diff --git a/protected/gatk-tools-protected/src/main/java/org/broadinstitute/gatk/tools/walkers/haplotypecaller/HaplotypeCaller.java b/protected/gatk-tools-protected/src/main/java/org/broadinstitute/gatk/tools/walkers/haplotypecaller/HaplotypeCaller.java
index cf34b1fb4..2ee5752f0 100644
--- a/protected/gatk-tools-protected/src/main/java/org/broadinstitute/gatk/tools/walkers/haplotypecaller/HaplotypeCaller.java
+++ b/protected/gatk-tools-protected/src/main/java/org/broadinstitute/gatk/tools/walkers/haplotypecaller/HaplotypeCaller.java
@@ -497,10 +497,11 @@ public class HaplotypeCaller extends ActiveRegionWalker<List<VariantContext>, In
     protected boolean mergeVariantsViaLD = false;
 
     @Advanced
-    @Argument(fullName="tryPhysicalPhasing", shortName="tryPhysicalPhasing", doc="If specified, we will add physical (read-based) phasing information", required = false)
-    protected boolean tryPhysicalPhasing = false;
+    @Argument(fullName="doNotRunPhysicalPhasing", shortName="doNotRunPhysicalPhasing", doc="If specified, we will not try to add physical (read-based) phasing information", required = false)
+    protected boolean doNotRunPhysicalPhasing = false;
 
-    public static final String HAPLOTYPE_CALLER_PHASING_KEY = "HCP";
+    public static final String HAPLOTYPE_CALLER_PHASING_ID_KEY = "PID";
+    public static final String HAPLOTYPE_CALLER_PHASING_GT_KEY = "PGT";
 
     // -----------------------------------------------------------------------------------------------
     // arguments for debugging / developing the haplotype caller
@@ -634,12 +635,11 @@ public class HaplotypeCaller extends ActiveRegionWalker<List<VariantContext>, In
         if ( emitReferenceConfidence() ) {
 
             if (SCAC.genotypingOutputMode == GenotypingOutputMode.GENOTYPE_GIVEN_ALLELES)
-                throw new UserException.BadArgumentValue("ERC/gt_mode","you cannot request reference confidence output and Genotyping Giving Alleles at the same time");
+                throw new UserException.BadArgumentValue("ERC/gt_mode","you cannot request reference confidence output and GENOTYPE_GIVEN_ALLELES at the same time");
 
             SCAC.genotypeArgs.STANDARD_CONFIDENCE_FOR_EMITTING = -0.0;
             SCAC.genotypeArgs.STANDARD_CONFIDENCE_FOR_CALLING = -0.0;
 
-
             // also, we don't need to output several of the annotations
             annotationsToExclude.add("ChromosomeCounts");
             annotationsToExclude.add("FisherStrand");
@@ -651,6 +651,9 @@ public class HaplotypeCaller extends ActiveRegionWalker<List<VariantContext>, In
             if (!SCAC.annotateAllSitesWithPLs)
                 logger.info("All sites annotated with PLs forced to true for reference-model confidence output");
             SCAC.annotateAllSitesWithPLs = true;
+        } else if ( ! doNotRunPhysicalPhasing ) {
+            doNotRunPhysicalPhasing = true;
+            logger.info("Disabling physical phasing, which is supported only for reference-model confidence output");
         }
 
         if ( SCAC.AFmodel == AFCalcFactory.Calculation.EXACT_GENERAL_PLOIDY )
@@ -678,7 +681,7 @@ public class HaplotypeCaller extends ActiveRegionWalker<List<VariantContext>, In
         if( SCAC.genotypingOutputMode == GenotypingOutputMode.GENOTYPE_GIVEN_ALLELES && consensusMode )
             throw new UserException("HaplotypeCaller cannot be run in both GENOTYPE_GIVEN_ALLELES mode and in consensus mode. Please choose one or the other.");
 
-        genotypingEngine = new HaplotypeCallerGenotypingEngine( getToolkit(), SCAC, tryPhysicalPhasing);
+        genotypingEngine = new HaplotypeCallerGenotypingEngine( getToolkit(), SCAC, !doNotRunPhysicalPhasing);
         // initialize the output VCF header
         final VariantAnnotatorEngine annotationEngine = new VariantAnnotatorEngine(Arrays.asList(annotationClassesToUse), annotationsToUse, annotationsToExclude, this, getToolkit());
 
@@ -699,8 +702,10 @@ public class HaplotypeCaller extends ActiveRegionWalker<List<VariantContext>, In
                 VCFConstants.DEPTH_KEY,
                 VCFConstants.GENOTYPE_PL_KEY);
 
