The article by Adrian Bird explores the evolving understanding of epigenetics, a field that has gained significant attention in recent years. Epigenetics is often described as the study of heritable changes in gene function that do not involve changes in DNA sequence, but it encompasses a wide range of phenomena with different roots and definitions. Conrad Waddington defined it as the study of epigenesis, focusing on how genotypes give rise to phenotypes during development. Arthur Riggs and colleagues, however, defined it more narrowly as heritable changes in gene function that cannot be explained by DNA sequence changes. The molecular basis of heritable epigenetics has been studied in various organisms, with DNA methylation and the Polycomb/Trithorax systems being key examples. These systems can lead to stable transmission of epigenetic traits across generations. Classic cases of epimutations, such as the peloric variant of toadflax flowers, demonstrate how stable changes in gene expression can occur without mutations. Despite the paucity of data from animal studies, epigenetics has captured public imagination due to its potential to explain long-term environmental impacts on physiology and behavior. Studies have shown that environmental factors and aging can lead to long-lasting epigenetic effects, such as changes in DNA methylation patterns in monozygotic twins and the impact of maternal care on offspring DNA methylation. The article also discusses the challenges and complexities of defining epigenetics, particularly the requirement for heritability. It proposes a revised definition that focuses on the structural adaptation of chromosomal regions to register, signal, or perpetuate altered activity states. This definition aims to be inclusive of various chromosomal marks and processes while avoiding the strict constraints of heritability. Overall, the article highlights the exciting and multifaceted nature of epigenetics, emphasizing its potential to bridge the gap between genetics and environmental influences on development and disease.The article by Adrian Bird explores the evolving understanding of epigenetics, a field that has gained significant attention in recent years. Epigenetics is often described as the study of heritable changes in gene function that do not involve changes in DNA sequence, but it encompasses a wide range of phenomena with different roots and definitions. Conrad Waddington defined it as the study of epigenesis, focusing on how genotypes give rise to phenotypes during development. Arthur Riggs and colleagues, however, defined it more narrowly as heritable changes in gene function that cannot be explained by DNA sequence changes. The molecular basis of heritable epigenetics has been studied in various organisms, with DNA methylation and the Polycomb/Trithorax systems being key examples. These systems can lead to stable transmission of epigenetic traits across generations. Classic cases of epimutations, such as the peloric variant of toadflax flowers, demonstrate how stable changes in gene expression can occur without mutations. Despite the paucity of data from animal studies, epigenetics has captured public imagination due to its potential to explain long-term environmental impacts on physiology and behavior. Studies have shown that environmental factors and aging can lead to long-lasting epigenetic effects, such as changes in DNA methylation patterns in monozygotic twins and the impact of maternal care on offspring DNA methylation. The article also discusses the challenges and complexities of defining epigenetics, particularly the requirement for heritability. It proposes a revised definition that focuses on the structural adaptation of chromosomal regions to register, signal, or perpetuate altered activity states. This definition aims to be inclusive of various chromosomal marks and processes while avoiding the strict constraints of heritability. Overall, the article highlights the exciting and multifaceted nature of epigenetics, emphasizing its potential to bridge the gap between genetics and environmental influences on development and disease.