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The objective in the current study was to associate different genotypes for the gene that codifies the α S1 -casein with the profile of milk caseins and the morphology of the mammary gland epithelial cells in goats. The genotypes HH, HF, EE and EF for the α S1 -casein gene (H = any high allele) were assessed based on the differences in the size of fragments amplified using polymerase chain reaction (PCR) and PCR-restriction fragment length polymorphism (PCR-RFLP). The milk casein fractions were measured by reversed phase HPLC. Mammary gland biopsies of animals with each of these genotypes were submitted to histological and ultrastructural microscopy analysis for the evaluation of secretory epithelial cells. The amount of total casein in milk from goats with genotype HH was only higher than in milk from those animals with the genotype EF, while the α S1 -casein (α S1 -Cn) fraction was different among all genotypes. The largest α S1 -Cn fraction was found in milk from the animals with the genotype HH, and it progressively decreased in milk from animals with the genotypes HF, EE and EF. Other milk casein fractions (β-casein, α S2 -casein and κ-casein) had a characteristic composition for each genotype. The κ-casein (κ-Cn) fraction was greater whenever the F allele was found, and the β-casein fraction was greater in EE homozygous animals. The ultrastructural analysis showed that mammary gland secretory cells from goats with the genotype EE and EF have highly dilated rough endoplasmic reticulum (RER) cisternae, whereas the dilation pattern in the animals with the genotype HF was intermediate between those with the genotype HH, and EE/EF. Although the amount of α S1 -casein is critical for an efficient transport of other caseins by RER, the defective F allele affects the other milk casein fractions differently from the defective E allele, probably due to the lack of phosphorylation sites on the protein encoded by the F allele, which would affect, in turn, the way caseins interact during milk micelle formation, reflecting on casein fractions in milk.

O alto polimorfismo encontrado no lócus do gene da αS1-caseína em caprinos, classificado em quatro níveis de expressão - alto, médio, baixo e nulo -, está associado à produção de 3,6; 1,6; 0,6 e 0g/L/alelo, respectivamente. O estudo foi realizado para investigar possíveis variações na produção de leite e seus constituintes, no perfil de caseínas e na lipólise da gordura. Quarenta e quatro cabras foram distribuídas em cinco genótipos: dois homozigotos, um para alta (AA) e outro para produção intermediária (EE), e três heterozigotos chamados AE, AF e EF, para αs1-caseína. Para a lipólise, o leite foi subamostrado em quatro alíquotas que sofreram tratamento térmico no momento da ordenha e após 24h de resfriamento. Diferenças entre genótipos foram observadas para a produção de caseína e de suas frações. As demais variáveis não diferiram entre genótipos. O genótipo AA apresentou os maiores conteúdos de caseína (28,6g/L) e de αS1-cn (22,3%). Os demais genótipos apresentaram média de 20,4g/L. Os grupos AE e AF apresentaram média de 12,1, EE-10,1 e EF-9,1% de αS1-cn. O resfriamento do leite por 24 horas aumentou a taxa de lipólise no leite. A genotipagem das cabras para αS1-cn pode ser usada como ferramenta de seleção com objetivo de obter produtos lácteos com distintos perfis de proteínas.