranged in a long line. In 1911 he put forth this chromosome theory of inheritance based on genes. The next question was "how did the genes actually work?"

As early as the late nineteenth century, it was known that a unique type of acid named deoxyribonucleic acid (DNA) existed in every cell nucleus. It had been long ignored, however, and was considered unimportant to heredity. By 1944, though, better research and improved technology enabled biologists to realize that DNA was actually the key chemical at the center of heredity. Research continued and by 1952, biologists were able to demonstrate that DNA was indeed the genetic material for which they had been searching. However, since its structure was still unknown, it was not possible to describe how such an apparently simple molecule of DNA could contain the vast and complicated information or code that was needed to develop a human being.

WATSON AND CRICK COMPLETE THE PUZZLE

The final breakthrough to this puzzle was achieved in 1953 by the American biochemist James Watson and his colleague, the English biochemist Francis Harry Compton Crick. That year they were able to successfully construct a "double helix" model of a DNA molecule that solved the puzzle. DNA, they explained, is made up of two long strands connected by "base pairs" (like rungs on a ladder), and that the entire model looks like a curving, twisting ladder or a spiral staircase. Watson and Crick also found that the bases always paired up in a specific order, so that if they knew the sequence of one strand, they could accurately tell the sequence of the other. Finally, they discovered that the order of the chemical bases represented a code that was translated by the cell and used as a guide to make proteins.

Since their achievement, scientists have learned that DNA is the blueprint for all life on Earth. It is now known that almost every cell in our bodies has a set of chromosomes that store this DNA. It also is known that each DNA base is like a letter in the alphabet, and that a sequence of bases forms a message. These messages are called genes, and each gene instructs the cell that contains it on how to make a specific protein. We also know that while all living things have their own unique code of DNA, the molecule that forms that code is basically the same for all.

Finally, we know that it is through chromosomes that genetic information is passed from one generation to another. Today's genetics has gone beyond all of these discoveries and has begun to put them to good use. Now that scientists actually are able to look at the genes themselves, they can know what sequences produce what effect. This sometimes allows scientists to move genes around, fix mistakes, and even transfer them between species. These many years of research culminated in 1997, when scientists in Scotland produced a lamb that was cloned from a cell nucleus taken from the udder of a sheep.

As the twenty-first century begins, science is preparing to cope with what promises to be a difficult and probably very controversial period as it becomes able to artificially alter genes through a process called genetic engineering. This powerful new tool can produce great benefits, such as curing a genetic disease like Alzheimer's, but it also has the potential for serious misuse. Genetics may be a young science, but it promises to put its stamp on the twenty-first century.