Wool (sheep) and cashmere (goat) fibres have unique biological, physical, and chemical properties and these fibres are becoming more important as the demand for natural products increases. However, these complex protein fibres are at times compromised by natural variability in their properties, and this can impact their use and value. Genetic improvement via selection and breeding can partly overcome this problem, enabling the farming of sheep and goats that produce more desirable fibre. This review explores the challenges in improving wool and cashmere fibre characteristics using genetics, with a focus on improving our understanding of the key protein components of fibres, wool keratins and keratin-associated proteins (KAPs). Despite progress in our knowledge of these proteins, gaining a better understanding of them and how they affect these fibres remains an ongoing challenge. This is not straight-forward, given the large number of similar yet unique genes that produce the proteins and the gaps that remain in their identification and characterisation. More research is required to clarify gene and protein sequence variability and the location and patterns of gene expression, which in turn limits our understanding of fibre growth and variation. Several aspects that currently hinder our progress in this quest include the incomplete identification of all the genes and weaknesses in the approaches used to characterise them, including newer omics technologies. We describe future research directions and challenges, including the need for ongoing gene identification, variation characterisation, and gene expression analysis and association studies to enable further improvement to these valuable natural fibres.

The wool of sheep consists of structurally intricate natural fibres that can be processed and manufactured into a range of products. It is prized for its insulation, moisture-buffering capability, flame resistance, and biodegradability. These features arise from its unique fibre architecture and specialised protein composition, which set it apart from most other natural and synthetic fibres. However, despite these novel characteristics, wool fibre variation hampers its uses and reduces its ability to compete with other fibres. This review summarises our current knowledge of wool fibre biology. It begins with a description of wool’s functional properties and performance attributes, then explores the structural foundations of these properties, the molecular basis of fibre trait variation, and prospects for improving fibre quality using genetic approaches. Particular attention is given to the wool keratin and keratin-associated protein genes, their spatiotemporal expression patterns, and genetic polymorphism that may influence fibre characteristics. Opportunities for the genetic improvement of sheep are discussed, including the use of genetic modification and marker-assisted selection. Challenges in interpreting gene–trait associations, particularly from high-throughput omics studies, are highlighted, along with the need for functionally validated genetic markers. Potential trade-offs between wool characteristics and other production and reproductive traits are considered, emphasising the need for balanced breeding approaches. By integrating insights from structural biology, molecular genetics, and breeding strategies, this review provides a foundation for wool fibre improvement.

Sheep’s wool is known to have unique biological, physical and chemical properties. The fibre primarily consists of proteins, but these have amino acid sequence variation, and at the phenotypic level wool fibre varies considerably. This can affect its utility and value. Unravelling the genetic factors that underpin the protein and phenotypic variability is crucial if we are to contemplate improving wool quality. Accordingly, this study investigates the high glycine and tyrosine content keratin-associated protein 19-5 gene (KRTAP19-5) in sheep. PCR-single strand confirmation polymorphism analysis, coupled with DNA sequencing of a region spanning whole coding sequence, revealed six sequence variants containing seven single nucleotide polymorphisms (SNPs). Five of the SNPs were located within the coding region, with four leading to amino acid changes if expressed. In 247 Chinese Tan sheep derived from 10 sire-lines, and renowned for their distinct ‘spring-like’ crimped wool at up to approximately 35 days after birth, one of the variants was found to be associated with decreased curvature of the fine wool fibres in the fleece. No associations were detected with other fibre traits or with variation in the heterotypic hair fibres of the Tan sheep. While these findings may be useful for developing gene markers to alter mean wool fibre curvature and improve sheep breeding, many other genes and environmental factors are known to contribute to variation in fibre traits.

Keratin-associated proteins (KAPs) are structural components of wool fibres. High-glycine/tyrosine (HGT)-KAPs are a subset of the KAP family, and their abundance in fibres varies. In this study, we report the discovery of an ovine HGT-KAP gene to which we assigned the name KRTAP36-2. Polymerase chain reaction and single-strand conformation polymorphism (PCR-SSCP) analyses revealed four variants of this gene in a screening population of 170 sheep from a variety of breeds. The DNA sequencing of the variants revealed four single-nucleotide polymorphisms (SNPs) and a dinucleotide deletion. Three of these SNPs were in the coding region, and one of these was non-synonymous and potentially led to the amino acid substitution p.Cys27Gly near the middle of the protein. The remaining SNP was located near the putative TATA box, and the di-nucleotide deletion was near the putative transcription initiation site. The effect of this variation in KRTAP36-2 was investigated in 274 Southdown × Merino lambs that were the progeny of five sires. Variation was only found to be associated with wool yield, that is, the proportion of the greasy fleece that remained as clean fleece upon scouring (expressed as a percentage). This may have some value in increasing wool production.

Understanding the genetic basis of wool traits is crucial for improving wool production. In this study, we investigated the ovine KAP13 gene family, which in humans contains multiple members, while only one member has been identified to date in sheep. Three ovine KRTAP13 genes, likely representing KRTAP13-1, KRTAP13-2, and KRTAP13-4, were identified through sequence analysis and phylogenetic comparisons. These genes are positioned on chromosome 1, between KRTAP15-1 and KRTAP27-1, in a pattern that is like the arrangement in humans but not identical. Analyses revealed multiple sequence variants of each gene in 356 sheep from a variety of wool, meat, and dual-purpose breeds. The effect of these genes on four fibre traits: mean fibre curvature (MFC), mean fibre diameter (MFD), coefficient of variation of fibre diameter (CVFD), and fibre diameter standard deviation (FDSD), was assessed in 240 lambs of the Chinese Tan sheep breed. An allele of KRTAP13-2 was revealed to be associated with a decrease in FDSD and CVFD in heterotypic fibres. No associations were found between KRTAP13-4 variation and wool traits, and an association analysis for KRTAP13-1 was not conducted because no variation was found in this gene in the Chinese Tan sheep studied. These findings suggest a potential role for KRTAP13-2 in regulating wool traits, particularly fibre diameter uniformity in larger heterotypic hair fibres, and suggest its potential use as a marker for improving wool traits.

Chinese Tan sheep lambs are recognised for having tight ‘spring-like’ curly wool when young, but this phenotype disappears with age. This wool consists of shorter, fine wool fibres (which are usually unmedullated) and heterotypic hair fibres (which are frequently medullated), which are referred to as ‘halo hair’. Both the wool and hair fibres consist of α-keratin proteins embedded in a keratin-associated protein (KAP) matrix. Of these KAPs, the KAP20-1 gene (designated KRTAP20-1) and its effect on four fibre traits (mean fibre curvature, mean fibre diameter, fibre diameter standard deviation, and coefficient of variation of fibre diameter) of Tan lambs was studied. Seven previously identified KRTAP20-1 variants (A, B, D, E, F, G, and H) of KRTAP20-1 were revealed, but the previously identified variant C was not present. Of the seven variants detected, only two (A and G) were common and present at frequencies greater than 5%, and the effect of these on the fibre traits of the finer wool fibres was assessed. It was found that variant G was associated with an increased mean fibre curvature in these wool fibres. This suggests that KRTAP20-1 might possibly be expressed differentially in the two fibre types, which may be of future value in breeding.