This example displays a family tree of human blood groups, breaking down the major types (A, B, AB, O) into their positive and negative variants. The hierarchy is rendered as an animated SVG tree using `d3.stratify` and `d3.tree()`, with circles representing each node and colored by generation depth. Transitions animate the links and nodes as they appear. Text labels are added via SVG `text` elements. The data is hardcoded in the script, and the layout is computed with a fixed size of 950 by 250 pixels.