C/CTwo copies of the ancestral allele. Published research associates this genotype with lactase non-persistence — lactase production typically declines after childhood, which can mean reduced tolerance for dairy in adulthood.
C/TOne copy of each allele. Research associates this genotype with a good chance of continued lactase production into adulthood, though individual tolerance still varies.
T/TTwo copies of the derived allele. Published research associates this genotype with lactase persistence — continued lactase production into adulthood is common.
Lactose tolerance in practice depends on more than one gene and can change over time. If dairy causes discomfort, talk to a doctor before changing your diet.
Source: Enattah et al. 2002, Nature Genetics — identification of a variant associated with adult-type hypolactasia.