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Curvature in mammalian fibers, such as wool and human hair, is an important feature of the functional trait of coat structure—it affects mechanical resilience and thermo‐insulation. However, to examine the relationship between fiber curvature, ultrastructure and protein composition fiber diameter variability has to be minimal. To achieve this we utilised the progeny of straight‐wool domestic sheep mutant rams (crimp mutants) and wild‐type ewes. Proteomic and structural results of the resulting mutant/wild‐type twin pairs confirmed that straight crimp mutant wool had a normal cuticle and the same cortical protein and ultrastructural building blocks as wild‐type (crimpy) fibers but differed in the layout of its cortical cells and in the relative proportions of keratin (K) and keratin‐associated proteins (KAPs). In the case of the crimp mutants (straight fibers), the orthocortex was distributed in a fragmented, annular ring, with some orthocortical cells near the central medulla, a pattern similar to that of straight hairs from humans and other mammals. Crimp mutant fibers were noted for the reduced abundance of some proteins in the high glycine–tyrosine class normally associated with the orthocortex, specifically the KAP6, KAP7, and KAP8 families, while proteins from the KAP16 and KAP19 were found in increased abundance. In addition to this, the type I keratin, K38, which is also associated with the orthocortex, was also found at lower abundance in the mutant fibers. Conversely, proteins from the ultra‐high sulfur class normally associated with the paracortex, specifically the KAP4 and KAP9 families, were found in higher abundance. [ABSTRACT FROM AUTHOR]