The LCT gene controls whether you produce lactase (enzyme that breaks down lactose) as an adult. The genetic variant that allows people to still drink milk as an adult is found in about 90% of Caucasians. But Caucasians are the exception here, and most other populations don’t carry the variant that produces lactase as an adult.
Check your genetic data for rs4988235 (23andMe v.4, v.5; AncestryDNA):
Want more details? Check out the full article on Lactose Intolerance Genes
*SNP stands for Single Nucleotide Polymorphism, which is when one of the nucleotide bases (the A, C, G, or Ts) is replaced by a different nucleotide base in a gene.