What you carry at this position in your DNA and what research says about it.
Lactose Tolerance: 6/10 evidence, 12 publications.
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Sample genotype (not yours): C/C
Lactose Intolerant
Lactose digestion is severely limited in adulthood due to a natural decline in lactase enzyme production. Consuming dairy products is likely to cause digestive discomfort.
What this genotype means: High likelihood of lactose intolerance; Higher gut Bifidobacterium levels when consuming dairy; Lower preference for dairy-rich foods.
Sample genotype (not yours): C/T
Lactose Tolerant
Lactase enzyme production is maintained into adulthood, allowing for the efficient digestion of lactose. Typical dairy products can be consumed without digestive discomfort.
What this genotype means: Ability to digest dairy in adulthood; Typical gut microbiome composition; Slightly higher preference for milky sweets.
Sample genotype (not yours): T/T
High levels of the lactase enzyme are produced throughout adulthood. Milk and other dairy products can be digested seamlessly without symptoms.
What this genotype means: Full lactase persistence in adulthood; No dairy-related digestive discomfort; Slightly higher circulating chylomicron lipids.
Gene function and biological role
Gene Function: The MCM6 gene encodes a protein that is a key component of the MCM2-7 complex, which is essential for the initiation of eukaryotic genome replication. Beyond its role in DNA replication, MCM6 contains crucial regulatory enhancer elements that control the expression of the neighboring LCT gene, which encodes the lactase enzyme. These enhancer regions act as molecular switches that determine whether lactase production continues into adulthood or declines after weaning. Genetic variations within these introns of MCM6 are the primary drivers of lactase persistence or adult-type hypolactasia in various human populations. This regulatory relationship makes MCM6 one of the most famous examples of gene-culture co-evolution and positive selection in human history.
Variant Impact: The rs182549 variant is an intronic single nucleotide polymorphism located within the MCM6 gene that influences the transcription of the adjacent LCT gene. A base change from the ancestral C allele to the derived T allele alters a transcription factor binding site, preventing the post-weaning downregulation of lactase. This single nucleotide change is in near-perfect linkage disequilibrium with another well-known lactase persistence variant, rs4988235. Together, these variants form a haplotype that is highly predictive of lactose tolerance in populations of European descent. In contrast, other distinct mutations in the same regulatory region drive lactase persistence in pastoralist communities of African and Middle Eastern origin [1].
Key Insight: Undigested lactose in the gut of lactose-intolerant individuals acts as a prebiotic, significantly boosting beneficial Bifidobacterium levels.
Location: MCM6. Chromosome 2 · 2q21.3 · 135.9 Mb
Origin Story: The T allele of rs182549 rapidly rose in frequency in Europe over the last 10,000 years, co-evolving with the cultural adoption of dairy farming and cattle domestication, which provided a critical survival advantage during periods of famine.
Estimated prevalence by population
Global 42.1%; European 12.9%; African 77.0%; East Asian 99.6%; South Asian 67.4%; Americas 59.1%; Ashkenazi 75.0%; Finnish 17.7%; Middle Eastern 88.1%; Amish 5.4%.
Estimated CC genotype frequency.
Global 35.5%; European 46.0%; African 21.5%; East Asian 0.4%; South Asian 29.4%; Americas 35.5%; Ashkenazi 23.2%; Finnish 48.7%; Middle Eastern 11.5%; Amish 35.6%.
Estimated CT genotype frequency.
Global 22.4%; European 41.1%; African 1.5%; East Asian < 0.1%; South Asian 3.2%; Americas 5.3%; Ashkenazi 1.8%; Finnish 33.6%; Middle Eastern 0.4%; Amish 59.1%.
Estimated TT genotype frequency.
Showing effects for C/C genotype
Nutrition & Metabolism
Food Intolerances
Lactose Digestion. Evidence: 6/10. Lactose Intolerant.
Organ Systems
Digestive & Gut Disorders
Gut Bifidobacterium. Evidence: 6/10. Source: PMID 33462485. Higher Abundance.
Neurology & Mental Health
Behavioral & Personality Traits
Sweet Food Cravings. Evidence: 6/10. Source: PMID 35585065. Typical Liking.
Cardiovascular & Metabolic
Lipids & Cholesterol
Chylomicron Lipids. Evidence: 6/10. Source: PMID 41044249. Typical Levels.
