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CHAPTER ONE: GENETIC PRINCIPLES OVERVIEW In this section we describe the biology underlying the genetics of fish. Individual topics are explained in sufficient detail to allow subsequent discussion of genetic issues in fisheries and hatchery management. Individuals interested in a more complete treatment of any particular issue should consult a recent text. Gardner and Snustad (1981) provide good discussions of topics in molecular genetics. Hartl (1980) is useful for population genetics, and Falconer (1981) is a general reference for quantitative genetics and breeding. Kirpichnikov (1981) gives a thorough treatment of the genetics and breeding of fish. Areas of recent research are reviewed in Wilkins and Gosling (1983). The volume edited by Turner (1984) is a good reference for topics related to the evolutionary genetics of fish. MOLECULAR GENETICS AND CYTOGENETICS Molecular genetics and cytogenetics are foundations for the genetics of individuals and populations. Molecular genetics is the study of genetic processes at the molecular level. Cytogenetics is the study of genetics at the level of chromosomes in cells. The field of molecular genetics has grown tremendously in recent years and, as a result, many new techniques are available for study and manipulation of genes in fishes. Some familiarity with molecular genetics and cytogenetics is essential in order to understand genetic processes and to appreciate the new tools that are available. DNA All of the genetic information in an individual fish is contained in molecules of deoxyribonucleic acid (DNA). Molecules of DNA are composed of subunits called nucleotides. Each nucleotide contains a compound called a base. There are four kinds of nucleotides in DNA because there are four different bases (adenine, guanine, thymine, and cytosine). DNA molecules, as shown in Figure 1, consist of a long ladder of paired nucleotides. A natural twist in the ladder gives the DNA molecule a double helix structure. Nucleotides form base pairs in the double helix in a specific manner (Figure 1). Where thymine is found in one strand of the helix, only adenine will be found in the same position of the opposite strand. Similarly, where guanine is found in one strand, only cytosine will be found in the same position of the opposite strand. The two strands of the helix are said to be complementary because of the way nucleotides form base pairs. Whenever a cell divides, the DNA must be replicated in order to provide each daughter cell with a complete set of genes. An advantage of complementary base pairing is evident during replication of the DNA molecule. During replication, the two strands of the DNA helix are separated by enzymes so that each strand is available to serve as a template for a new molecule (Figure 1). Individual nucleotides are aff ixed to each template. Two complete and identical DNA molecules result. The complementary pairing of bases ensures that the replication of DNA is essentially error free. 5