An interesting question is that how can we claim that we are descendants of the 3 Avos and 4 Imahos when part of Klal Yisroel does not come from Rochel or Leah? Probably the obvious answer is that when Rochel gave Bilhah to Yaakov she said that this will produce descendants for herself, so we see that the children of the Shfachos are considered to be from the corresponding Em.
