Casein polymorphisms in the goat
1997
Grosclaude, F. | Martin, P. (INRA, Jouy-en-Josas (France). Lab. de genetique biochimique et de cytogenetique)
The caprine alphas1-casein locus is remarkable for its high degree of polymorphism and for the unusual existence of large differences in the level of protein synthesis between its alleles. Among the 11 currently identified alleles, six (A, B1, B2, B3, B4 and C) are associated with a "high" level of alphas1-casein (around 3.5 g per litre per allele), one (E) with a "medium" level (1.1 g per litre), two (F and G) with a "low"-level (0.45 g per litre); the two remaining alleles (01 and 02) are null alleles and produce no alphas1-casein. The B1 allele is the original allele for this species. The alleles giving reduced protein levels are thus defective mutants. The type of mutations is quite variable: single nucleotide substitution or deletion, or large insertions or deletions. An experiment was undertaken which determined that the polymorphism of alphas1-casein significantly affected (P less than 0.01) the true protein content of milk (AA more than AE, AF more than EE, EF more than FF), while the effects on the amount of milk produced was only that AA goats produced less milk than AE, AF, EE and EF goats (P less than 0.01). Unexpectedly, the polymorphism of alphas1-casein significantly affected (P0.01) the fat content of the milk (AA, AE, AE more than EF and FF), a phenomenon whose explanation was thought to be related to the cellular mechanisms behind the secretion of milk components. Observations made on the endoplasmic reticulum of the mammary epithelial cells of goats homozygous for the defective alleles suggest that a dysfunction exists in the secretory mechanisms. The case of a French traditional cheese is studied and the distribution of the alleles responsible for the polymorphism in the local breeds of the Mediterranean area is evoked.
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