Transgressive segregation, where hybrid offspring exhibit phenotypes extreme relative to both parents, is common in hybrid populations. A review of 171 studies shows that 91% report at least one transgressive trait, with 44% of 1229 traits being transgressive. Transgression is most frequent in intraspecific crosses involving inbred, domesticated plant populations and least in interspecific crosses. In plants, transgressive segregation is widespread, with 58% of 579 traits showing transgression, and more common in domesticated crosses than in wild ones. In animals, transgressive segregation is less frequent, with 31% of 650 traits showing transgression. Domesticated animal hybrids are more likely to exhibit transgressive segregation than wild ones, and intraspecific hybrids are more likely than interspecific ones. Transgressive segregation is often due to complementary gene action, where alleles from each parent have opposing effects, leading to extreme phenotypes in hybrids. This mechanism is supported by QTL studies in plants, showing that complementary genes are a major cause of transgression. Transgressive segregation may contribute to niche divergence and phenotypic novelty in hybrid lineages, and may facilitate adaptation. However, the role of transgression in adaptive evolution remains unclear, as some explanations, like developmental instability, are unlikely to be heritable. The genetic basis of transgressive segregation is largely distinct from that of heterosis. Factors predicting transgressive segregation include genetic divergence between parental lines, the use of inbred lines, and the presence of complementary genes. Transgressive segregation is more common in crosses between inbred or selfing lineages, and in crosses with genetically divergent parents. Traits with a history of directional selection are less likely to exhibit transgressive segregation than those with a history of genetic drift or stabilizing selection. The relationship between genetic differentiation and transgressive segregation is complex, and further research is needed to fully understand the evolutionary role of transgressive segregation.Transgressive segregation, where hybrid offspring exhibit phenotypes extreme relative to both parents, is common in hybrid populations. A review of 171 studies shows that 91% report at least one transgressive trait, with 44% of 1229 traits being transgressive. Transgression is most frequent in intraspecific crosses involving inbred, domesticated plant populations and least in interspecific crosses. In plants, transgressive segregation is widespread, with 58% of 579 traits showing transgression, and more common in domesticated crosses than in wild ones. In animals, transgressive segregation is less frequent, with 31% of 650 traits showing transgression. Domesticated animal hybrids are more likely to exhibit transgressive segregation than wild ones, and intraspecific hybrids are more likely than interspecific ones. Transgressive segregation is often due to complementary gene action, where alleles from each parent have opposing effects, leading to extreme phenotypes in hybrids. This mechanism is supported by QTL studies in plants, showing that complementary genes are a major cause of transgression. Transgressive segregation may contribute to niche divergence and phenotypic novelty in hybrid lineages, and may facilitate adaptation. However, the role of transgression in adaptive evolution remains unclear, as some explanations, like developmental instability, are unlikely to be heritable. The genetic basis of transgressive segregation is largely distinct from that of heterosis. Factors predicting transgressive segregation include genetic divergence between parental lines, the use of inbred lines, and the presence of complementary genes. Transgressive segregation is more common in crosses between inbred or selfing lineages, and in crosses with genetically divergent parents. Traits with a history of directional selection are less likely to exhibit transgressive segregation than those with a history of genetic drift or stabilizing selection. The relationship between genetic differentiation and transgressive segregation is complex, and further research is needed to fully understand the evolutionary role of transgressive segregation.