Here are the facts

White Americans average 0.37% non-white blood. 0.19% African , 0.18% Amerindian
Source:
http://www.unz.com/isteve/nyt-white-black-a-murky-distinction-grows-still-murkier/

Afrikaners and Boers average 5-7% non-white ancestry, depending on which study you believe. Most of this non-white ancestry is sub-Saharan African in origin.
Source: https://www.unz.com/gnxp/admixture-in-south-african-afrikaners/

South Africans produced Sandra Laing, a genetic throwback which occurred when negro genes in South Africans decided to reappear one generation. The South African apartheid government concluded she was a throwback and not a product of adultery with a black. Therefore the South African government passed legislation to keep Sandra Laing legally white. https://en.m.wikipedia.org/wiki/Sandra_Laing

White Americans have never once produced a nigger throwback child.

South Africans had the pencil test. A person who was known to have 25% black and 10% Indian ancestry would be declared white as long as a pencil didn't get stuck in their hair. https://en.m.wikipedia.org/wiki/Pencil_test_(South_Africa)

White Americans had the one drop rule test. A person who was known to be 1/16, 1/32, 1/64 or even 1/128 black was black. Therefore such a person would face felony criminal prosecution if they intermarried or had sex with a pure white. They could only legally reproduce with blacks.
https://en.m.wikipedia.org/wiki/One-drop_rule

Which group is the actual mutts? White Americans or white South Africans?