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For example humans who come from an ancestry which started domesticating cattle, and consuming dairy products have digestive systems which generate an enzyme called lactase which helps in digesting milk by digesting the sugar lactose in the milk.
However a significant population of humans are also lactose intolerant, which means they cannot digest milk products which contain lactose, because their body does not produce lactase.
Some evolutionary gene modifications might prove fatal when exposed, but are nevertheless are useful while they remain dormant and hence are continued to be carried.
For instance, Sickle Cell anemia is a fatal disease where RBCs sickle up causing life threatening complications.
O-ve means a blood group which has neither A nor B antigens, nor Rh D antigen.