Neanderthal DNA is, for the most part, dormant. And at at a level of only 1 to 4 percent in modern humans, it would have no effect. My guess is lactase persistence, which is is the continued activity of the lactase enzyme in adulthood. Since lactase's only function is the digestion of lactose in milk, in most mammal species, the activity of the enzyme is dramatically reduced after weaning. In some human populations, though, lactase persistence has recently evolved as an adaptation to the consumption of nonhuman milk and dairy products beyond infancy. Lactase persistence is a text-book example of natural selection in humans: it has been reported to present stronger selection pressure than any other known human gene. The distribution of the lactase persistence phenotype, or the ability to digest lactose into adulthood, is not homogeneous in the world. Lactase persistence-frequencies are highly variable. In Europe, the distribution of the lactase persistence phenotype is clinal, with frequencies ranging from 15–54% in the south-east to 89–96% in the north-west.