The image shows the location of genes A, B, C, and D on a chromosome. A chromosome. Gene A is above the attachment point. Gene B is just below the attachment point, followed by Genes C and d.

A maternal chromosome has a trait combination of ABCD, while a paternal chromosome has a trait combination of abcd. During meiosis, the gametes that are formed have trait combinations of ABCD, ABcd, abCD, and abcd. What led to the increase in trait variation in the resulting gametes?

crossing-over between sister chromatids in prophase II
independent assortment of sister chromatids in anaphase II
crossing-over between the maternal and paternal chromosomes in prophase I
independent assortment of maternal and paternal chromosomes in anaphase I