This review discusses the evolution of heteromorphic sex chromosomes in various species, focusing on young systems where only parts of the chromosome are nonrecombining, while the rest is pseudoautosomal. It highlights the progressive suppression of recombination in evolutionary independent cases, driven by selection. The review emphasizes the role of genetic degeneration, repetitive sequences, and the pseudoautosomal region in sex chromosome evolution. It discusses how selection during the adaptation of a chromosome to its role as a Y chromosome can drive recombination suppression. The review also explores the mechanisms behind recombination suppression, including the effects of sexually antagonistic genes, chromosome rearrangements, and transposable elements. It discusses the role of inversions and translocations in the evolution of sex chromosomes, and how these processes contribute to the formation of neo-sex chromosomes. The review also addresses the time scale of genetic degeneration, showing that Y chromosomes can partially degenerate within a few million generations. It highlights the accumulation of transposable elements in nonrecombining regions and the implications for gene function and expression. The review concludes that sex chromosomes can evolve in various ways, with recombination suppression favored when genes interact with sex in ways that affect fitness. It emphasizes the importance of studying sex-linked genes and genetic maps to understand the evolution of sex chromosomes. The review also discusses the challenges of determining the age of sex chromosomes in plants and fish, and the need for further research to understand the evolutionary processes involved.This review discusses the evolution of heteromorphic sex chromosomes in various species, focusing on young systems where only parts of the chromosome are nonrecombining, while the rest is pseudoautosomal. It highlights the progressive suppression of recombination in evolutionary independent cases, driven by selection. The review emphasizes the role of genetic degeneration, repetitive sequences, and the pseudoautosomal region in sex chromosome evolution. It discusses how selection during the adaptation of a chromosome to its role as a Y chromosome can drive recombination suppression. The review also explores the mechanisms behind recombination suppression, including the effects of sexually antagonistic genes, chromosome rearrangements, and transposable elements. It discusses the role of inversions and translocations in the evolution of sex chromosomes, and how these processes contribute to the formation of neo-sex chromosomes. The review also addresses the time scale of genetic degeneration, showing that Y chromosomes can partially degenerate within a few million generations. It highlights the accumulation of transposable elements in nonrecombining regions and the implications for gene function and expression. The review concludes that sex chromosomes can evolve in various ways, with recombination suppression favored when genes interact with sex in ways that affect fitness. It emphasizes the importance of studying sex-linked genes and genetic maps to understand the evolution of sex chromosomes. The review also discusses the challenges of determining the age of sex chromosomes in plants and fish, and the need for further research to understand the evolutionary processes involved.