[FIX] Clarify that dataset_description.Genetics object is required fo…

…r genetics data (#1442)

* [FIX] Clarify that dataset_description.Genetics object is required for genetics data

also clarify that Genetics.Dataset is required if Genetics is supplied

* clarify: info on data is supplied, not data itself

* Update src/modality-specific-files/genetic-descriptor.md

* list required first

src/modality-specific-files/genetic-descriptor.md

@@ -19,11 +19,12 @@ and can be used for practical guidance when curating a new dataset.
 
 ## Dataset Description
 
-Genetic descriptors are encoded as an additional, OPTIONAL entry in the
+If information on associated genetic data is supplied as part of a BIDS dataset,
+these "genetic descriptors" are encoded as an additional, REQUIRED entry in the
 [`dataset_description.json`](../modality-agnostic-files.md#dataset_descriptionjson)
 file.
 
-Datasets linked to a genetic database entry include the following REQUIRED or OPTIONAL
+Datasets linked to a genetic database entry include the following REQUIRED and OPTIONAL
 keys in the `Genetics` sub-[object][] of `dataset_description.json`:
 
 <!-- This block generates a table describing subfields within a metadata field.

src/schema/objects/metadata.yaml

@@ -1144,19 +1144,20 @@ Genetics:
   description: |
     An object containing information about the genetics descriptor.
   type: object
+  required_fields: [Dataset]
   properties:
-    Database:
-      name: Database
+    Dataset:
+      name: Dataset
       description: |
         [URI](SPEC_ROOT/common-principles.md#uniform-resource-indicator)
-        of database where the dataset is hosted.
+        where data can be retrieved.
       type: string
       format: uri
-    Dataset:
-      name: Dataset
+    Database:
+      name: Database
       description: |
         [URI](SPEC_ROOT/common-principles.md#uniform-resource-indicator)
-        where data can be retrieved.
+        of database where the dataset is hosted.
       type: string
       format: uri
     Descriptors: