الملخص الإنجليزي
In most mammals and in some humans, the level of the enzyme lactase declines after weaning. However, some humans develop a condition known as lactase persistence (LP) where the levels of lactase remain high during adulthood as a result of a mutation that took place thousands of years ago. This mutation occurred in the enhancer region of the LCT gene which is located in the neighboring MCM6 gene. The evolution of lactase persistence is believed to be a result of natural selection due. to the development of cattle domestication and milk consumption among pastoralist groups. The frequency of lactase persistence varies from one geographic region to another, and is found to be very high in North Europe and in pastoralist Arab and African groups. In Oman, the highest frequency of lactase persistence was observed in the Southern region of Oman, Dhofar. The Single nucleotide polymorphisms (SNPs) responsible for lactase persistence vary from one ethnic group to another, and the one found at a very high frequency of 0.74 in Dhofar is known as the G-13915 polymorphism. Because of this high allele frequency, it is hypothesized that this mutation originated in Southern Arabia as a result of a strong recent selective pressure. In order to trace the origin of the G-13915 polymorphism and to estimate its age in Dhofar, samples from three Omani sub-populations; Northern Omanis, Omanis of Asian origin and Southern Omanis (Dhofarians) were collected. The samples were analyzed for ten Short Tandem Repeats (STRs) that cover an area of 118 Mb of the q arm of chromosome 2 and for fourteen SNPs that cover an area of 3.5 Mb and flank LCT and MCM6 genes. Data obtained were used to construct haplotypes that compared the three Omani sub-populations and traced the age and origin of the G 13915 mutation. The results of the study revealed that Dhofarians are different from the other two subpopulations at the LCT locus in both; the haplotypes they carry and the genetic distance between them and the other groups. The genetic distance between Dhofarians and Northern Omanis (Fst = 0.529, p<0.001) is greater than between Northern Omanis and Omanis of Asian origin (Fst = 0.0307, p<0.05) at the G-13915 locus. However, Evidence for natural selection was obtained in Dhofar for the G-13915 mutation, which had the highest Fst value than any other marker typed in the study (Fst = 0.372, p = 0.005). The age of the mutation in Dhofar was found to be 2178 5550 years, which supports the hypothesis that the mutation has originated in Southern Arabia and is probably due to selective pressure.
