From a genetics standpoint, the link between these cousin marriages and increased risk of genetic disorders in the offspring of such marriages has become strong. The reason for that is in the laws of probabilities. The causes behind the differences between individuals, is mostly due to recombination of genes during the process of meiosis in cells. Other than that, mutational events and independent assortments also cause genetic variations, that differs one individual from the other. However, these variations are of a very small percentage as all humans share roughly 99 percent of their genomes with others. It’s the approximate one percent that brings about the differences between us.
Looking at the average DNA that is shared among relatives, a person shares 50 percent of their DNA with their parents and 50 percent with their siblings. As the degrees shift from 1st to 4th, the percentage of shared DNA drops whereby you end up sharing 12.5 percent of your DNA with your first-cousins.
That shared DNA is significant when those cousins inter-marry. The problem is that the common gene pool from which genetic variation arises becomes smaller and smaller the more one marries within a family. And through such restricted genetic pools, the recessive genes that cause autosomal recessive disorders become dominant and get expressed in the offspring.