1) error in DNA copying
2)biparental reproduction process
3)mixing of two different genetic materials 

Sexual reproduction is a process in which the genetic material from two individuals (male and female) combine to form the offspring. Two reasons for the appearance of variation among the progeny formed by sexual reproduction may be as follows:
1) The progeny formed by sexual reproduction is the result of the combination of the genetic material from two different individuals. So, the offspring acquires half of the traits from the mother and half from the father. Thus, the combination of various traits results in the variation in the offspring due to which the offspring is different from both the parents.2) Crossing over is the process in which the DNA is exchanged between the homologous chromosomes during meiosis. It occurs during gamete production, and as a result of this the gametes produced by an individual do not have the same genetic material as of the individual. This adds to the variation as the genetic material of an organism is recombined before formation of gametes and the recombined gametes fuse to form the zygote which gives rise to the offspring. So, the offspring will not have exact set of genes from the parents but will have recombined set from each of them