So Madara was able to create life with Senju DNA when he made white zetsu, he was also able to create a zetsu which he was able to pour his Uchiha will into. We've seen how white zetsu is obviously lighter hearted like the senju characters we know and black zetsu is ominous like the uchiha. My theory is that just like Madara, the so6p created the elder and younger son on his deathbed using his much more advanced skills since he had the combination of the two. it has been theorized before that the so6p created the two sons but I think Madara creating black zetsu proves it. This is also the explanation for how he had two sons with totally different genes that didn't carry eachothers' recessive genes.