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Environment and Heredity

Two factors decide most of the characteristics of the organism, heredity and environment. Heredity is the transmissions of an organism's characteristics to its offspring. Organisms also develop certain characteristics which are induced through their response to their environment. Hereditary traits may be passed on from one generation to another, but environmentally induced traits cannot.

One-cell animals and plants can reproduce by dividing themselves into two new organisms. An amoeba, for example, reproduce in this manner. When it becomes a certain size, its cell nucleus and cytoplasm divide and form two amoebas. A more complex, many-cell organism, the hydra, can form a bud which eventually breaks loose to become another individual.

Many plants and animals produce sex cells, know as egg cells and sperms. When sperms encounter egg cells, one of the sperms enters and egg cell (fertilization) and the substance of the two is combined. Each fertilized egg cell produces a new organism.

Ordinarily the characteristics of one generation are inherited by the next, like begetting like. But though this is generally true, numerous exceptions may be observed in all populations. For example, a trait of parents may disappear in their children and reappear in the grandchildren. Or a new trait may suddenly appear and prove to be inheritable. Plant and animal breeders have for thousands of years selected their stock to develop more desirable traits or varieties. Until the nineteenth century, however, nothing was known about the laws and mechanisms responsible for these observations and practical breeding potentials.

figure 4-3. Photomicrograph of onion root tip cells at about 800X magnification. Several cells are elongated, with the chromosomes separating in the process called mitosis, which produces two identical sets of chromosomes in the course of cell division.