How does genetic diversity affect biodiversity in an area provide an example

When humans farm large plantation of one cash crop, they reduce the biodiversity of the region because they reduce the genetic diversity of the species in the environment. A reduction in genetic diversity means that the genetic variability of the species is reduced  hence there are fewer sub-species in the environment. This ultimately reduces the biodiversity of the ecosystem -  which is the variability among all the living organisms in the environment.

this is true, assuming that the allele is completely recessive and incomplete dominance is not a factor.

not sure if this what   you'r looking for would be percent composition