In chapter 14 from what lineage does Roxy claim they are?

Roxy scolds Tom for having black lineage in him:

t's de nigger in you, dat's what it is. Thirty-one parts o' you is white, en on'y one part nigger, en dat po' little one part is yo' soul. 'Tain't wuth savin'; 'tain't wuth totin' out on a shovel en throwin' en de gutter. You has disgraced yo' birth. What would yo' pa think o' you? It's enough to make him turn in his grave."