diff --git a/malariagen_data/anoph/base_params.py b/malariagen_data/anoph/base_params.py
index 11c3e6b74..5cefff779 100644
--- a/malariagen_data/anoph/base_params.py
+++ b/malariagen_data/anoph/base_params.py
@@ -271,19 +271,21 @@ def validate_sample_selection_params(
 ]
 
 min_minor_ac: TypeAlias = Annotated[
-    int,
+    Union[int, float],
     """
     The minimum minor allele count. SNPs with a minor allele count
-    below this value will be excluded.
+    below this value will be excluded. Can also be a float, which will
+    be interpreted as a fraction.
     """,
 ]
 
 max_missing_an: TypeAlias = Annotated[
-    int,
+    Union[int, float],
     """
     The maximum number of missing allele calls to accept. SNPs with
     more than this value will be excluded. Set to 0 to require no
-    missing calls.
+    missing calls. Can also be a float, which will be interpreted as
+    a fraction.
     """,
 ]
 
diff --git a/malariagen_data/anoph/snp_data.py b/malariagen_data/anoph/snp_data.py
index 7db7e91f3..8cb398552 100644
--- a/malariagen_data/anoph/snp_data.py
+++ b/malariagen_data/anoph/snp_data.py
@@ -1659,18 +1659,26 @@ def biallelic_snp_calls(
             # Apply conditions.
             if max_missing_an is not None or min_minor_ac is not None:
                 loc_out = np.ones(ds_out.sizes["variants"], dtype=bool)
+                an = ac_out.sum(axis=1)
 
                 # Apply missingness condition.
                 if max_missing_an is not None:
-                    an = ac_out.sum(axis=1)
                     an_missing = (ds_out.sizes["samples"] * ds_out.sizes["ploidy"]) - an
-                    loc_missing = an_missing <= max_missing_an
+                    if isinstance(max_missing_an, float):
+                        an_missing_frac = an_missing / an
+                        loc_missing = an_missing_frac <= max_missing_an
+                    else:
+                        loc_missing = an_missing <= max_missing_an
                     loc_out &= loc_missing
 
                 # Apply minor allele count condition.
                 if min_minor_ac is not None:
                     ac_minor = ac_out.min(axis=1)
-                    loc_minor = ac_minor >= min_minor_ac
+                    if isinstance(min_minor_ac, float):
+                        ac_minor_frac = ac_minor / an
+                        loc_minor = ac_minor_frac >= min_minor_ac
+                    else:
+                        loc_minor = ac_minor >= min_minor_ac
                     loc_out &= loc_minor
 
                 ds_out = ds_out.isel(variants=loc_out)
diff --git a/tests/anoph/test_snp_data.py b/tests/anoph/test_snp_data.py
index 19576a6d3..e31bf35d0 100644
--- a/tests/anoph/test_snp_data.py
+++ b/tests/anoph/test_snp_data.py
@@ -1387,3 +1387,69 @@ def test_biallelic_snp_calls_and_diplotypes_with_conditions(
             max_missing_an=max_missing_an,
             n_snps=n_snps_available + 10,
         )
+
+
+@parametrize_with_cases("fixture,api", cases=".")
+def test_biallelic_snp_calls_and_diplotypes_with_conditions_fractional(
+    fixture, api: AnophelesSnpData
+):
+    # Fixed parameters.
+    contig = random.choice(api.contigs)
+    all_sample_sets = api.sample_sets()["sample_set"].to_list()
+    sample_sets = random.choice(all_sample_sets)
+    site_mask = random.choice((None,) + api.site_mask_ids)
+
+    # Parametrise conditions.
+    min_minor_ac = random.uniform(0, 0.05)
+    max_missing_an = random.uniform(0.05, 0.2)
+
+    # Run tests.
+    ds = check_biallelic_snp_calls_and_diplotypes(
+        api=api,
+        sample_sets=sample_sets,
+        region=contig,
+        site_mask=site_mask,
+        min_minor_ac=min_minor_ac,
+        max_missing_an=max_missing_an,
+    )
+
+    # Check conditions are met.
+    ac = ds["variant_allele_count"].values
+    an = ac.sum(axis=1)
+    ac_min = ac.min(axis=1)
+    assert np.all((ac_min / an) >= min_minor_ac)
+    an_missing = (ds.sizes["samples"] * ds.sizes["ploidy"]) - an
+    assert np.all((an_missing / an) <= max_missing_an)
+    gt = ds["call_genotype"].values
+    ac_check = allel.GenotypeArray(gt).count_alleles(max_allele=1)
+    assert np.all(ac == ac_check)
+
+    # Run tests with thinning.
+    n_snps_available = ds.sizes["variants"]
+    # This should always be true, although depends on min_minor_ac and max_missing_an,
+    # so the range of values for those parameters needs to be chosen with some care.
+    assert n_snps_available > 2
+    n_snps_requested = random.randint(1, n_snps_available // 2)
+    ds_thinned = check_biallelic_snp_calls_and_diplotypes(
+        api=api,
+        sample_sets=sample_sets,
+        region=contig,
+        site_mask=site_mask,
+        min_minor_ac=min_minor_ac,
+        max_missing_an=max_missing_an,
+        n_snps=n_snps_requested,
+    )
+    n_snps_thinned = ds_thinned.sizes["variants"]
+    assert n_snps_thinned >= n_snps_requested
+    assert n_snps_thinned <= 2 * n_snps_requested
+
+    # Ask for more SNPs than available.
+    with pytest.raises(ValueError):
+        api.biallelic_snp_calls(
+            sample_sets=sample_sets,
+            region=contig,
+            site_mask=site_mask,
+            min_minor_ac=min_minor_ac,
+            max_missing_an=max_missing_an,
+            n_snps=n_snps_available + 10,
+        )