The birth of the gene

Around 1900 a number of scientists observed ‘Mendelian ratios’ in plant and animal breeding experiments. Mendelian ratios can be observed when two varieties of a plant or animal, each of which reliably displays some observable characteristic, such as the height of a plant or the colour of its flowers, are crossed to produce hybrid offspring. In the first generation, one of the two parental characteristics disappears, and all the offspring show the other characteristic. All plants in the first generation may be tall, even if only one parent plant was tall and the other was short. Or all the offspring may have red flowers, even if one parent had red flowers and the other white. If this happens it will appear that only one of the two parental characters has been passed to the next generation, and the other character has been lost. But if these first-generation offspring are crossed with one another, the second generation will display both the characters seen in the two original varieties, and will display them in the Mendelian ratio of 3:1. Three-quarters of the second generation show the character which was universal in the first generation, while one quarter show the character which disappeared in that generation. There is a compelling explanation of this and other, more complex Mendelian ratios which hypothesises that each organism contains two factors that determine which character it will display. One factor comes from each parent, and if an organism inherits two different factors, one is always expressed preferentially over the other (Figure 2.1).