Short answers, pulled from the story.
Modern science regards race as a social construct rather than a biological reality. The U.S. National Academies of Sciences, Engineering, and Medicine declared in 2023 that researchers should not use race as a proxy for describing human genetic variation, calling it a "misleading and harmful surrogate" for population genetic differences.
According to scholars Bancel and colleagues, the modern race concept was invented and rationalized somewhere between 1730 and 1790. The term entered common usage in the 16th century, but the systematic linking of physical traits to moral and intellectual hierarchies developed through the age of European colonialism and the Atlantic slave trade.
The Human Genome Project found that an average of 85% of statistical genetic variation exists within local populations, with only about 8% separating large groups living on different continents. Craig Venter, upon examining genome mapping data in 2000, concluded that the types of variation do not support the notion of genetically defined races.
The one-drop rule defined anyone with any known African ancestry as Black, regardless of appearance. During the Reconstruction era this convention became widespread, and by the early 20th century it was made statutory in many states. It was specific to the United States and to those with African ancestry.
Brazil did not use rigid descent rules like the one-drop rule. Racial identity was tied to physical appearance rather than ancestry, meaning full siblings could belong to different racial groups. Over a dozen racial categories were recognized, grading into each other continuously, and a child was never automatically assigned the racial type of one or both parents.
Clinal variation refers to the gradual change of physical and genetic traits across geography rather than in discrete jumps between bounded groups. Anthropologist C. Loring Brace showed that skin color grades continuously from northern Europe southward to equatorial Africa with no visible boundary, and that different traits produce different, non-overlapping gradients, making any fixed set of racial boundaries dependent on which traits are chosen.