Melanosis in Cattle: Genetic Basis of Melanocytic Hyperplasia

Melanocytic hyperplasia (melanosis) in cattle is a hereditary condition causing dark brown to black coat color due to increased melanocytes in skin.

Genetics

• Autosomal recessive inheritance.
• Mutation in KITLG (stem cell factor) gene.
• Homozygous recessive lethal in some breeds (embryonic/fetal loss).
• Heterozygotes may show intermediate pigmentation.

Phenotype

• Dark brown to black coat.
• Often horned; some polled.
• Markedly increased melanocytes in epidermis, dermis, hair follicles.
• No systemic illness; purely pigmentary.

Breeds

Reported in Holstein-Friesian, Simmental, and crosses. Not a disease—genetic variant.

Differential

• Melanoma (malignant): solitary masses, not diffuse hyperplasia.
• Adrenal hyperplasia (ACTH excess): hyperpigmentation of mucosa, not coat.