Mendelian Laws of Inheritance Table of Contents Source Citation The Mendelian laws of inheritance lay down the basic principles of genetics. They state that characteristics are not inherited in a random way, but instead follow predictable, mathematical patterns. Mendelian laws were formulated by Austrian monk and botanist (a person specializing in the study of plants) Gregor Johann Mendel (1822­1887) in 1865, but went unnoticed for nearly a half century. Before Mendel, many scientists had realized that certain traits, or characteristics, are passed on from one generation to the next, but in the middle of the nineteenth century, no one had any idea as to how these traits are passed on. In 1857 Mendel was able to combine his interest in both botany and mathematics by undertaking a long­term study breeding garden peas. For the next eight years, Mendel conducted a scientific study of how traits pass from one generation to the next. By using ordinary garden peas­­like those we eat today and call sweet peas­­Mendel was able to easily breed for what are called "pure traits." This means that a self­pollinated plant (a plant that contains both male and female reproductive organs and is able to transfer pollen between these parts) with pure traits always produces offspring like itself. For example, a purebred plant that produces yellow pods always produces yellow pods. Mendel then selected pea varieties that differed in single traits (such as height or pod color), and then he crossed them with plants that had a different trait (crossing tall plants with short, or yellow pods with green). After crossing a pure tall with a pure short, he recorded the number of each type harvested and saved the seeds produced by each plant for later planting. While Mendel was conducting these experiments, neither he nor anyone else had any idea that such things as chromosomes (coiled structures in a cell's nucleus that carries the cell's heredity information) and genes (basic units of heredity) exist, although he eventually decided that plants must contain something he called "factors" and "particles of inheritance." He came to this conclusion because of the pattern of results he eventually saw. The first thing that Mendel discovered after crossing a pure tall plant with a pure short one was that it did not result in the production of medium­size offspring. Instead, in the first generation, all the plants were tall. However, after allowing these plants to self­pollinate, he saw that the next generation produced plants that were a mix of tall and short. In fact, three­quarters were tall (which he called a dominant factor), and one­quarter were short (which he called a recessive factor). Mendel continued crossing hundreds of plants and kept careful records. Eventually he was able to state that a regular 3 to 1 ratio existed for the number of dominant versus recessive traits. This led him to realize that there must be laws or rules that explain this mathematical ratio. Mendel' first law of inheritance Two labeled diagrams showing Mendel's first law of inheritance, the law of segregation. Illustration by Hans & Cassidy. Courtesy of Gale Research. Continued work and study eventually allowed him to formulate what are now called the Mendelian laws of inheritance. He stated that the characteristics of an organism are passed on from one generation to the next by particles (which he called factors and we call genes). These genes exist in pairs, which are really different versions of the same genetic instructions. In this pair, one of the two factors comes from the male parent and the other comes from the female parent (each contributes equally). Finally, traits do not blend but remain distinct, and they combine and sort themselves out according to fixed rules. Mendel also stated that dominant genes are always expressed, but that recessive factors from both parents have to be present before they are expressed. Recessive factors can therefore be present in an individual but not have any effect on its characteristics. Mendel published his findings in an obscure journal, and although his laws had laid the foundation for the new science of genetics, his work remained unknown for nearly two decades after his death. In 1900 Mendel's work was separately discovered by three different botanists (in three different countries) who realized that Mendel had discovered the laws of genetics long before they had. Although each published his own version of these laws, each man cited Mendel as the real discoverer. All three honorably stated that their work was merely a confirmation of what Mendel had accomplished in 1865. Source Citation: "Mendelian Laws of Inheritance." U*X*L Complete Life Science Resource. Ed. Leonard C. Bruno and Julie Carnagie. Online. Detroit: U*X*L, 2009. Science Resource Center. Gale. 14 December 2009 <http://galenet.galegroup.com/servlet/SciRC?ste=1&docNum=CV2641700145>