2.2. Lacaune Sheep

The Lacaune breed originates from the Roquefort area of Southern France. It is a main dairy sheep breed in French and has been selected for milk use during the last 40 years and now is a very important dairy sheep breed all over the world ^[5][11]. Lacaune ewes produce milk with higher total solids but in slightly less volume than the East Friesians. The milk produced from Lacaune was processed into the famous Roquefort cheese ^[6][12]. 

2.3. Sarda Sheep

The Sarda sheep is a domestic sheep from Italy and is also a famous milk sheep breed across the world. It is also known by some other names like Cagliari and Sardinian sheep. The breed is indigenous to the island of Sardinia, and raised throughout Italy and other Mediterranean countries, particularly in Tunisia. 

3. Comparison of Cow, Goat, and Sheep Milk Contents

Compared with other kinds of milk, the content of fat, protein, lactose percent, and vitamin and mineral contents of sheep milk are higher than those of goat milk and cow milk ^[7][29]. Park ^[8][30] found that the total fat content of sheep’s milk was 51% and 54% higher than that of goat’s milk and cow milk, respectively, and sheep’s milk was richer in unsaturated fatty acids. Unsaturated fatty acids in food are the source of high-quality animal fat that is necessary to the human body and plays a very important role in regulating blood lipids, clearing thrombosis, enhancing immunity, improving eyesight, and strengthening the brain ^[9][31]. Moreover, the trans fatty acid content in sheep milk was 6 times of goat milk, and the unsaturated fatty acid content of sheep milk was also three times of other small ruminants in Schroeder. 

4. Genes Affecting Sheep Milk Performance

Sheep milk traits have many different phenotypic diversities that have attracted the attention of various researchers worldwide to investigate its hidden genetic and genomic determinism. More and more genomic-based investigation approaches have been used in genetic comparisons of different populations ^[10][34]. This can identify genes regulating the investigated traits, and with the advancement of agricultural technologies, the understanding of genetic bases underlying target traits has increased drastically ^[11][12][35,36]. Many studies with diverse breeds of dairy sheep have indicated casein gene (CSN) as a promising candidate gene for milk compositions traits ^[13][14][37,38]. Moreover, the SCD gene, SOCS2 and SLC2A2 gene were also chosen as significant candidate genes affecting milk fat, milk protein, or milk quality in recent studies ^[15][16][17][39,40,41]. 

5. SLC2A2 Gene—Transport Nutrition from Blood to Milk

SLC2A2 (Solute Carrier Family member 2) is a member of solute carrier (SLC) family. The solute carrier family is made up of different transporter families that include ion channels and exchangers, its passive transporters SLC2A2 is located on chromosomes 1 and belongs to the passive transporters, which are related to the biological process of carbohydrate metabolic process and insulin secretion regulation ^[18][42]. Many researchers have found this gene is related to milk synthesis and milk lactation ^[19][20][21][43,44,45].  Besides, in secretory tissues such as the mammary gland, Zn²⁺ transporters of SLC families are involved in specific functions like insulin synthesis and the secretion of some digestive proenzymes, transferring nutrients from blood to milk.

6. CSN2 Gene—Benefit for the Lactose Intolerance

Casein (CSN) is the main protein component of milk in most milking species and accounts for about 80% of total milk protein ^[22][51]. Its functions include transporting calcium and phosphate to milk to provide enough calcium and phosphorus for bone formation in young lambs and to meet amino acid requirements. Casein includes α S1-casein (CSN1S1), α S2-casein (CSN1S2), β-casein (CSN2), and k-casein (CSN3) ^[13][23][24][37,52,53]. Sheep casein contains 45% β-casein, represented by two phosphorylated forms, β1-casein and β2-casein, which have similar amino acid compositions as bovine β-caseins and have a significant effect on milk protein ^[25][54]. Interestingly, people also found that milk containing A2 β-casein (CSN2) only causes fewer and slighter symptoms in lactose intolerance (LI) than milk containing both A1 and A2 β-caseins ^[26][27][55,56].

7. SCD Gene—Promote Unsaturated Fatty Acids Yield

For many years, people have paid important attention to unsaturated fatty acids in preventing the risk of cancer. Fatty acids are one of the most important bioactive components in mammalian milk ^[28][59]. Due to their high nutritional value and function on the physicochemical processes of the body, they are necessary for the development of the nervous system and the growth of a young body. These discoveries increased the demand of sheep milk, as sheep milk fat contains several components that provide humans great health benefits, notably a large quantity of unsaturated fatty acids and conjugated linoleic acid (CLA) ^[29][60]. Most unsaturated fatty acids in ruminant milk are synthesized in the mammary gland by the action of an enzyme called stearoyl-CoA desaturase (SCD) working on circulating vaccenic acid synthesis.

8. SOCS2—An Indicator of Mastitis in Mammary Gland

Mastitis is an infectious disease mainly caused by staphylococcus aureus and streptococcus invading the mammary gland. Genetic control of susceptibility to mastitis has been widely proved in dairy ruminants, but the genetic basis underlying this disease is still largely unknown. Initially, Rachel Rupp et al. ^[30][64] found the identification of a mutation in the Suppressor of Cytokine Signal 2 gene (SOCS2) by implementing a genome-wide association study. This mutation was shown to cause a deficiency on functional activity of the SOCS2 protein, which suggested an impairment of negative control on the JAK/STAT signal pathways in mastitis animals ^[16][31][40,65]. 

9. Other Detected Genes Related to Milk Traits

Since 2017, various genomic-based studies have been conducted to figure out the genetic basis on sheep milk to find the specific genomic variant(s) contributing to sheep milk production ^[32][33][34][50,70,71]. For example, Sutera, A.M. carried out GWAS for milk production traits in 469 Valle del Belice sheep using repeated measures, and the research found SNP rs425417915, which is located in an intronic region of TTC7B (tetratricopeptide repeat domain 7B) gene, to be associated with both fat percent and protein percent, playing a critical role in lipid metabolism in cattle and was reported to be associated with fat yield (FY) composition in sheep ^[35][72].