We resemble our parents because of a complex of biological processes which we call heredity. Parents and children tend to be similar in many characteristics - structural, physiological and psychological. However, the young are not exact duplicates of their parents and usually differ in many traits. This difference we call variation.
The science of the study of heredity is called genetics, and the most famous name connected with it is that of Gregor Johann Mendel (1822-1884), and Austrian monk whose discoveries laid the foundation of the science.
Mendel studied plants, especially the garden pea and showed that inherited characteristics were the result of paired elementary units of heredity, which we now call genes. These he symbolized by letters. A characteristic of one parent, such as tallness, which appeared in the progeny to the exclusion of another, was said to be dominant. A characteristic which tended not to reappear was called recessive.
Later studies have shown that the genes are carried by structures called chromosomes. The human body begins with the union of two sex cells, and the body of a new born infant has some 200,000 million cells. These cells arise through the division of other cells. When they divide, their nuclei (or tiny central bodies) divide also by a remarkable process called mitosis.
During mitosis the nuclei resolve themselves into structures called chromosomes, which divide lengthwise. All body cells have true copies of all chromosomes that were present in the fertilized egg from which the body developed.