Applying Mendal's LAW OF SEGREGATION. Allele segregates while gamete formation. Applying this rule to Nepalese we get, Allele for chetri, tamang, bahun, newar, sarki, damai or whatever it is segregates while coming to US. As a result, same allele or like molecules attract each other. People having same allele are likely to attract each other and don't often bind with other types of allele. If these people having different allele wants to bind then a new species will be formed which we call a "mutant". And most of the time mutants are harmful. So, raazstone, if these Nepalese people do not segregate ,they will create mutants which are harmful for the society.