diff --git a/frontend/src/lib/person/genealogy/StammbaumTree.svelte b/frontend/src/lib/person/genealogy/StammbaumTree.svelte
index 7d6caab8..a957ccd4 100644
--- a/frontend/src/lib/person/genealogy/StammbaumTree.svelte
+++ b/frontend/src/lib/person/genealogy/StammbaumTree.svelte
@@ -18,6 +18,7 @@ import {
 	ZOOM_STEP_KB
 } from '$lib/person/genealogy/panZoom';
 import { panZoomGestures } from '$lib/person/genealogy/panZoomGestures';
+import { buildLineageIndex, highlightLineage } from '$lib/person/genealogy/layout/highlightLineage';
 import StammbaumGenerationRail from '$lib/person/genealogy/StammbaumGenerationRail.svelte';
 import StammbaumConnectors from '$lib/person/genealogy/StammbaumConnectors.svelte';
 import StammbaumNode from '$lib/person/genealogy/StammbaumNode.svelte';
@@ -63,6 +64,17 @@ let {
 
 const layout = $derived.by<Layout>(() => buildLayout(nodes, edges));
 
+// Lineage highlight (#703). The adjacency index is rebuilt only when the edges
+// change; the cheap walk re-runs whenever the selection changes. A null
+// highlight (no selection) means full strength everywhere — nothing dims.
+const lineageIndex = $derived(buildLineageIndex(edges));
+const highlight = $derived.by(() =>
+	selectedId ? highlightLineage(lineageIndex, selectedId) : null
+);
+const isNodeActive = (id: string) => highlight === null || highlight.active.has(id);
+const isConnectorActive = (aId: string, bId: string) =>
+	highlight === null || highlight.isConnectorActive(aId, bId);
+
 // Stammbaum gutter (#689). 100 px column on the left of the canvas on md+
 // viewports, carrying the G{n} label per generation row. Hidden entirely on
 // phones (canvas is already overflow-scroll; 100 px of permanent chrome is
@@ -288,7 +300,11 @@ function handleCanvasKey(event: KeyboardEvent) {
 			{/each}
 		{/if}
 
-		<StammbaumConnectors edges={edges} positions={layout.positions} />
+		<StammbaumConnectors
+			edges={edges}
+			positions={layout.positions}
+			isConnectorActive={isConnectorActive}
+		/>
 
