Book excerpt: Although Gregor Mendel is considered the father of genetics, he has never taken the credit for his principles on heredity. Mendel's treatises, though they were part of the collection of the largest European libraries in the 19th century, were only rediscovered in 1900, 16 years after his death. Mendel's revolutionary ideas would have given greater strength to the formulation of Charles Darwin's ideas about common descent and gradual evolution through natural selection presented in 1859 in "The Origin of Species." However, Darwin was not totally ignorant of the possibility of genetic heredity. He even described “invisible characters” emerging in atavistic situations and named his hypothetical particle of heredity as “gemmules.” It is remarkable that the “invisible characters” and “gemmules” referred by Darwin are what we now know as genes – a term coined in 1909 by Wilhelm Johannsen that was widely accepted. During the 1930s and 40s, the findings of great proponents of genetics and evolution such as Mendel, Darwin, Wallace, Fisher, Haldane, Wright, Dobzhansky, Mayr, and several others were brought together to form the neo-Darwinian synthesis. In addition, in the 40s, genetics started its molecular revolution, which in the late 70s, driven by sequencing technology, gave rise to the genomics era. It took approximately 100 years to formulate the theoretical foundations of genetics to understand how information is transmitted to the next generations. Now, less than 45 years after the beginning of the genomic era, science is fully capable of identifying complete genomes. Among animals, fishes are one of the most relevant groups in genetic studies. Although fish studies were important in applying and corroborating Mendel's findings in the first decades of the 19th century, these studies contributed little to the development of classical genetics. However, fish have been of great importance for the development of molecular genetics. Several species such as Carassius auratus, Oryzias latipes, and Danio rerio (among several others of productive interest such as Salmo salar, Oreochromis niloticus, and Cyprinus carpio) have been used around the world as biological models. These models can be used for the study of genes and genomes, epigenetics, and genetic expression. Genetic studies using fish, in addition to increasing genetic knowledge about the species, also serve for a better general understanding of the physiology of metabolic pathways, diseases, evolution, systematics, dispersion, creation, and selection of individuals and lineages. Considering this, this Research Topic aims to bring together studies that present applications of fish as targets in genetic studies.