Question

Why is it said that marriages between close relatives is dangerous for the next generation?

Answer 1

Human beings have many hundred recessive traits which in homozygous state cause hereditary diseases. Being non-infective, the hereditary diseases are nearly incurable. The only way to avoid them is to try for nonappearance of these harmful traits in homozygous state. In the heterozygous condition, the recessive traits remain suppressed due to presence of dominant alleles.
Homozygosity is more common if marriages are performed between relatives of 4-6 generations. The same should, therefore, be avoided. Out crossing or marriages between unrelated individuals for 4-6 generations will continue heterozygosity in the offspring.