Food for thought! ... If a member of the Hapsburg (Habsburg) royal clan committed incest, rape, or seduced a young girl like Juana, whould the "royals" be tempted to hide such an event? - her first pregnancy produced a boy - Emperor to be, Charles V (for the Austrians) or Carlos I (for the Spaniards) - this boy was "defective" - had a large jaw and hanging large lower lip (he drooled, did not eat in public and had a speech impediment) - the same characteristic or trait were noted in multiple prior and future members of the Habsburg clan - these traits were inherited by men and women from either a mother of father similarly impacted (autosomal dominant highly penetrant gene mutation). This sort of combination of anomalies is virtually always inherited (new gene mutations are very rare). Here is the question! Neither Juan nor her consort Felipe of Austria manifested such anomalies (as illlustrated here and in a multitude of companion images posted along). Juan was declared "loca" (or demented) and confined by the Austrian court for the rest of her life! Would you believe a "crazy" woman if she told you that her son was illegitimate (see the word bastard)? - A medical geneticist of today would at least analyze Juanas and Felipes genes and ... those of Felipes father Maximilian who did have the same anomalies as his grandson Carlos I or Charles V.
P.S. you may find of interest the rest of the gallery of images/notes concerning the "Habsburg Jaw" (for ample links please focus on Maximillian) and "word-ideas" inherent in Paternity and Patriotism.