Candidate genes affecting sheep and goat milk quality

Abstract

The candidate gene approach is now exploited in sheep and goats, aiming to identify the molecular mechanisms, which make milks of different quality. In this paper, the relevant literature on this topic is reviewed. The most intensively analysed genes are milk protein genes, where, up to now, major effects were assessed for the α s1-casein, in both sheep and goats. For the α s2-casein, only in goats it was possible to associate the detected alleles with different casein synthesis levels: normal, low or null. For the β-casein, several variants were detected in goats, some of which are associated with a very low casein content, but extensive studies on the frequency and the effect of such variants are still to be conducted. The k-casein was thoroughly analysed at the genomic level in many breeds of goats, and 16 alleles were identified. However, no positive association of any allele with milk characteristics was so far reported. Conflicting results were obtained on the effect of β-lactoglobulin in sheep, while, in goats, variants were also extensively studied, but a positive association was found only with milk yield. Also the structure of the α-lactalbumin gene was partially investigated, but its possible effect on milk quality is still uncertain either in sheep or goats. Other candidate genes, that are being now extensively studied, are the genes encoding enzymes directly involved in fatty acid metabolism: ACACA in sheep; SCD in sheep and goats; LPL and DGAT1 in sheep, for which preliminary results of their effect on milk quality are given. The list of the studied candidate genes, that are being investigated according to their function, include FABP3, AMY, CGN, ACP6, ANAXA9, AMY2, gGH and Lactoferrin, although research on these genes is still at the beginning. It is expected, that new candidate genes will be identified by microarray analysis on total mRNA of the mammary gland, and that further field studies will be carried on, to demonstrate the association between genetic variants and milk quality. Nutritional and functional properties of milk still need to be more deeply investigated.