Human recombination is a significant source of genetic variation.

Which would most likely decrease the genetic variation in the human population?

Loss of genetic variation(:Novanet:)

If the human population was reduced to a very small number of interbreeding individual then this small population, denied outbreeding, would have very little genetic variation. Humans, who went through a bottleneck event about 70,000 years ago, are considered a " small " species because they have little genetic variation in comparison to many other species.

Google cheetah to see how this concept works.

mutations cause genetic variation, and vice-versa. If there is a genetic variation (or lack of one), then this can effect the severity of the mutation.