Animal ShellsSnailsPigmentationPolymorphism, GeneticPolymorphism, Single NucleotideGenetic Predisposition to DiseaseGene FrequencyAllelesHaplotypesCase-Control StudiesGenotypePolymerase Chain ReactionPolymorphism, Single-Stranded ConformationalGenetic Association StudiesLinkage DisequilibriumAsian Continental Ancestry GroupGenetic VariationSequence Analysis, DNABase SequenceGenetic MarkersHomozygoteMolecular Sequence DataRisk FactorsPhenotypeMethylenetetrahydrofolate Reductase (NADPH2)European Continental Ancestry GroupAmplified Fragment Length Polymorphism AnalysisChinaHeterozygotePromoter Regions, GeneticExonsOdds RatioDNA PrimersChromosome MappingMicrosatellite RepeatsGenome-Wide Association StudyDNA Mutational AnalysisMutationGenetic LinkagePeptidyl-Dipeptidase AGenetics, Population