What causes genetic variation in meiosis?

• Chiasma formation and crossing over- portions of non-sister chromatids of homologous chromosomes may break and rejoin. Exchange of alleles between homologous chromosomes ⇒ new combination of alleles 
• In metaphase I – (independent assortment) orientation of homologous chrm pairs at equator is random and how one homologous pair orientates does not affect how other homologous pair orientates. Metaphase II- orientation of non-identical sister chromatids of one chromosome at equator is random. Each daughter cell will receive a random mix of maternal and paternal chromosomes. 
• Random fusion of two gametes will result in 223x223 different possible types of offspring in humans. Can also be non-disjunction of homologous chromosomes in mei I or non-disjunction of sister chromatids during mei II resulting in polyploidy.