EDIT

    @Override

    public Pager<Media> getMediaInHierarchy(SpecimenOrObservationBase<?> rootOccurence, boolean collectOriginalMedia, boolean collectDerivateMedia, Integer pageSize,

            Integer pageNumber, List<String> propertyPaths) {

    	 List<Media> media = new ArrayList<>();

         //media specimens

         if(rootOccurence.isInstanceOf(MediaSpecimen.class)){

             MediaSpecimen mediaSpecimen = HibernateProxyHelper.deproxy(rootOccurence, MediaSpecimen.class);

             media.add(mediaSpecimen.getMediaSpecimen());

         }

         // pherograms & gelPhotos

         if (rootOccurence.isInstanceOf(DnaSample.class)) {

             DnaSample dnaSample = CdmBase.deproxy(rootOccurence, DnaSample.class);

             Set<Sequence> sequences = dnaSample.getSequences();

             //we do show only those gelPhotos which lead to a consensus sequence

             for (Sequence sequence : sequences) {

                 Set<Media> dnaRelatedMedia = new HashSet<>();

                 for (SingleRead singleRead : sequence.getSingleReads()){

                     AmplificationResult amplification = singleRead.getAmplificationResult();

                     dnaRelatedMedia.add(amplification.getGelPhoto());

                     dnaRelatedMedia.add(singleRead.getPherogram());

                     dnaRelatedMedia.remove(null);

                 }

                 media.addAll(dnaRelatedMedia);

             }

         }

         if(rootOccurence.isInstanceOf(DerivedUnit.class)){

             DerivedUnit derivedUnit = HibernateProxyHelper.deproxy(rootOccurence, DerivedUnit.class);

             if (collectDerivateMedia) {

	             for (DerivationEvent derivationEvent : derivedUnit.getDerivationEvents()) {

	                 for (DerivedUnit childDerivative : derivationEvent.getDerivatives()) {

	                	 //collectOriginalMedia should only called for the first derived unit

	                     media.addAll(getMediaInHierarchy(childDerivative, false, true, pageSize, pageNumber, propertyPaths).getRecords());

	                 }

	             }

             }

             if (collectOriginalMedia) {

            	 for (SpecimenOrObservationBase original : derivedUnit.getOriginals()) {            		 	

	                	 //collect media to the top of the tree 

	                     media.addAll(getMediaInHierarchy(original, true, false, pageSize, pageNumber, propertyPaths).getRecords());

	             }

	         }

         }

         return new DefaultPagerImpl<>(pageNumber, Long.valueOf(media.size()), pageSize, media);

    }

    	return getMediaInHierarchy(rootOccurence, false, true, pageSize,

                pageNumber, propertyPaths);