The units of heredity are the genes, which are carried in the chromosomes in a definite pattern like beads on a string. Each gene occupies a definite position. Most genes are very stable and rarely change their type. Each gene has a definite function in the development of the plant or animal.
Each kind of plant or animal has a definite number of chromosomes. The fruit fly has 8 chromosomes in every cell, the oak has 24 chromosomes, the mouse has 40 and man 48 chromosomes. Each of these chromosomes carries many genes.
Normally each cell has its chromosomes in pairs. A sex cell carries only one chromosome from each of these pairs. When a male and female sex cell unite, the offspring gets half its chromosomes from the father and half from the mother. How the chromosomes are sorted among the sex cells is largely a matter of chance. It is therefore a matter of pure chance whether a parent hands on one or the other of a pair of genes to any particular offspring.
If an organism has received an unlike pair of genes from its parents, the effect of one may mask the other. Thus if an ordinary wild rabbit is crossed to an albino rabbit, the offspring will all be coloured, colour being "dominant" over albino. In some cases a compromise may be the result.
In summary: Each gene has its own specific effect on the make-up of the plant or animal. How strongly this effect shows itself depends partly on the environment. The plant or the animal grows and fulfils itself, shaped jointly by its heredity and its environment.
PLANT GROWTH. In plants, only the general shape of the plant is fixed in advance; the number and place of its limbs and organs is not fixed as it is in most animals.
In plants, the growth of all parts unfolds at each stage from special shoots or growing points. The active cells at these growing points seem to influence the development of the whole plant, root and branch. Much research centres on these growing points and on the chemicals made by the active cells there, as well as in the new man-made chemicals which are able to influence the growth of plants.