Animal Molecular Breeding 2024, Vol.14, No.2, 165-177 http://animalscipublisher.com/index.php/amb 167 Gene expression regulation is another critical aspect of coat color determination. Transcriptome analysis of rabbit skin tissues has shown that the expression levels of key pigmentation genes, such as TYR and TYRP1, vary between different coat color phenotypes. This differential expression can result in the production of different amounts and types of melanin, leading to the observed coat color variations (Qin et al., 2016). 3 Common Coat Color Variants in Domestic Rabbits 3.1 Description of major coat colors Domestic rabbits exhibit a variety of coat colors, with some of the most common being albino, agouti, and black. The albino variant is characterized by a complete lack of pigmentation, resulting in white fur and red eyes due to the visibility of blood vessels in the absence of melanin. This phenotype is often associated with mutations in the tyrosinase (TYR) gene, which is crucial for melanin production (Utzeri et al., 2021). The agouti coat color, on the other hand, features a banded pattern of pigmentation, where each hair has multiple colors, typically with a dark base, a lighter middle band, and a dark tip. This pattern is regulated by the agouti signaling protein (ASIP) gene, which influences the distribution of eumelanin and pheomelanin along the hair shaft (Zhang et al., 2023). The black coat color is a result of high eumelanin production, giving the fur a uniform dark appearance. This phenotype is often linked to the melanocortin 1 receptor (MC1R) gene, which plays a significant role in the regulation of melanin synthesis (Fontanesi et al., 2006). 3.2 Genetic mutations responsible for each variant The genetic basis for these coat color variants involves specific mutations in key pigmentation genes. For the albino phenotype, mutations in the TYR gene are critical. Disruptive mutations in TYR, such as missense mutations p.T373 K and p.E294 G, impair the enzyme's function, leading to a lack of melanin production and resulting in albinism (Utzeri et al., 2021). The agouti coat color is primarily influenced by the ASIP gene. Variations in this gene, particularly those affecting its expression or function, result in the characteristic banded hair pattern seen in agouti rabbits (Zhang et al., 2023). The black coat color is associated with mutations in the MC1R gene. Specific alleles, such as the c.280_285del6 deletion, have been identified in black-coated rabbits, which enhance the receptor's activity, leading to increased eumelanin production (Fontanesi et al., 2006). Additionally, the melanophilin (MLPH) gene has been implicated in coat color dilution, where mutations such as the c.585delG frameshift mutation result in a diluted pigmentation phenotype, affecting both eumelanin and pheomelanin7 8. 3.3 Inheritance patterns of coat colors The inheritance patterns of these coat color variants follow Mendelian principles, with some traits being dominant and others recessive. The albino phenotype, caused by mutations in the TYR gene, is typically inherited in an autosomal recessive manner. This means that two copies of the mutant allele are required for the albino phenotype to be expressed (Table 1) (Utzeri et al., 2021). The agouti coat color, influenced by the ASIPgene, follows a more complex inheritance pattern due to its interaction with other pigmentation genes. However, it is generally considered a dominant trait, where a single copy of the agouti allele can produce the banded hair pattern (Zhang et al., 2023). The black coat color, associated with the MC1R gene, can be inherited as a dominant trait when the c.280_285del6 allele is present. This allele is dominant over other MC1R alleles, such as the c.304_333del30 allele, which is associated with red/fawn/yellow coat colors (Fontanesi et al., 2006). The dilution of coat color, linked to the MLPHgene, is inherited in an autosomal recessive manner, requiring two copies of the mutant allele for the diluted phenotype to manifest (Fontanesi et al., 2014; Demars et al., 2018). 4 Recent Advances in Genetic Research 4.1 Genome-wide association studies (GWAS) on coat color Genome-wide association studies (GWAS) have significantly advanced our understanding of the genetic basis of coat color variations in domestic rabbits. These studies involve scanning the entire genome to identify genetic variants associated with specific traits, such as coat color. One notable study focused on Chinese Rex rabbits, where researchers collected blood samples from 250 rabbits with six different coat colors and performed genome sequencing using a restriction site-associated DNA sequencing approach. They identified 91 546 single nucleotide
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