You should read
-W.E.B. Du Bois, Black Reconstruction in America
-Oliver C. Cox, Caste, Class, and Race: A Study in Social Dynamics
-Friedrich Engels, The Origin of the Family, Private Property and the State
-Lise Vogel, Marxism and the Oppression of Women: Toward a Unitary Theory
Racial and Gender discrimination is a direct product of class relations not a second system operating alongside it.