-        if ( tryPhysicalPhasing )
-            headerInfo.add(new VCFFormatHeaderLine(HAPLOTYPE_CALLER_PHASING_KEY, VCFHeaderLineCount.UNBOUNDED, VCFHeaderLineType.String, "Physical phasing information, each unique ID within a given sample (but not across samples) connects alternate alleles as occurring on the same haplotype"));
+        if ( ! doNotRunPhysicalPhasing ) {
+            headerInfo.add(new VCFFormatHeaderLine(HAPLOTYPE_CALLER_PHASING_ID_KEY, 1, VCFHeaderLineType.String, "Physical phasing ID information, where each unique ID within a given sample (but not across samples) connects records within a phasing group"));
+            headerInfo.add(new VCFFormatHeaderLine(HAPLOTYPE_CALLER_PHASING_GT_KEY, 1, VCFHeaderLineType.String, "Physical phasing haplotype information, describing how the alternate alleles are phased in relation to one another"));
+        }
 
         // FILTER fields are added unconditionally as it's not always 100% certain the circumstances
         // where the filters are used.  For example, in emitting all sites the lowQual field is used
diff --git a/protected/gatk-tools-protected/src/main/java/org/broadinstitute/gatk/tools/walkers/haplotypecaller/HaplotypeCallerGenotypingEngine.java b/protected/gatk-tools-protected/src/main/java/org/broadinstitute/gatk/tools/walkers/haplotypecaller/HaplotypeCallerGenotypingEngine.java
index 0a78367d3..77c308cfc 100644
--- a/protected/gatk-tools-protected/src/main/java/org/broadinstitute/gatk/tools/walkers/haplotypecaller/HaplotypeCallerGenotypingEngine.java
+++ b/protected/gatk-tools-protected/src/main/java/org/broadinstitute/gatk/tools/walkers/haplotypecaller/HaplotypeCallerGenotypingEngine.java
@@ -57,6 +57,7 @@ import org.broadinstitute.gatk.tools.walkers.genotyper.OutputMode;
 import org.broadinstitute.gatk.utils.GenomeLoc;
 import org.broadinstitute.gatk.utils.GenomeLocParser;
 import org.broadinstitute.gatk.utils.Utils;
+import org.broadinstitute.gatk.utils.collections.Pair;
 import org.broadinstitute.gatk.utils.genotyper.ReadLikelihoods;
 import org.broadinstitute.gatk.utils.haplotype.EventMap;
 import org.broadinstitute.gatk.utils.haplotype.Haplotype;
@@ -77,16 +78,16 @@ public class HaplotypeCallerGenotypingEngine extends GenotypingEngine<HaplotypeC
 
     private MergeVariantsAcrossHaplotypes crossHaplotypeEventMerger;
 
-    private final boolean tryPhysicalPhasing;
+    private final boolean doPhysicalPhasing;
 
     /**
      * {@inheritDoc}
      * @param toolkit {@inheritDoc}
      * @param configuration {@inheritDoc}
      */
-    public HaplotypeCallerGenotypingEngine(final GenomeAnalysisEngine toolkit, final HaplotypeCallerArgumentCollection configuration, final boolean tryPhysicalPhasing) {
+    public HaplotypeCallerGenotypingEngine(final GenomeAnalysisEngine toolkit, final HaplotypeCallerArgumentCollection configuration, final boolean doPhysicalPhasing) {
         super(toolkit,configuration);
-        this.tryPhysicalPhasing = tryPhysicalPhasing;
+        this.doPhysicalPhasing = doPhysicalPhasing;
     }
 
     /**
@@ -97,7 +98,7 @@ public class HaplotypeCallerGenotypingEngine extends GenotypingEngine<HaplotypeC
      */
     public HaplotypeCallerGenotypingEngine(final GenomeAnalysisEngine toolkit, final HaplotypeCallerArgumentCollection configuration, final Set<String> sampleNames) {
         super(toolkit,configuration,sampleNames);
-        tryPhysicalPhasing = false;
+        doPhysicalPhasing = true;
     }
 
 
@@ -254,7 +255,7 @@ public class HaplotypeCallerGenotypingEngine extends GenotypingEngine<HaplotypeC
                     if (logger != null) logger.info("Genotyping event at " + loc + " with alleles = " + mergedVC.getAlleles());
                 }
 
-                ReadLikelihoods<Allele> readAlleleLikelihoods = readLikelihoods.marginalize(alleleMapper,genomeLocParser.createPaddedGenomeLoc(genomeLocParser.createGenomeLoc(mergedVC),ALLELE_EXTENSION));
+                ReadLikelihoods<Allele> readAlleleLikelihoods = readLikelihoods.marginalize(alleleMapper, genomeLocParser.createPaddedGenomeLoc(genomeLocParser.createGenomeLoc(mergedVC), ALLELE_EXTENSION));
                 if (configuration.isSampleContaminationPresent())
                     readAlleleLikelihoods.contaminationDownsampling(configuration.getSampleContamination());
 
@@ -286,7 +287,7 @@ public class HaplotypeCallerGenotypingEngine extends GenotypingEngine<HaplotypeC
             }
         }
 
