The relationship between race and genetics is relevant to the controversy concerning race classification. In everyday life many societies classify populations into groups based on phenotypical traits and impressions of probable geographic ancestry and socio-economic status—these are the groups usually called "races" in countries like the United States. Because the patterns of variation of human genetic traits can be both abrupt and clinal, with a gradual change in trait frequency between population clusters, it is possible to statistically correlate clusters of physical traits with individual geographic ancestry. The frequencies of alleles tend to form clusters where populations live closely together and interact over periods of time. This is due to endogamy within kin groups and lineages