diff --git a/tests/test_formats.py b/tests/test_formats.py
index 270d9438..b4cdb4f8 100644
--- a/tests/test_formats.py
+++ b/tests/test_formats.py
@@ -1279,16 +1279,53 @@ def test_bad_format_version(self):
             with sd.copy() as copy:
                 copy.data.attrs[tsinfer.FORMAT_VERSION_KEY] = 100, 0
 
-    def test_ancestral_allele(self):
+    @pytest.mark.parametrize(
+        "position, genotypes, alleles, ancestral_allele, expected_ancestral_state, "
+        "expected_recode_alleles, expected_genotypes",
+        [
+            (0, [0, 0, 1, 2], ["A", "B", "C"], 2, "C", ("C", "A", "B"), [1, 1, 2, 0]),
+            (
+                0,
+                [0, 1, 2, 3],
+                ["A", "C", "G", "T"],
+                0,
+                "A",
+                ("A", "C", "G", "T"),
+                [0, 1, 2, 3],
+            ),
+            (0, [0, -1, 1, 2], ["A", "B", "C"], 0, "A", ("A", "B", "C"), [0, -1, 1, 2]),
+            (0, [0, -1, 1, 2], ["A", "B", "C"], 2, "C", ("C", "A", "B"), [1, -1, 2, 0]),
+            (0, [0, 0, 0, 0], ["A"], 0, "A", ("A",), [0, 0, 0, 0]),
+            (0, [-1, -1, -1, -1], ["A", "B"], 0, "A", ("A", "B"), [-1, -1, -1, -1]),
+        ],
+    )
+    def test_ancestral_allele(
+        self,
+        position,
+        genotypes,
+        alleles,
+        ancestral_allele,
+        expected_ancestral_state,
+        expected_recode_alleles,
+        expected_genotypes,
+    ):
         with tsinfer.SampleData() as sd:
-            sd.add_site(0, [0, 0, 1, 2], alleles=["A", "B", "C"], ancestral_allele=2)
+            sd.add_site(
+                position, genotypes, alleles=alleles, ancestral_allele=ancestral_allele
+            )
         v = next(sd.variants(recode_ancestral=True))
-        assert v.site.alleles == ("A", "B", "C")
-        assert v.site.ancestral_allele == 2
-        assert v.site.ancestral_state == "C"
-        assert v.alleles == ("C", "A", "B")
-        assert list(v.genotypes) == [1, 1, 2, 0]
-        assert [h[0] for _, h in sd.haplotypes(recode_ancestral=True)] == [1, 1, 2, 0]
+        assert v.site.alleles == tuple(alleles + [None] if -1 in genotypes else alleles)
+        assert v.site.ancestral_allele == ancestral_allele
+        assert v.site.ancestral_state == expected_ancestral_state
+        assert v.alleles == tuple(
+            list(expected_recode_alleles) + [None]
+            if -1 in genotypes
+            else expected_recode_alleles
+        )
+        assert list(v.genotypes) == expected_genotypes
+        assert [
+            h[0] for _, h in sd.haplotypes(recode_ancestral=True)
+        ] == expected_genotypes
 
     def test_missing_ancestral_allele(self):
         with tsinfer.SampleData() as sd: