As Gregor Mendel noted in his seminal paper of 1865, it had been known for some time that hybrids are usually not exactly intermediate between the parental species. Mendel introduced the term 'dominant' to refer to characters that are transmitted almost unchanged to the hybrids, and 'recessive' to those that become latent. In the meantime, more than a century of genetics has shown that dominance is a ubiquitous phenomenon with most deleterious mutations being recessive. Dominance is not only ubiquitous, but clearly of great evolutionary importance because it masks the effects of deleterious mutations as long as they are rare and are found predominantly in heterozygous individuals. Mutations are rarely completely dominant or recessive, but usually intermediate, also called codominant. Complete absence of dominance is uncommon, however. The degree of dominance plays an important role in the explanation of several evolutionary phenomena. These include the evolution of sexual reproduction and recombination, the evolution of selfing, Haldane's rule on inviable species hybrids, and the maintenance ofgenetic variation ofmetric traits. It is also well established now that dominance is a property of the phenotype and not the gene because genes that affect several traits can be dominant for one trait and recessive for another.

In 1928, R. A. Fisher raised the question why most deleterious mutations are recessive and proposed a model for the evolutionary modification of dominance relations. This was motivated by early experiments by W. Bateson and others which had demonstrated that dominance relations can be modified by changing the genetic background or, later, by selection. Fisher's theory was criticized by Sewall Wright for reasons discussed below. As an alternative, he suggested that the explanation ofdominance has a strong physiological component. In agreement with Wright, J. B. S. Haldane held that dominance provides a factor of safety, in that a single dose of the most active allele would, suitably regulated, provide enough of the gene product for normal viability. Fisher's hypothesis led to fierce controversies between proponents of 'physiological' and

'evolutionary' explanations. In 1981, Kacser and Burns developed a molecular explanation using metabolic control theory which, by many, was taken as evidence that an evolutionary explanation of dominance is obsolete. Still, there are many demonstrated cases of evolution of dominance in natural systems. However, the demonstrated cases of evolution of dominance in natural systems, some of them discussed below, require an explanation. In the meantime, alternative molecular theories, also based on biochemical principles, have been proposed. Some of them provide the potential for evolutionary modification of dominance. Despite its central importance and a huge body of empirical and theoretical work, no generally valid explanation for dominance has been provided until today. In fact, disagreement among scientists about several issues related to its explanation abound.

We shall start with a brief summary of empirical and experimental facts. Then we shall highlight and evaluate the various explanations of dominance (molecular basis of dominance), the theories that have been proposed for the evolution of dominance, as well as some of the main controversies. Finally, we will suggest a possible reconciliation.

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