Marriages between blood relations have been common in countries like India, Middle East, China, Japan and a few North African countries. Its almost normal in some villages of Andhra Pradesh and Tamil Nadu in Southern parts of India. Pharoahs in Egypt used to perform marriages between own brothers and sisters just to preserve their royal blood.
Although the moral aspects of marrying a blood relation might be debated widely, its the medical aspect that needs to be attended.
Physical disabilities and deformities, mental affictions, genetic mutations, leukemias and a few auto-immune diseases are some of the dreaded consequences.
Marriages between first cousins forms the bulk of these in the present day. Genetically, first cousins have 12.5% of genetic matter common between them. Which leads to a huge risk of developing genetic diseases due to presence of two disease-causing recessive genes, which would otherwise had remained dormant.
Apparently, the WHO has been in talks with the Governments of affected countries to pass a law that would make consanguineous marriages illegal, if not punishable.