We know that a new individual develops when a male cell (sperm) combines with a female cell (ovum). Soon after the sperm penetrates the ovum, the head of the sperm divides and releases twenty-four small bodies in the nucleus of the cell (chromosomes). These chromosomes pair with those of the ovum which also number twenty-four. The new individual, therefore, starts life with forty-eight chromosomes, each different from the others in shape and size. Through a process of division and redivision, each cell in the new individual eventually has an exact duplicate of the original set of chromosomes. This explains why a child resembles his father's family in some respects and his mother's family in others, for the chromosomes carry the characteristics of the families concerned.
Each chromosomes is made up of small elements called genes which are inherited from the parents. Each pair of genes, one from the father and one from the mother, is responsible for a particular aspect of a child's development. Each pair determines some physical feature of the child.
The genes contain the hereditary factors which determine eye and hair color and quality, height, shape of the head and facial features, skin color and other physical features. There are traits which are strong or dominant and traits which are weak or recessive
When a gene carrying a dominant trait is paired with a gene carrying a recessive trait, the dominant trait is transmitted to the child. This explains why a child, for example, has his father's height or his mother's nose. Other paired genes join to produce a seeming combination of both traits which explains a skin color somewhere between that of the father's very dark skin, and the mother's which is very fair.
Heredity also influences the nervous system and affects the child's mental development. Let us distinguish two important terms: aptitude and ability. Aptitude is a pattern of traits needed for learning a particular task. Ability is the power to do the task. Our abilities depend to a considerable extent upon our aptitudes. Aptitudes are inherited; abilities are acquired through practice. If you are an excellent musician, painter or sculptor, will your children be excellent artists, too? If you are a bright student, will your children also be bright student?
Aptitudes for the arts or for scholarship are inherited, but artistic or scholastic abilities must be built upon aptitudes through proper training and experience. Aptitude alone does not mean ability to perform the task. Aptitudes set a limit on ability, but training and experience will help determine how far you can develop your abilities. Abilities are developed; not inherited. Things that you do to improve yourself mentally or physically cannot be passed on to your offspring through heredity.
There are hereditary factors transmitted to the child by parents for traits they do not show themselves. This explains why a child may not resemble either parent but resembles relatives on both sides. The parents, of course, received their physical characteristics from the chromosomes of their own parents who in turn received theirs from the respective parents, and so on, as far back as we can imagine. This explains why members of a family do not always resemble each other as a result of the various combinations of the genes. You can also understand why these members of the family are at the same time more like each other than the people outside their family.