-        final List<VariantContext> phasedCalls = tryPhysicalPhasing ? phaseCalls(returnCalls, calledHaplotypes) : returnCalls;
+        final List<VariantContext> phasedCalls = doPhysicalPhasing ? phaseCalls(returnCalls, calledHaplotypes) : returnCalls;
         return new CalledHaplotypes(phasedCalls, calledHaplotypes);
     }
 
@@ -303,8 +304,8 @@ public class HaplotypeCallerGenotypingEngine extends GenotypingEngine<HaplotypeC
         final Map<VariantContext, Set<Haplotype>> haplotypeMap = constructHaplotypeMapping(calls, calledHaplotypes);
 
         // construct a mapping from call to phase set ID
-        final Map<VariantContext, Integer> phaseSetMapping = new HashMap<>();
-        final int uniqueCounterEndValue = constructPhaseSetMapping(calls, haplotypeMap, phaseSetMapping);
+        final Map<VariantContext, Pair<Integer, String>> phaseSetMapping = new HashMap<>();
+        final int uniqueCounterEndValue = constructPhaseSetMapping(calls, haplotypeMap, calledHaplotypes.size() - 1, phaseSetMapping);
 
         // we want to establish (potential) *groups* of phased variants, so we need to be smart when looking at pairwise phasing partners
         return constructPhaseGroups(calls, phaseSetMapping, uniqueCounterEndValue);
@@ -349,17 +350,19 @@ public class HaplotypeCallerGenotypingEngine extends GenotypingEngine<HaplotypeC
      *
      * @param originalCalls    the original unphased calls
      * @param haplotypeMap     mapping from alternate allele to the set of haplotypes that contain that allele
+     * @param totalAvailableHaplotypes the total number of possible haplotypes used in calling
      * @param phaseSetMapping  the map to populate in this method
      * @return the next incremental unique index
      */
     protected static int constructPhaseSetMapping(final List<VariantContext> originalCalls,
                                                   final Map<VariantContext, Set<Haplotype>> haplotypeMap,
-                                                  final Map<VariantContext, Integer> phaseSetMapping) {
+                                                  final int totalAvailableHaplotypes,
+                                                  final Map<VariantContext, Pair<Integer, String>> phaseSetMapping) {
 
         final int numCalls = originalCalls.size();
         int uniqueCounter = 0;
 
-        // use the haplotype mapping to connect variants that are always present on the same haplotypes
+        // use the haplotype mapping to connect variants that are always/never present on the same haplotypes
         for ( int i = 0; i < numCalls - 1; i++ ) {
             final VariantContext call = originalCalls.get(i);
             final Set<Haplotype> haplotypesWithCall = haplotypeMap.get(call);
@@ -369,18 +372,46 @@ public class HaplotypeCallerGenotypingEngine extends GenotypingEngine<HaplotypeC
             for ( int j = i+1; j < numCalls; j++ ) {
                 final VariantContext comp = originalCalls.get(j);
                 final Set<Haplotype> haplotypesWithComp = haplotypeMap.get(comp);
+                if ( haplotypesWithComp.isEmpty() )
+                    continue;
 
