that's a really good question, I don't have the exactl answer but I will tell you my opinion , I think because each cell have diploid 2n number of chromosomes, this means each cell have 2 homologous chromosomes these chormosomes might have different alleles for the same trait, and by undergoing meiosis the number decrease to 1n hiploid so one chromosome of the two homologous one will disappear from the cell, this could lead to appearance of a recessive character to appear, I hope that this help although basically crossing over is the main reason for genetic variation in the cell so...