 		<!-- Nodes -->
 		{#each nodes as node (node.id)}
@@ -298,6 +314,7 @@ function handleCanvasKey(event: KeyboardEvent) {
 					node={node}
 					pos={pos}
 					selected={selectedId === node.id}
+					dimmed={!isNodeActive(node.id)}
 					onSelect={onSelect}
 				/>
 			{/if}
diff --git a/frontend/src/lib/person/genealogy/StammbaumTree.svelte.test.ts b/frontend/src/lib/person/genealogy/StammbaumTree.svelte.test.ts
index 7b1bc107..b84b12d1 100644
--- a/frontend/src/lib/person/genealogy/StammbaumTree.svelte.test.ts
+++ b/frontend/src/lib/person/genealogy/StammbaumTree.svelte.test.ts
@@ -894,3 +894,136 @@ describe('StammbaumTree generation rail (#689, #692)', () => {
 		await vi.waitFor(() => expect(railLabels()).toContain('Generation 3'));
 	});
 });
+
+describe('StammbaumTree lineage highlight (#703)', () => {
+	// A three-generation family. Selecting "Vater" highlights his pedigree
+	// (Grossvater, Grossmutter), his descendant (Kind), and the spouses of those
+	// blood people (Mutter, his married-in wife). "Tante" is a collateral sibling
+	// of Vater and must dim.
+	type Edge = {
+		id: string;
+		personId: string;
+		relatedPersonId: string;
+		personDisplayName: string;
+		relatedPersonDisplayName: string;
+		relationType: 'PARENT_OF' | 'SPOUSE_OF';
+	};
+	const edge = (
+		personId: string,
+		relatedPersonId: string,
+		relationType: 'PARENT_OF' | 'SPOUSE_OF'
+	): Edge => ({
+		id: `${personId}-${relationType}-${relatedPersonId}`,
+		personId,
+		relatedPersonId,
+		personDisplayName: '',
+		relatedPersonDisplayName: '',
+		relationType
+	});
+
+	const NODES = [
+		{ id: 'gf', displayName: 'Grossvater', familyMember: true },
+		{ id: 'gm', displayName: 'Grossmutter', familyMember: true },
+		{ id: 'vater', displayName: 'Vater', familyMember: true },
+		{ id: 'mutter', displayName: 'Mutter', familyMember: true },
+		{ id: 'kind', displayName: 'Kind', familyMember: true },
+		{ id: 'tante', displayName: 'Tante', familyMember: true }
+	];
+	const EDGES = [
+		edge('gf', 'gm', 'SPOUSE_OF'),
+		edge('gf', 'vater', 'PARENT_OF'),
+		edge('gm', 'vater', 'PARENT_OF'),
+		edge('gf', 'tante', 'PARENT_OF'),
+		edge('gm', 'tante', 'PARENT_OF'),
+		edge('vater', 'mutter', 'SPOUSE_OF'),
+		edge('vater', 'kind', 'PARENT_OF'),
+		edge('mutter', 'kind', 'PARENT_OF')
+	];
+
+	function nodeOpacity(displayName: string): string | null {
+		const g = document.querySelector(`g[role="button"][aria-label="${displayName}"]`);
+		if (!g) throw new Error(`No node group rendered for ${displayName}`);
+		return g.getAttribute('opacity');
+	}
+	const DIM = '0.4';
+
+	it('renders every node at full strength when nothing is selected (AC1)', () => {
+		render(StammbaumTree, {
+			nodes: NODES,
+			edges: EDGES,
+			selectedId: null,
+			panZoom: { x: 0, y: 0, z: 1 },
+			showGutter: false,
+			onSelect: () => {}
+		});
+
+		for (const n of NODES) expect(nodeOpacity(n.displayName)).toBeNull();
+	});
+
+	it('keeps the bloodline + spouses full and dims collaterals when a person is selected (AC2)', () => {
+		render(StammbaumTree, {
+			nodes: NODES,
+			edges: EDGES,
+			selectedId: 'vater',
+			panZoom: { x: 0, y: 0, z: 1 },
+			showGutter: false,
+			onSelect: () => {}
+		});
+
+		// Anchor, ancestors, descendant, and the spouses of blood people stay full.
+		for (const name of ['Vater', 'Grossvater', 'Grossmutter', 'Kind', 'Mutter']) {
+			expect(nodeOpacity(name)).toBeNull();
+		}
+		// The collateral sibling dims.
+		expect(nodeOpacity('Tante')).toBe(DIM);
+	});
+
+	it('recomputes the highlight for a newly selected person and clears the previous one (AC6)', async () => {
+		const { rerender } = render(StammbaumTree, {
+			nodes: NODES,
+			edges: EDGES,
+			selectedId: 'vater',
+			panZoom: { x: 0, y: 0, z: 1 },
+			showGutter: false,
+			onSelect: () => {}
+		});
+		expect(nodeOpacity('Tante')).toBe(DIM);
+		expect(nodeOpacity('Vater')).toBeNull();
+
+		// Select Tante: her lineage is now active (her parents stay full), while
+		// Vater's descendant branch (Kind, Mutter) drops out of the active set.
+		await rerender({
+			nodes: NODES,
+			edges: EDGES,
+			selectedId: 'tante',
+			panZoom: { x: 0, y: 0, z: 1 },
+			showGutter: false,
+			onSelect: () => {}
+		});
+		expect(nodeOpacity('Tante')).toBeNull();
+		expect(nodeOpacity('Kind')).toBe(DIM);
+		expect(nodeOpacity('Mutter')).toBe(DIM);
+	});
+
+	it('returns the whole tree to full strength when the selection is cleared (AC7)', async () => {
+		const { rerender } = render(StammbaumTree, {
+			nodes: NODES,
+			edges: EDGES,
+			selectedId: 'vater',
+			panZoom: { x: 0, y: 0, z: 1 },
+			showGutter: false,
+			onSelect: () => {}
+		});
+		expect(nodeOpacity('Tante')).toBe(DIM);
+
+		await rerender({
+			nodes: NODES,
+			edges: EDGES,
+			selectedId: null,
+			panZoom: { x: 0, y: 0, z: 1 },
+			showGutter: false,
+			onSelect: () => {}
+		});
+		for (const n of NODES) expect(nodeOpacity(n.displayName)).toBeNull();
+	});
+});