-                // if the variants are in phase, record that fact
-                if ( haplotypesWithCall.size() == haplotypesWithComp.size() && haplotypesWithCall.containsAll(haplotypesWithComp) ) {
-                    // if it's part of an existing group, use that group's unique ID
-                    if ( phaseSetMapping.containsKey(call) ) {
-                        phaseSetMapping.put(comp, phaseSetMapping.get(call));
-                    // otherwise, create a new group
-                    } else {
-                        phaseSetMapping.put(call, uniqueCounter);
-                        phaseSetMapping.put(comp, uniqueCounter);
+                // if the variants are together on all haplotypes, record that fact.
+                // another possibility is that one of the variants is on all possible haplotypes (i.e. it is homozygous).
+                final boolean callIsOnAllHaps = haplotypesWithCall.size() == totalAvailableHaplotypes;
+                final boolean compIsOnAllHaps = haplotypesWithComp.size() == totalAvailableHaplotypes;
+                if ( (haplotypesWithCall.size() == haplotypesWithComp.size() && haplotypesWithCall.containsAll(haplotypesWithComp)) || callIsOnAllHaps || compIsOnAllHaps ) {
+
+                    // create a new group if these are the first entries
+                    if ( ! phaseSetMapping.containsKey(call) ) {
+                        phaseSetMapping.put(call, new Pair<>(uniqueCounter, callIsOnAllHaps ? "1|1" : "0|1"));
+                        phaseSetMapping.put(comp, new Pair<>(uniqueCounter, compIsOnAllHaps ? "1|1" : "0|1"));
                         uniqueCounter++;
                     }
+                    // otherwise it's part of an existing group so use that group's unique ID
+                    else if ( ! phaseSetMapping.containsKey(comp) ) {
+                        final Pair<Integer, String> callPhase = phaseSetMapping.get(call);
+                        phaseSetMapping.put(comp, new Pair<>(callPhase.first, compIsOnAllHaps ? "1|1" : callPhase.second));
+                    }
+                }
+                // if the variants are apart on *all* haplotypes, record that fact
+                else if ( haplotypesWithCall.size() + haplotypesWithComp.size() == totalAvailableHaplotypes ) {
+
+                    final Set<Haplotype> intersection = new HashSet<>();
+                    intersection.addAll(haplotypesWithCall);
+                    intersection.retainAll(haplotypesWithComp);
+                    if ( intersection.isEmpty() ) {
+                        // create a new group if these are the first entries
+                        if ( ! phaseSetMapping.containsKey(call) ) {
+                            phaseSetMapping.put(call, new Pair<>(uniqueCounter, "0|1"));
+                            phaseSetMapping.put(comp, new Pair<>(uniqueCounter, "1|0"));
+                            uniqueCounter++;
+                        }
+                        // otherwise it's part of an existing group so use that group's unique ID
+                        else if ( ! phaseSetMapping.containsKey(comp) ){
+                            final Pair<Integer, String> callPhase = phaseSetMapping.get(call);
+                            phaseSetMapping.put(comp, new Pair<>(callPhase.first, callPhase.second.equals("0|1") ? "1|0" : "0|1"));
+                        }
+                    }
                 }
             }
         }
@@ -397,7 +428,7 @@ public class HaplotypeCallerGenotypingEngine extends GenotypingEngine<HaplotypeC
      * @return a non-null list which represents the possibly phased version of the calls
      */
     protected static List<VariantContext> constructPhaseGroups(final List<VariantContext> originalCalls,
-                                                               final Map<VariantContext, Integer> phaseSetMapping,
+                                                               final Map<VariantContext, Pair<Integer, String>> phaseSetMapping,
                                                                final int indexTo) {
         final List<VariantContext> phasedCalls = new ArrayList<>(originalCalls);
 
@@ -407,7 +438,7 @@ public class HaplotypeCallerGenotypingEngine extends GenotypingEngine<HaplotypeC
             final List<Integer> indexes = new ArrayList<>();
             for ( int index = 0; index < originalCalls.size(); index++ ) {
                 final VariantContext call = originalCalls.get(index);
-                if ( phaseSetMapping.containsKey(call) && phaseSetMapping.get(call) == count )
+                if ( phaseSetMapping.containsKey(call) && phaseSetMapping.get(call).first == count )
                     indexes.add(index);
             }
             if ( indexes.size() < 2 )
@@ -418,7 +449,8 @@ public class HaplotypeCallerGenotypingEngine extends GenotypingEngine<HaplotypeC
 
             // update the VCs
             for ( final int index : indexes ) {
-                final VariantContext phasedCall = phaseVC(originalCalls.get(index), uniqueID);
+                final VariantContext originalCall = originalCalls.get(index);
+                final VariantContext phasedCall = phaseVC(originalCall, uniqueID, phaseSetMapping.get(originalCall).second);
                 phasedCalls.set(index, phasedCall);
             }
         }
@@ -452,12 +484,13 @@ public class HaplotypeCallerGenotypingEngine extends GenotypingEngine<HaplotypeC
      *
      * @param vc   the variant context
      * @param ID   the ID to use
+     * @param phaseGT the phase GT string to use
      * @return phased non-null variant context
      */
-    private static VariantContext phaseVC(final VariantContext vc, final String ID) {
+    private static VariantContext phaseVC(final VariantContext vc, final String ID, final String phaseGT) {
         final List<Genotype> phasedGenotypes = new ArrayList<>();
         for ( final Genotype g : vc.getGenotypes() )
-            phasedGenotypes.add(new GenotypeBuilder(g).attribute(HaplotypeCaller.HAPLOTYPE_CALLER_PHASING_KEY, ID).make());
+            phasedGenotypes.add(new GenotypeBuilder(g).attribute(HaplotypeCaller.HAPLOTYPE_CALLER_PHASING_ID_KEY, ID).attribute(HaplotypeCaller.HAPLOTYPE_CALLER_PHASING_GT_KEY, phaseGT).make());
         return new VariantContextBuilder(vc).genotypes(phasedGenotypes).make();
     }
 
diff --git a/protected/gatk-tools-protected/src/test/java/org/broadinstitute/gatk/tools/walkers/haplotypecaller/HaplotypeCallerGVCFIntegrationTest.java b/protected/gatk-tools-protected/src/test/java/org/broadinstitute/gatk/tools/walkers/haplotypecaller/HaplotypeCallerGVCFIntegrationTest.java
index fca9b91fb..14924bdc3 100644
--- a/protected/gatk-tools-protected/src/test/java/org/broadinstitute/gatk/tools/walkers/haplotypecaller/HaplotypeCallerGVCFIntegrationTest.java
+++ b/protected/gatk-tools-protected/src/test/java/org/broadinstitute/gatk/tools/walkers/haplotypecaller/HaplotypeCallerGVCFIntegrationTest.java
@@ -68,11 +68,11 @@ public class HaplotypeCallerGVCFIntegrationTest extends WalkerTest {
 
         // this functionality can be adapted to provide input data for whatever you might want in your data
         tests.add(new Object[]{NA12878_PCRFREE, ReferenceConfidenceMode.NONE, PCRFreeIntervals, "9db87ae56df22456f3c08024384f3e5e"});
-        tests.add(new Object[]{NA12878_PCRFREE, ReferenceConfidenceMode.BP_RESOLUTION, PCRFreeIntervals, "4a006f9e39cacc3ae84384ad56ca22a2"});
-        tests.add(new Object[]{NA12878_PCRFREE, ReferenceConfidenceMode.GVCF, PCRFreeIntervals, "fe014592c9fa8d442508a96eb09e0c93"});
+        tests.add(new Object[]{NA12878_PCRFREE, ReferenceConfidenceMode.BP_RESOLUTION, PCRFreeIntervals, "90dc8950e28e042097817d785022482e"});
+        tests.add(new Object[]{NA12878_PCRFREE, ReferenceConfidenceMode.GVCF, PCRFreeIntervals, "a64e2d8e60a1be9cb0b4306c85e96393"});
         tests.add(new Object[]{NA12878_WEx, ReferenceConfidenceMode.NONE, WExIntervals, "d5c07fa3edca496a84fd17cecad06230"});
-        tests.add(new Object[]{NA12878_WEx, ReferenceConfidenceMode.BP_RESOLUTION, WExIntervals, "330685c734e277d70a44637de85ad54d"});
-        tests.add(new Object[]{NA12878_WEx, ReferenceConfidenceMode.GVCF, WExIntervals, "b01a2c222a3f00a675f5534c3b449919"});
+        tests.add(new Object[]{NA12878_WEx, ReferenceConfidenceMode.BP_RESOLUTION, WExIntervals, "66019a0914f905522da6bd3b557a57d1"});
+        tests.add(new Object[]{NA12878_WEx, ReferenceConfidenceMode.GVCF, WExIntervals, "de763f6c9a5aec4586a2671941e4c96d"});
 
         return tests.toArray(new Object[][]{});
     }
@@ -137,7 +137,7 @@ public class HaplotypeCallerGVCFIntegrationTest extends WalkerTest {
     public void testWrongGVCFNonVariantRecordOrderBugFix() {
         final String commandLine = String.format("-T HaplotypeCaller --pcr_indel_model NONE -R %s -I %s -L %s -ERC GVCF --no_cmdline_in_header -variant_index_type %s -variant_index_parameter %d",
                 b37KGReference, WRONG_GVCF_RECORD_ORDER_BUGFIX_BAM, WRONG_GVCF_RECORD_ORDER_BUGFIX_INTERVALS, HaplotypeCaller.OPTIMAL_GVCF_INDEX_TYPE, HaplotypeCaller.OPTIMAL_GVCF_INDEX_PARAMETER);
-        final WalkerTestSpec spec = new WalkerTestSpec(commandLine + " -o %s", Arrays.asList("d595dae0e3f993047c22c6e520e190aa"));
+        final WalkerTestSpec spec = new WalkerTestSpec(commandLine + " -o %s", Arrays.asList("e6a4e571abb59b925d59a38d244f0abe"));
         spec.disableShadowBCF();
         executeTest("testMissingGVCFIndexingStrategyException", spec);
     }
@@ -149,7 +149,7 @@ public class HaplotypeCallerGVCFIntegrationTest extends WalkerTest {
     public void testNoCallGVCFMissingPLsBugFix() {
         final String commandLine = String.format("-T HaplotypeCaller --pcr_indel_model NONE -R %s -I %s -L %s -ERC GVCF --no_cmdline_in_header -variant_index_type %s -variant_index_parameter %d",
                 b37KGReference, NOCALL_GVCF_BUGFIX_BAM, NOCALL_GVCF_BUGFIX_INTERVALS, HaplotypeCaller.OPTIMAL_GVCF_INDEX_TYPE, HaplotypeCaller.OPTIMAL_GVCF_INDEX_PARAMETER);
-        final WalkerTestSpec spec = new WalkerTestSpec(commandLine + " -o %s", Arrays.asList("2a91768430a84dbae8d40242dee8c9b8"));
+        final WalkerTestSpec spec = new WalkerTestSpec(commandLine + " -o %s", Arrays.asList("187e43b1034478edced1eb5c664bac34"));
         spec.disableShadowBCF();
         executeTest("testNoCallGVCFMissingPLsBugFix", spec);
     }
diff --git a/protected/gatk-tools-protected/src/test/java/org/broadinstitute/gatk/tools/walkers/haplotypecaller/HaplotypeCallerGenotypingEngineUnitTest.java b/protected/gatk-tools-protected/src/test/java/org/broadinstitute/gatk/tools/walkers/haplotypecaller/HaplotypeCallerGenotypingEngineUnitTest.java
index 19f07eccd..635a5997d 100644
--- a/protected/gatk-tools-protected/src/test/java/org/broadinstitute/gatk/tools/walkers/haplotypecaller/HaplotypeCallerGenotypingEngineUnitTest.java
+++ b/protected/gatk-tools-protected/src/test/java/org/broadinstitute/gatk/tools/walkers/haplotypecaller/HaplotypeCallerGenotypingEngineUnitTest.java
@@ -56,6 +56,7 @@ import htsjdk.samtools.reference.ReferenceSequenceFile;
 import htsjdk.variant.variantcontext.*;
 import org.broadinstitute.gatk.utils.BaseTest;
 import org.broadinstitute.gatk.utils.*;
+import org.broadinstitute.gatk.utils.collections.Pair;
 import org.broadinstitute.gatk.utils.fasta.CachingIndexedFastaSequenceFile;
 import org.broadinstitute.gatk.utils.haplotype.EventMap;
 import org.broadinstitute.gatk.utils.haplotype.Haplotype;
@@ -416,11 +417,15 @@ public class HaplotypeCallerGenotypingEngineUnitTest extends BaseTest {
         final Allele altC = Allele.create("C", false);
         final Allele altT = Allele.create("T", false);
 
+        final VariantContext vc1 = new VariantContextBuilder().chr("20").start(1).stop(1).alleles(Arrays.asList(ref, altC)).make();
         final VariantContext vc2 = new VariantContextBuilder().chr("20").start(2).stop(2).alleles(Arrays.asList(ref, altC)).make();
         final VariantContext vc3 = new VariantContextBuilder().chr("20").start(3).stop(3).alleles(Arrays.asList(ref, altT)).make();
         final VariantContext vc4 = new VariantContextBuilder().chr("20").start(4).stop(4).alleles(Arrays.asList(ref, altC)).make();
         final List<VariantContext> calls = Arrays.asList(vc2, vc3, vc4);
 
+        final Haplotype pos1 = new Haplotype("CAAAA".getBytes());
+        pos1.setEventMap(new EventMap(Arrays.asList(vc1)));
+        pos1.getEventMap().put(1, vc1);
         final Haplotype pos2 = new Haplotype("ACAAA".getBytes());
         pos2.setEventMap(new EventMap(Arrays.asList(vc2)));
         pos2.getEventMap().put(2, vc2);
@@ -440,44 +445,69 @@ public class HaplotypeCallerGenotypingEngineUnitTest extends BaseTest {
         pos234.getEventMap().put(3, vc3);
         pos234.getEventMap().put(4, vc4);
 
-        // test no phased variants
-        final Set<Haplotype> haplotypes2NoPhase = new HashSet<>();
-        haplotypes2NoPhase.add(pos2);
-        haplotypes2NoPhase.add(pos24);
-        final Set<Haplotype> haplotypes3NoPhase = new HashSet<>();
-        haplotypes3NoPhase.add(pos3);
-        final Set<Haplotype> haplotypes4NoPhase = new HashSet<>();
-        haplotypes4NoPhase.add(pos4);
-        haplotypes4NoPhase.add(pos24);
-        final Map<VariantContext, Set<Haplotype>> haplotypeMapNoPhase = new HashMap<>();
-        haplotypeMapNoPhase.put(vc2, haplotypes2NoPhase);
-        haplotypeMapNoPhase.put(vc3, haplotypes3NoPhase);
-        haplotypeMapNoPhase.put(vc4, haplotypes4NoPhase);
-        tests.add(new Object[]{calls, haplotypeMapNoPhase, 0, 0});
+        final Map<VariantContext, Set<Haplotype>> haplotypeMap = new HashMap<>();
 
-        // test 2 phased variants
-        final Set<Haplotype> haplotypes2SomePhase = new HashSet<>();
-        haplotypes2SomePhase.add(pos24);
-        final Set<Haplotype> haplotypes4SomePhase = new HashSet<>();
-        haplotypes4SomePhase.add(pos24);
-        final Map<VariantContext, Set<Haplotype>> haplotypeMapSomePhase = new HashMap<>();
-        haplotypeMapSomePhase.put(vc2, haplotypes2SomePhase);
-        haplotypeMapSomePhase.put(vc3, haplotypes3NoPhase);
-        haplotypeMapSomePhase.put(vc4, haplotypes4SomePhase);
-        tests.add(new Object[]{calls, haplotypeMapSomePhase, 2, 1});
+        // test no phased variants #1
+        final Set<Haplotype> haplotypes2 = new HashSet<>();
+        haplotypes2.add(pos2);
+        haplotypeMap.put(vc2, haplotypes2);
+        tests.add(new Object[]{Arrays.asList(vc2), new HashMap<>(haplotypeMap), 2, 0, 0, 0, 0});
 
