What are the three main ways that variations in genotype arise in a population

These three factors affect how variations in genotype arise in a population:
1) Sexual recombination- one-half of the parent 1 and one-half of parent 2’s genes are combined to form a brand new gene for the offspring resulting to a new type of combination that is not existing before.
2) Gene crossover- this is the trade-in of genetic material from two cells that each contains a pair or 1 maternal strand and 1 paternal strand of the parents chromosomes (Homologous).
3) Chromosome aberration- it is a mutation in the chromosome wherein there is an extra or missing part of the DNA.  

Gravitational force i believe