Showing effects for C/T genotype
Lactose Digestion. Evidence: 6/10. Lactose Tolerant.
Gut Bifidobacterium. Evidence: 6/10. Source: PMID 33462485. Typical Abundance.
Sweet Food Cravings. Evidence: 6/10. Source: PMID 35585065. Higher Liking.
Chylomicron Lipids. Evidence: 6/10. Source: PMID 41044249. Slightly Higher.
Showing effects for T/T genotype
What this genotype means for the people who carry it
Sample genotype: CC (Homozygous)
Lactose digestion is severely limited in adulthood due to a natural decline in lactase enzyme production. When dairy is consumed, undigested lactose passes into the large intestine, where it is fermented by gut bacteria. This fermentation process often leads to symptoms like bloating, gas, abdominal pain, and diarrhea. Interestingly, the presence of undigested lactose in the colon serves as a prebiotic, significantly increasing the abundance of beneficial Bifidobacterium species [1]. This shift in the gut microbiome is particularly pronounced in those who continue to consume small amounts of dairy despite malabsorption [1]. Because this variant is located in a regulatory region of the MCM6 gene, it acts as a genetic switch that downregulates the neighboring LCT gene after weaning. For those of European ancestry, this genotype is a highly reliable predictor of adult-type hypolactasia. However, in sub-Saharan African and East Asian populations, other genetic variants are more predictive of lactose digestion capacity [2].
High likelihood of lactose intolerance; Higher gut Bifidobacterium levels when consuming dairy; Lower preference for dairy-rich foods.
Sample genotype: CT (Heterozygous)
Lactase enzyme production is maintained into adulthood, allowing for the efficient digestion of lactose in the small intestine. This genetic trait, known as lactase persistence, is inherited in a dominant manner, meaning a single copy of the T allele is sufficient to prevent lactose intolerance. Because lactose is fully broken down into glucose and galactose, very little reaches the large intestine to undergo bacterial fermentation. Consequently, typical symptoms of dairy sensitivity like bloating and gas are avoided. The gut microbiome reflects this efficient digestion, showing a typical, lower abundance of Bifidobacterium compared to lactose-intolerant genotypes [1]. This genotype is also associated with a higher preference and liking for milky sweets and dairy-rich foods [2]. Metabolically, the regular consumption of dairy enabled by this genotype can lead to slightly higher circulating levels of lipids and cholesterol in extremely large VLDL particles [3]. This co-evolution of diet, genetics, and metabolism represents a classic example of recent human adaptation.
Ability to digest dairy in adulthood; Typical gut microbiome composition; Slightly higher preference for milky sweets.
Sample genotype: TT (Homozygous)
Adults with this genotype produce high levels of the lactase enzyme, ensuring seamless digestion of milk and other dairy products. This ability to digest lactose persists throughout life because the T allele prevents the normal post-weaning shutdown of the LCT gene. Since lactose is completely absorbed in the small intestine, it does not reach the colon to feed gas-producing bacteria. This protects against the digestive discomfort, bloating, and diarrhea associated with lactose intolerance. Because there is no undigested lactose reaching the large intestine, the gut microbiome maintains a typical, lower baseline of Bifidobacterium species [1]. Having this genotype is strongly linked to a higher dietary intake of dairy and an increased liking for milky sweets [2]. From a metabolic standpoint, the resulting dairy-rich diet is associated with slightly higher levels of total lipids and free cholesterol in chylomicrons and VLDL particles [3]. This genetic variant represents one of the strongest examples of positive natural selection in human history, particularly among pastoralist populations.
Full lactase persistence in adulthood; No dairy-related digestive discomfort; Slightly higher circulating chylomicron lipids.
Variants with shared genetic architecture
rs4988235 · MCM6 Gene · Chr 2 · Lactose Digestion
rs1446585 · R3HDM1 Gene · Chr 2 · Colorectal Cancer Risk
rs7041 · GC Gene · Chr 4 · Vitamin D Levels
Lactase persistence and lipid pathway selection in the Maasai.
Large-scale association analyses identify host factors influencing human gut microbiome composition.
A genetic map of human metabolism across the allele frequency spectrum.
Content updated August 2026.
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