Trying to find information on my great grandpa?

My grandfather does not know who his biological father is and his biological mother refuses to tell my grandfather who his father is. Is there by chance any way my grandfather can find out who his real dad is without DNA testing. The only other person who knew who my great grandfather was has passed away and now we have no way of knowing. Any help is appreciated.

what percent of each ethnicity am I?

So my dad is 50% filipino and 50% chinese. And my mom is full filipino. so would that make me 75% filipino and 25% Chinese?

Some genes are recessive so they do not emerge very often. You clearly take after that grandparent and that side of your family. If you have a light-skinned grandparent, then their child (your mother, for the sake of ease) will carry those same genes for lighter skin but they may not be expressed (visible) because they are over-ridden by the genes she got from their other parent (your other grandparent). If your father also carries the lighter-skinned gene but does not show it, three of four children will be dark skinned, one of four will be lighter skinned.

Assuming the records are in the U.S. Federal Government, call your nearest National Archives and Record Administration branch. Ask to speak to a military archivist, and ask them your questions.... they should be able to direct you to source material.

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Williams means son of William, so in Italian Guglielmo is the Italian form of the masculine name William BOTH languages have foundations in Latin language and Vilhelmus in the Latin language version of the same name William and "son of" is figlio di translated to the Italian language........ however as Wendy has already said Williams is an English language surname so if you are assuming or were told your ancestors " changed their name when migrating" take that with a large pinch of salt as it is unlikely, you need to find records of BOTH names to prove that and that is likely to be a naturalisation record, when you find that then you will know and only then