So that is what is confusing me. If your ducks had the runner pattern allele (that is what Ancona have) they would be spotted, it is a dominant trait. Since the parents aren’t spotted they shouldn’t be able to have it?!
https://www.backyardchickens.com/articles/mallard-derived-duck-color-genetics-basics.74277/ This is a link to a wonderful article by Pyxis explaining genetics. Both parents would have to carry white to get a white baby. So, if both parents were Peking crosses, it would work. I do not know...
I believe you are seeing hidden recessive traits and your ducks are indeed mix breeds. The black allele from a Swedish or Cayuga is dominant and will hide whatever is also there. Or, You could have Pekin mixes. The white allele is recessive and codes for no color put in the feathers. So, if...