ABSTRACT
The aim of this study was to investigate the effect of CSN1S1 gene polymorphism, time of lactation and parity on milk quantity and quality. A total number of 165 Zaraibi goats (105 kids, 7 bucks and 53 does) after kidding season of February/ March 2013 were used in this experiment. The electrophoretic pattern of digested fragments of PCR amplified goat CSN1S1 products with XmnI enzyme showed the presence of 9 genotypes, 4 homozygous (AA, BB, CC and DD) and the other 5 heterozygous (AC, AD, BC, BD and CD). The nine genotypes were derived from 4 alleles; three strong alleles (A, B and C) with frequencies (17.76, 27.63 and 26.97%, respectively) and one intermediate allele (D) with frequency 27.63%. BB and CC were the most common homozygous genotypes detected in the present study with similar frequency (18.42%) followed by DD (17.76%), BD (13.82%), AA (10.53%), AC (10.53%), BC (4.61%), AD (3.95%) and CD (1.97%). Daily milk yield, percentage of milk fat, protein, total solid and solid not fat significantly affected by CSN1S1 genotypes. Zaraibi does carrying AD variant showed the highest levels of daily milk yield, fat, protein, total solid and solid not fat. Sequence analysis showed that only the deletion of nucleotide C in allele A caused substitution of TTC to TTG forming Leu (Leucine) instead of Phe (Phenyl alanine) in mature protein. Meanwhile, no polymorphism detected between nucleotide 1 and 274 at amplified sequence of exon 9, so this region is considered as a highly conserved among alleles (A, B, C and D).
Daily milk yields in Zaraibi does were significantly high at the first week after kidding, peaked at the 2nd week then decrease till the end of lactation. Moreover, the highest milk yields were recorded in does at the 7th and 6th parities, while the lowest values observed in does at the 1st and 2nd parities. In addition, percentages of fat and total solids were the highest at day 90 (end of suckling period) than during days 120 and 210 of lactation period. Milk protein was the highest at end of lactation (day 210), while lactose levels showed the lowest values. In conclusion, studying genetic polymorphism for Zaraibi goats CSN1S1 gene is important for identifying favorable genotypes associated with high milk yields, protein and fat. Breeding program would be changed toward selecting animals carrying the strong and mild alleles for milk protein and fat required for economic goat milk industry.