In Mendel's pea plants, some were either tall or short and the seeds were either green or yellow. There were no intermediate forms between thee characteristics. They are easily distinguishable and are not affected by environmental conditions. Other examples are: the ability to roll the tongue or taste phenylthiourea, ABO blood groups in man, and normal and vestigial wings in Drosophila. Such discontinuous variation is brought about by one or only a few genes.
However, not all traits fall into clear cut classes. Man is not just dark or fair; nor is he either short or tall. There are many intermediate skin colors and height. Such characteristics show a continuous variation from one extreme to the other. A continuous variation is brought about by the combined (or additive) effect of many genes.
There may be genes for dark skin and genes for fair skin. The more dark skin genes a person has, the darker will be his skin color or vice versa. Continuous variations are also affected by environmental conditions. For example, a greater exposure to sunlight may cause a person to have a darker skin color. Other examples of continuous variation considered to be present in man are intelligence and weight.
However, not all traits fall into clear cut classes. Man is not just dark or fair; nor is he either short or tall. There are many intermediate skin colors and height. Such characteristics show a continuous variation from one extreme to the other. A continuous variation is brought about by the combined (or additive) effect of many genes.
There may be genes for dark skin and genes for fair skin. The more dark skin genes a person has, the darker will be his skin color or vice versa. Continuous variations are also affected by environmental conditions. For example, a greater exposure to sunlight may cause a person to have a darker skin color. Other examples of continuous variation considered to be present in man are intelligence and weight.
