diff --git a/vortex-bench/src/statpopgen/builder.rs b/vortex-bench/src/statpopgen/builder.rs
index 4e11b4e43e4..8d772d5cf9b 100644
--- a/vortex-bench/src/statpopgen/builder.rs
+++ b/vortex-bench/src/statpopgen/builder.rs
@@ -11,6 +11,7 @@ use arrow_array::builder::Float32Builder;
 use arrow_array::builder::Int32Builder;
 use arrow_array::builder::ListBuilder;
 use arrow_array::builder::StringBuilder;
+use arrow_array::builder::UInt8Builder;
 use arrow_array::builder::UInt64Builder;
 use arrow_schema::ArrowError;
 use arrow_schema::SchemaRef;
@@ -125,7 +126,7 @@ pub struct GnomADBuilder<'a> {
     /// `1|1`. We do not support these in the GT field.
     ///
     /// Every list is the same length; however, individual positions may be missing.
-    pub GT_builder: ListBuilder<UInt64Builder>,
+    pub GT_builder: ListBuilder<UInt8Builder>,
     /// The genotype quality.
     ///
     /// A small non-negative integer indicating our confidence in this genotype. It is usually the
@@ -444,3 +445,33 @@ impl<'a> GnomADBuilder<'a> {
         )
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use arrow_schema::DataType;
+    use noodles_vcf::Header;
+
+    use super::GnomADBuilder;
+
+    /// Regression test: the GnomAD schema declares `GT` as `list(u8)`, so the builder must
+    /// emit a `List<UInt8>` array. Previously the builder used `UInt64`, so `finish` failed
+    /// inside `RecordBatch::try_new` with a schema/array type mismatch.
+    #[test]
+    fn gt_builder_matches_u8_schema() -> anyhow::Result<()> {
+        let header = Header::default();
+        let schema = super::super::schema::schema_from_vcf_header(&header);
+
+        // `finish` validates every column against the schema via `RecordBatch::try_new`,
+        // so this only succeeds when the GT builder's element type matches `list(u8)`.
+        let batch = GnomADBuilder::new(&header, schema).finish()?;
+
+        let gt = batch
+            .column_by_name("GT")
+            .expect("GT column must be present");
+        let DataType::List(item) = gt.data_type() else {
+            panic!("GT must be a List, got {:?}", gt.data_type());
+        };
+        assert_eq!(item.data_type(), &DataType::UInt8, "GT items must be u8");
+        Ok(())
+    }
+}
diff --git a/vortex-bench/src/statpopgen/schema.rs b/vortex-bench/src/statpopgen/schema.rs
index e19afaa09a1..ec00e5a138c 100644
--- a/vortex-bench/src/statpopgen/schema.rs
+++ b/vortex-bench/src/statpopgen/schema.rs
@@ -34,7 +34,8 @@ pub fn schema_from_vcf_header(header: &Header) -> SchemaRef {
         .into_iter()
         .chain(info_fields)
         .chain([
-            Arc::new(Field::new("GT", list(UInt64), true)),
+            // GT is NULL, 0, 1, or 2
+            Arc::new(Field::new("GT", list(UInt8), true)),
             Arc::new(Field::new("GQ", list(Int32), true)),
             Arc::new(Field::new("DP", list(Int32), true)),
             Arc::new(Field::new("AD", list(list(Int32)), true)),
diff --git a/vortex-bench/src/statpopgen/vcf_conversion.rs b/vortex-bench/src/statpopgen/vcf_conversion.rs
index 357e81a93e3..f9a3ab1f844 100644
--- a/vortex-bench/src/statpopgen/vcf_conversion.rs
+++ b/vortex-bench/src/statpopgen/vcf_conversion.rs
@@ -128,7 +128,7 @@ pub fn value_list_string<'a>(
     })
 }
 
-pub fn parse_genotype(gt: Option<EntryValue>) -> VortexResult<Option<u64>> {
+pub fn parse_genotype(gt: Option<EntryValue>) -> VortexResult<Option<u8>> {
     let Some(gt) = gt else {
         return Ok(None);
     };
@@ -140,7 +140,11 @@ pub fn parse_genotype(gt: Option<EntryValue>) -> VortexResult<Option<u64>> {
         .process_results(|iter| iter.map(|x| x.0).collect::<Vec<_>>())?[..]
     {
         [None, None] => Ok(None),
-        [Some(l), Some(r)] => Ok(Some(l as u64 + r as u64)),
+        // GT dosage is the number of alternate alleles: 0, 1, or 2.
+        [Some(l), Some(r)] => match u8::try_from(l + r) {
+            Ok(dosage) => Ok(Some(dosage)),
+            Err(_) => vortex_bail!("genotype allele sum {} does not fit in u8", l + r),
+        },
         _ => vortex_bail!("wtf {:?}", gt),
     }
 }