Imagine the world of genetics. We can do amazing things and one of them is to modify the genes of animals. Now imagine I could modify a pig in a way that the result is more like a cow and not like a pig. For simplicity use a percent scale from 100% cow to 100% pig. In between would be a cig (mixture of cow and pig 50%-50%). Where is the treshold that allows you to eat the resulting animal?
As long as there is a hint of a doubt that an animal is in whatever form an offspring of a pig it would be considered as impure by the majority of scholars and therefore its meat as haram for consummation.
This is due to the fact that ibn Qudamah said:
There's no difference in the madhhab on the impureness of the dog and pig and whatever was derived from them that is najiss by itself, and the remaining (of their food) and its sweat and all that comes out from it.
As stated in my answer on Is it haram to touch a pig , to smell it, or to go even near it?
Now let's get more precise on your inquiry, as impurity covers only the majority and doesn't present any solution for those who deviate, which even have a strong backup as there's no clear proof for the impurity of a pig.
In the osol al fiqh there's a fiqh rule saying "if haram and halal come together the haram was dominant"
إذا اجتمع الحلال والحرام غلب الحرام
(See for example here in as-Suyuti's al-Ashbah wa an-Nadhair الأشباه والنظائر)
Note that al-Bayhaqi (in his as-Sunan al-Kubra see here -in Arabic-) and Abdulrazzaq (in his mosanaf see here -in Aarbic-) compiled a hadith in a similar wording of this rule, but many scholars consider it as fabricated as imam as-Suyuti pointed out and imam al-Bayhaqi showed in his Sunan the weaknesses of his presented narration!
Applied for this case it would mean: if the meat of one of the parents of such an animal was haram then the meat of the off-spring would be haram too.
As stated in the examples of imam as-Suyuti for this rule (Same source as above and same page):
ومنها : من أحد أبويه مأكول ، والآخر غير مأكول . لا يحل أكله
My own translation take it carefully
Among that is if one of the parents was eaten (allowed to be eaten) and the other not, then it is not allowed to be eaten.