What causes genetic variation during human sexual reproductive processes?

The zygote cell divides many times through mitosis to form a blastocyst during embryo formation.

Adjacent chromatids in homologous chromosomes cross over during meiosis in sperm and egg formation.

Mitochondria, containing mitochondrial DNA, collect in the midpiece of sperm cells during sperm formation.

Spindle fibers form between centrioles before attaching to chromosomes during meiosis in gamete formation.