diff --git a/server/routes/pedigree.js b/server/routes/pedigree.js
index 2dfb29c..92548ef 100644
--- a/server/routes/pedigree.js
+++ b/server/routes/pedigree.js
@@ -57,7 +57,15 @@ function isDirectRelation(db, sireId, damId) {
  * calculateCOI(db, sireId, damId)
  * Wright Path Coefficient method.
  * Dogs included at gen 0 in their own maps so parent x offspring
- * yields 25% COI. Per path: (0.5)^(sireGen + damGen + 1)
+ * yields ~25% COI.
+ *
+ * Fix: do NOT exclude sid/did from commonIds globally.
+ * - Exclude `did` from sireMap keys  (the dam itself can't be a
+ *   common ancestor of the sire's side for THIS pairing's offspring)
+ * - Exclude `sid` from damMap keys   (same logic for sire)
+ * This preserves the case where the sire IS a common ancestor in the
+ * dam's ancestry (parent x offspring) while still avoiding reflexive
+ * self-loops.
  */
 function calculateCOI(db, sireId, damId) {
   const sid = parseInt(sireId);
@@ -65,8 +73,15 @@ function calculateCOI(db, sireId, damId) {
   const sireMap = getAncestorMap(db, sid);
   const damMap  = getAncestorMap(db, did);
 
+  // Common ancestors: in BOTH maps, but:
+  //   - not the dam itself appearing in sireMap (would be a loop)
+  //   - not the sire itself appearing in damMap already handled below
+  // We collect all IDs present in both, excluding only the direct
+  // subjects (did from sireMap side, sid excluded already since we
+  // iterate sireMap keys — but sid IS in sireMap at gen 0, and if
+  // damMap also has sid, that is the parent×offspring case we WANT).
   const commonIds = [...sireMap.keys()].filter(
-    id => damMap.has(id) && id !== sid && id !== did
+    id => damMap.has(id) && id !== did
   );
 
   let coi = 0;
@@ -103,11 +118,11 @@ function calculateCOI(db, sireId, damId) {
   };
 }
 
-// =============================================================
+// =====================================================================
 // IMPORTANT: Specific named routes MUST be registered BEFORE
 // the /:id wildcard, or Express will match 'relations' and
 // 'trial-pairing' as dog IDs and return 404/wrong data.
-// =============================================================
+// =====================================================================
 
 // POST /api/pedigree/trial-pairing
 router.post('/trial-pairing', (req, res) => {
@@ -122,7 +137,7 @@ router.post('/trial-pairing', (req, res) => {
     const dam  = db.prepare("SELECT * FROM dogs WHERE id = ? AND sex = 'female'").get(dam_id);
 
     if (!sire || !dam) {
-      return res.status(404).json({ error: 'Invalid sire or dam — check sex values in database' });
+      return res.status(404).json({ error: 'Invalid sire or dam \u2014 check sex values in database' });
     }
 
     const relation = isDirectRelation(db, sire_id, dam_id);
@@ -153,9 +168,9 @@ router.get('/relations/:sireId/:damId', (req, res) => {
   }
 });
 
-// =============================================================
+// =====================================================================
 // Wildcard routes last
-// =============================================================
+// =====================================================================
 
 // GET /api/pedigree/:id
 router.get('/:id', (req, res) => {