-        // test all phased variants
-        final Set<Haplotype> haplotypes2AllPhase = new HashSet<>();
-        haplotypes2AllPhase.add(pos234);
-        final Set<Haplotype> haplotypes3AllPhase = new HashSet<>();
-        haplotypes3AllPhase.add(pos234);
-        final Set<Haplotype> haplotypes4AllPhase = new HashSet<>();
-        haplotypes4AllPhase.add(pos234);
-        final Map<VariantContext, Set<Haplotype>> haplotypeMapAllPhase = new HashMap<>();
-        haplotypeMapAllPhase.put(vc2, haplotypes2AllPhase);
-        haplotypeMapAllPhase.put(vc3, haplotypes3AllPhase);
-        haplotypeMapAllPhase.put(vc4, haplotypes4AllPhase);
-        tests.add(new Object[]{calls, haplotypeMapAllPhase, 3, 1});
+        // test no phased variants #2
+        final Set<Haplotype> haplotypes3 = new HashSet<>();
+        haplotypes3.add(pos3);
+        haplotypeMap.put(vc3, haplotypes3);
+        tests.add(new Object[]{Arrays.asList(vc2, vc3), new HashMap<>(haplotypeMap), 3, 0, 0, 0, 0});
+
+        // test opposite phase
+        tests.add(new Object[]{Arrays.asList(vc2, vc3), new HashMap<>(haplotypeMap), 2, 2, 1, 1, 1});
+
+        // test no phased variants #3
+        final Set<Haplotype> haplotypes4 = new HashSet<>();
+        haplotypes4.add(pos4);
+        haplotypeMap.put(vc4, haplotypes4);
+        tests.add(new Object[]{calls, new HashMap<>(haplotypeMap), 3, 0, 0, 0, 0});
+
+        // test mixture
+        final Set<Haplotype> haplotypes24 = new HashSet<>();
+        haplotypes24.add(pos24);
+        haplotypeMap.put(vc2, haplotypes24);
+        haplotypeMap.put(vc4, haplotypes24);
+        tests.add(new Object[]{calls, new HashMap<>(haplotypeMap), 2, 3, 1, 2, 1});
+
+        // test 2 with opposite phase
+        final Set<Haplotype> haplotypes1 = new HashSet<>();
+        haplotypes1.add(pos1);
+        haplotypeMap.put(vc1, haplotypes1);
+        haplotypeMap.remove(vc3);
+        tests.add(new Object[]{Arrays.asList(vc1, vc2, vc4), new HashMap<>(haplotypeMap), 2, 3, 1, 1, 2});
+
+        // test homs around a het
+        final Set<Haplotype> haplotypes2hom = new HashSet<>();
+        haplotypes2hom.add(pos24);
+        haplotypes2hom.add(pos234);
+        final Set<Haplotype> haplotypes4hom = new HashSet<>();
+        haplotypes4hom.add(pos24);
+        haplotypes4hom.add(pos234);
+        final Set<Haplotype> haplotypes3het = new HashSet<>();
+        haplotypes3het.add(pos234);
+        haplotypeMap.put(vc2, haplotypes2hom);
+        haplotypeMap.put(vc3, haplotypes3het);
+        haplotypeMap.put(vc4, haplotypes4hom);
+        tests.add(new Object[]{calls, new HashMap<>(haplotypeMap), 2, 3, 1, 1, 0});
+
+        // test hets around a hom
+        final Set<Haplotype> haplotypes2het = new HashSet<>();
+        haplotypes2het.add(pos234);
+        final Set<Haplotype> haplotypes4het = new HashSet<>();
+        haplotypes4het.add(pos234);
+        final Set<Haplotype> haplotypes3hom = new HashSet<>();
+        haplotypes3hom.add(pos3);
+        haplotypes3hom.add(pos234);
+        haplotypeMap.put(vc2, haplotypes2het);
+        haplotypeMap.put(vc3, haplotypes3hom);
+        haplotypeMap.put(vc4, haplotypes4het);
+        tests.add(new Object[]{calls, new HashMap<>(haplotypeMap), 2, 3, 1, 2, 0});
 
