Heredity is defined as the resemblance among individuals related by descent, and variation is the occurrence of differences among individuals of the same species. Therefore, genetics is the science of the study of resemblances and differences that exist among individuals. Resemblances and differences among related individuals are primarily due to genes. Genes cause the production of enzymes which in turn control chemical reactions in the body, thus affecting body development and function. For normal body development and function, genes must occur in pairs.
Genes are a part of the chromosomes that reside in the nucleus of body cells.
Chromosomes in the nucleus of a particular cell contain the same genetic information as the chromosomes in every cell of the body. In the normal cell of a pig, or any mammal, chromosomes occur in distinct pairs. This contrasts with humans, who have 23 chromosome pairs for a total of 46. Since chromosomes occur in pairs, genes also exist in pairs. If there are more or less than two chromosomes in a pair, an abnormality will occur.