A review of genetic variation underlying chicken phenotypic diversity.

Chickens exhibit extensive phenotypic diversity, much of which is found within local landraces as well as historical and commercially utilized breeds across the globe. These breeds have preserved a wide array of observable and quantifiable traits, including differences in feather structure, pigmentation, comb morphology, and skeletal development. Progress in molecular genetics has identified the genetic variants underlying many of these traits, offering valuable insights into how individual genes, regulatory elements, and genetic pathways influence development and morphology. These discoveries also highlight the importance of chickens as a model organism for studying vertebrate embryogenesis, gene interactions, core cellular mechanisms, and as a distinctive animal model for rare human diseases. Much of the research published on the molecular genetic mechanisms of this phenotypic variability is scattered across diverse, often specialized scientific publications, making it difficult for interested individuals to easily access a consolidated view of known genotype-phenotype relationships in chickens. This review addresses that gap by assembling a summary of 46 chicken phenotypes for which the causative gene and underlying mutation have been identified. An additional 11 traits for which the associated chromosomal region is known but the precise causative variant remains unresolved are also included. Where available, photographic images of each phenotype are provided to support breed documentation efforts and serve as a visual reference for researchers and students. By compiling this information in a single, accessible source, the review aims to support a wide range of users while highlighting the unique contributions of chickens to functional genetics.