Polyploidization has played a key role in the evolution of plant lineages. The effect of formation pathway on neopolyploid morphology and fertility, however, is unknown. This thesis compares one-step and two-step allopolyploids with Brassica carinata, Brassica juncea or Brassica napus as the maternal parent and Sinapis arvensis as the paternal parent. One-step allopolyploids formed twice as frequently, though once a homoploid hybrid formed two-step allopolyploids were ten times as frequent as one-step allopolyploids. The two-step allopolyploids were more fertile and more reproductively isolated from parental species. The two types of allopolyploids differed morphologically, but in both types, polyploidy had a large initial effect on morphology while the effect of hybridity endured. Both types of allopolyploids, but especially the one-step allopolyploids, showed an unexpectedly high rate of DNA downsizing. In conclusion, allopolyploids formed through different formation pathways differ significantly in fertility, morphology, and genome stability.