Dominant and Recessive Traits

     Gregor Mendel (1822-84) was an Austrian monk who studied plants. Over the course of seven years he experimented with green and yellow sweet peas by breeding them with each other. At first, common sense told him that he would get an equal amount of green and yellow peas. The first year, however, the peas were all green. He planted these the following year, this time expecting them to be all green again. However, some came out yellow.  He tried the experiment again, this time breeding round peas with wrinkled. The first year they were all round. He planted the resultant seeds the following year and obtained a crop of many round peas and some wrinkled peas.

    Mendel obtained the same results when he bred tall plants with short. Although he did not understand why the yellow, wrinkled, and short

peas would not appear the first year, he noticed one consistent characteristic: they would always appear in the next generation. After a few years, Mendel found another consistent characteristic. The ratio between the green, round, and tall peas and the yellow, wrinkled, and short peas was always about 3:1. Mendel did not receive recognition for this discovery during his lifetime but his work laid the foundation for the science of genetics and the law that governs this aspect of heredity has been given his name.

Mutation

    Hugo de Vries (1848-1935) was a Dutch plant physiologist. (Physiology is the study of the functions and vital processes in living organisms.) His life overlapped Darwin's and he accepted his theory of evolution but with one exception. He believed that gradually accumulating small changes could explain micro-evolution, the evolution of small differences within a species such as between individual cats or dogs, but could not explain macro-evolution, the evolution of big differences between two unrelated species such as cats and dogs. He thought the latter could not be achieved by the slow accumulation of small changes but required much bigger evolutionary jumps.

    Like Mendel, de Vries looked for answers in plants. He grew a great variety of primroses near his home and in the course of an experiment that lasted twenty years he found a number that had traits their parents did not have. The traits appeared suddenly. A radical change had occurred, not gradually but all at once. After the initial change the new features appeared consistently, generation after generation.

    De Vries knew that molecules passed through cell walls. (Molecules are the smallest particles of an element or compound that can exist in a free state and still retain the characteristic of the element or compound. The molecules of elements consist of one atom or two or more similar atoms; those of compounds consist of two or more different atoms.) His understanding of molecules enabled him to imagine something very small that controlled development. He visualized tiny groups of atoms and molecules, which he called pangens (later called genes) that
determined the nature of a trait, a view that was proven correct by later discoveries. Although it was later found that what looked to him like mutations had in fact been hybrids. (A hybrid is an offspring of two different species of plants.) with traits showing up that had been hidden for a long time, he had arrived at the correct conclusion: He had discovered a source of sudden big variation whose existence Darwin had denied.

Studies carried out in the 1960s and '70s have uncovered that physical, chemical and biological systems can, under extreme conditions, undergo sudden major transformations and evolve into more complex and totally different structures.