         return tests.toArray(new Object[][]{});
     }
@@ -485,12 +515,25 @@ public class HaplotypeCallerGenotypingEngineUnitTest extends BaseTest {
     @Test(dataProvider="ConstructPhaseSetMappingProvider")
     public void testConstructPhaseSetMapping(final List<VariantContext> calls,
                                          final Map<VariantContext, Set<Haplotype>> haplotypeMap,
+                                         final int totalHaplotypes,
                                          final int expectedMapSize,
-                                         final int expectedNumGroups) {
-        final Map<VariantContext, Integer> actualPhaseSetMapping = new HashMap<>();
-        final int actualNumGroups = HaplotypeCallerGenotypingEngine.constructPhaseSetMapping(calls, haplotypeMap, actualPhaseSetMapping);
+                                         final int expectedNumGroups,
+                                         final int expectedNum01,
+                                         final int expectedNum10) {
+        final Map<VariantContext, Pair<Integer, String>> actualPhaseSetMapping = new HashMap<>();
+        final int actualNumGroups = HaplotypeCallerGenotypingEngine.constructPhaseSetMapping(calls, haplotypeMap, totalHaplotypes, actualPhaseSetMapping);
         Assert.assertEquals(actualNumGroups, expectedNumGroups);
         Assert.assertEquals(actualPhaseSetMapping.size(), expectedMapSize);
+
+        int num01 = 0, num10 = 0;
+        for ( final Pair<Integer, String> phase : actualPhaseSetMapping.values() ) {
+            if ( phase.second.equals("0|1") )
+                num01++;
+            else if ( phase.second.equals("1|0") )
+                num10++;
+        }
+        Assert.assertEquals(num01, expectedNum01);
+        Assert.assertEquals(num10, expectedNum10);
     }
 
     @DataProvider(name = "ConstructPhaseGroupsProvider")
@@ -509,27 +552,27 @@ public class HaplotypeCallerGenotypingEngineUnitTest extends BaseTest {
         final List<VariantContext> calls = Arrays.asList(vc1, vc2, vc3);
 
         // test no phased variants, empty map
-        final Map<VariantContext, Integer> nonePhased1 = new HashMap<>();
+        final Map<VariantContext, Pair<Integer, String>> nonePhased1 = new HashMap<>();
         tests.add(new Object[]{calls, nonePhased1, 0, 0, 0});
 
         // test no phased variants, full map, exception expected
-        final Map<VariantContext, Integer> nonePhased2 = new HashMap<>();
-        nonePhased2.put(vc1, 0);
-        nonePhased2.put(vc2, 1);
-        nonePhased2.put(vc3, 2);
+        final Map<VariantContext, Pair<Integer, String>> nonePhased2 = new HashMap<>();
+        nonePhased2.put(vc1, new Pair<>(0, "0/1"));
+        nonePhased2.put(vc2, new Pair<>(1, "0/1"));
+        nonePhased2.put(vc3, new Pair<>(2, "0/1"));
         tests.add(new Object[]{calls, nonePhased2, 3, -1, -1});
 
         // test 2 phased variants
-        final Map<VariantContext, Integer> twoPhased = new HashMap<>();
-        twoPhased.put(vc1, 0);
-        twoPhased.put(vc2, 0);
+        final Map<VariantContext, Pair<Integer, String>> twoPhased = new HashMap<>();
+        twoPhased.put(vc1, new Pair<>(0, "0/1"));
+        twoPhased.put(vc2, new Pair<>(0, "0/1"));
         tests.add(new Object[]{calls, twoPhased, 1, 1, 2});
 
         // test all phased variants
-        final Map<VariantContext, Integer> allPhased = new HashMap<>();
-        allPhased.put(vc1, 0);
-        allPhased.put(vc2, 0);
-        allPhased.put(vc3, 0);
+        final Map<VariantContext, Pair<Integer, String>> allPhased = new HashMap<>();
+        allPhased.put(vc1, new Pair<>(0, "0/1"));
+        allPhased.put(vc2, new Pair<>(0, "0/1"));
+        allPhased.put(vc3, new Pair<>(0, "0/1"));
         tests.add(new Object[]{calls, allPhased, 1, 1, 3});
 
         return tests.toArray(new Object[][]{});
@@ -537,7 +580,7 @@ public class HaplotypeCallerGenotypingEngineUnitTest extends BaseTest {
 
     @Test(dataProvider="ConstructPhaseGroupsProvider")
     public void testConstructPhaseGroups(final List<VariantContext> calls,
-                                         final Map<VariantContext, Integer> phaseMap,
+                                         final Map<VariantContext, Pair<Integer, String>> phaseMap,
                                          final int endIndex,
                                          final int expectedNumGroups,
                                          final int expectedGroupSize) {
@@ -553,8 +596,8 @@ public class HaplotypeCallerGenotypingEngineUnitTest extends BaseTest {
         int counter = 0;
         for ( final VariantContext call : actualPhasedCalls ) {
             for ( final Genotype g : call.getGenotypes() ) {
-                if ( g.hasExtendedAttribute(HaplotypeCaller.HAPLOTYPE_CALLER_PHASING_KEY) ) {
-                    uniqueGroups.add(g.getExtendedAttribute(HaplotypeCaller.HAPLOTYPE_CALLER_PHASING_KEY).toString());
+                if ( g.hasExtendedAttribute(HaplotypeCaller.HAPLOTYPE_CALLER_PHASING_ID_KEY) ) {
+                    uniqueGroups.add(g.getExtendedAttribute(HaplotypeCaller.HAPLOTYPE_CALLER_PHASING_ID_KEY).toString());
                     counter++;
                 }
             }