Genetics, the science of heredity and variation, is used to study the rural hen and highlight its characteristics by establishing the gene-performance relationship. Compared with hens from selected breeds, lines or strains, the rural hen, which is not selected using conventional and molecular scientific methods and is a natural incubator to ensure the longevity of its offspring, is far inferior in performance for production traits. Factor or formal genetics has enabled us to identify the effect of genes with a clearly visible importance on expressed traits. Methods for identifying genes with visible effects have been developed in this book. We have presented some of these genes of tropical importance and their benefits on poultry performance, both for meat production and for egg production (for breeding, consumption and sale). Certain variability in hens is often due to chromosomal changes occurring during cell division, especially meiosis, and is at the origin of the diversity often observed.