Genetically, consanguinity derives from the reduction in variation due to meiosis that occurs because of the smaller number of near ancestors. Since all humans share between 99.6% and 99.9% of their genome, consanguinity only affects a very small part of the sequence. If two siblings have a child, the child only has two rather than four grandparents. In these circumstances the probability that the child inherits two copies of a harmful recessive gene (allele) rather than one which would not have immediate effects is much increased.