This example visualizes the hierarchy of blood group types using a tree layout, with nodes representing groups like A, B, AB, and O, and their positive or negative variants. The visualization uses SVG circles and text labels, which animate into position via D3 transitions, while links between parent and child nodes also animate. The code applies d3.stratify() to convert flat data into a hierarchy and d3.tree() to compute node coordinates, with circle fill colors indicating generation depth. Rendered with D3 v6, the animation highlights the structure of blood group inheritance.