diff --git a/.gitignore b/.gitignore
index 1767f2597..6566ba43c 100644
--- a/.gitignore
+++ b/.gitignore
@@ -38,6 +38,11 @@ docs/build/
 docs/source/pynwb.*.rst
 
 
+# Virtual Environment
+venv/
+env/
+ENV/
+
 # setuptools
 build/
 dist/
diff --git a/docs/gallery/advanced_io/plot_editing.py b/docs/gallery/advanced_io/plot_editing.py
index b08b2adfd..4bcf8c55b 100644
--- a/docs/gallery/advanced_io/plot_editing.py
+++ b/docs/gallery/advanced_io/plot_editing.py
@@ -162,3 +162,41 @@
         "synthetic_timeseries_renamed",
         "/analysis/synthetic_timeseries_renamed",
     )
+
+##############################################
+# Adding datasets to existing groups
+# ----------------------------------
+# You can add new datasets to existing groups using PyNWB by calling ``set_modified()``.
+# Here's an example of adding a genotype to a Subject:
+
+from pynwb import NWBFile, NWBHDF5IO
+from pynwb.file import Subject
+
+# First, let's create a file with a Subject that is missing the genotype, which is optional
+nwbfile = NWBFile(
+    session_description="example file with subject",
+    identifier="EXAMPLE_ID",
+    session_start_time=datetime.now(tzlocal()),
+    session_id="LONELYMTN",
+    subject=Subject(
+        subject_id="mouse001",
+        species="Mus musculus",
+        age="P30D",
+    )
+)
+
+with NWBHDF5IO("test_edit3.nwb", "w") as io:
+    io.write(nwbfile)
+
+# Now add the genotype using PyNWB and set_modified()
+with NWBHDF5IO("test_edit3.nwb", "a") as io:
+    nwbfile = io.read()
+    nwbfile.subject.genotype = "Sst-IRES-Cre"
+    nwbfile.subject.set_modified()  # Required to mark the container as modified
+    io.write(nwbfile)
+
+# Verify the dataset was added
+with NWBHDF5IO("test_edit3.nwb", "r") as io:
+    nwbfile = io.read()
+    print(f"Subject genotype: {nwbfile.subject.genotype}")
+    # Output: Subject genotype: Sst-IRES-Cre