{
  "schemaVersion": "BASE-biological-assertion/1.1.0",
  "caseStudy": {
    "slug": "sickle-cell-malaria",
    "title": "Sickle Cell, Inheritance, and Malaria"
  },
  "context": {
    "scope": "Human red cells and Plasmodium falciparum; genotype- and oxygen-dependent effects",
    "organism": "Homo sapiens",
    "taxonId": "9606",
    "cellTypeId": "cell-type-erythrocyte",
    "cellStage": "Mature erythrocyte; oxygenation varies by tissue and assay",
    "conditionId": "disease-sickle-cell",
    "population": "HbAS, HbSS, and HbAA genotypes; malaria exposure differs by place and time",
    "exposureContext": "Clinical associations are population-level; polymerization and parasite-growth experiments model specified oxygen and cell conditions."
  },
  "groups": [
    {
      "id": "variant",
      "title": "From Allele to Hemoglobin",
      "description": "Connect the HBB sequence change to the altered protein complex.",
      "assertionIds": [
        "sickle.hbb-encodes-beta-globin",
        "sickle.hbs-is-gluval-substitution"
      ]
    },
    {
      "id": "red-cell",
      "title": "Oxygen, Polymerization & Genotype",
      "description": "Separate the molecular mechanism from the clinical phenotype.",
      "assertionIds": [
        "sickle.hbs-forms-deoxy-polymers",
        "sickle-genotype-shapes-clinical-category"
      ]
    },
    {
      "id": "malaria-selection",
      "title": "Malaria Evidence & Population Selection",
      "description": "Keep experiments, cohort associations, and evolutionary inference in their proper scopes.",
      "assertionIds": [
        "sickle-hbas-reduces-malaria-severity-in-cohorts",
        "sickle-hypoxia-can-restrict-parasite-growth",
        "sickle-selection-is-malaria-context-dependent"
      ]
    }
  ],
  "assertions": [
    {
      "id": "sickle.hbb-encodes-beta-globin",
      "version": 1,
      "subjectId": "gene-hbb",
      "predicate": "ENCODES",
      "objectId": "protein-beta-globin",
      "statement": "HBB encodes beta-globin, one of the globin subunits in adult hemoglobin.",
      "qualifier": "The same gene has multiple alleles; a gene-to-protein link alone does not specify hemoglobin composition or clinical outcome.",
      "evidenceKind": "database_annotation",
      "context": {
        "scope": "Human red cells and Plasmodium falciparum; genotype- and oxygen-dependent effects",
        "organism": "Homo sapiens",
        "taxonId": "9606",
        "cellTypeId": "cell-type-erythrocyte",
        "cellStage": "Mature erythrocyte; oxygenation varies by tissue and assay",
        "conditionId": "disease-sickle-cell",
        "population": "HbAS, HbSS, and HbAA genotypes; malaria exposure differs by place and time",
        "exposureContext": "Clinical associations are population-level; polymerization and parasite-growth experiments model specified oxygen and cell conditions."
      },
      "references": [
        {
          "referenceId": "ncbi-hbb",
          "relation": "supports",
          "sourceRecordId": "NCBI Gene:3043",
          "sourceVersion": "Record accessed 2026-09-27"
        },
        {
          "referenceId": "uniprot-beta-globin",
          "relation": "supports",
          "sourceRecordId": "UniProtKB:P68871",
          "sourceVersion": "Reviewed entry; release not recorded in this snapshot"
        }
      ]
    },
    {
      "id": "sickle.hbs-is-gluval-substitution",
      "version": 1,
      "subjectId": "variant-hbb-hbs",
      "predicate": "CHANGES_SEQUENCE_OF",
      "objectId": "protein-beta-globin",
      "statement": "The canonical HbS allele changes glutamate to valine at beta-globin residue 6 in mature-chain numbering (often written p.Glu7Val with precursor numbering).",
      "qualifier": "The residue-number difference reflects mature-chain versus precursor-protein conventions; it is one specific variant, not a complete description of HBB variation.",
      "evidenceKind": "database_annotation",
      "context": {
        "scope": "Human red cells and Plasmodium falciparum; genotype- and oxygen-dependent effects",
        "organism": "Homo sapiens",
        "taxonId": "9606",
        "cellTypeId": "cell-type-erythrocyte",
        "cellStage": "Mature erythrocyte; oxygenation varies by tissue and assay",
        "conditionId": "disease-sickle-cell",
        "population": "HbAS, HbSS, and HbAA genotypes; malaria exposure differs by place and time",
        "exposureContext": "Clinical associations are population-level; polymerization and parasite-growth experiments model specified oxygen and cell conditions."
      },
      "references": [
        {
          "referenceId": "ncbi-hbb",
          "relation": "supports",
          "sourceRecordId": "NCBI Gene:3043",
          "sourceVersion": "Record accessed 2026-09-27"
        },
        {
          "referenceId": "uniprot-beta-globin",
          "relation": "supports",
          "sourceRecordId": "UniProtKB:P68871; hemoglobin S substitution",
          "sourceVersion": "Reviewed entry; release not recorded in this snapshot"
        }
      ]
    },
    {
      "id": "sickle.hbs-forms-deoxy-polymers",
      "version": 1,
      "subjectId": "molecular-complex-hemoglobin-s",
      "predicate": "CAN_FORM",
      "objectValue": "Intracellular HbS polymers when sufficiently deoxygenated",
      "statement": "Deoxygenated hemoglobin S can assemble into intracellular polymers; the amount of polymer depends strongly on oxygen saturation and hemoglobin concentration.",
      "qualifier": "Polymer formation is not identical to visible sickling. HbF and other hemoglobin species, pH, cell hydration, and the duration of deoxygenation also modify the process.",
      "evidenceKind": "primary_study",
      "context": {
        "scope": "Human red cells and Plasmodium falciparum; genotype- and oxygen-dependent effects",
        "organism": "Homo sapiens",
        "taxonId": "9606",
        "cellTypeId": "cell-type-erythrocyte",
        "cellStage": "Mature erythrocyte; oxygenation varies by tissue and assay",
        "conditionId": "disease-sickle-cell",
        "population": "HbAS, HbSS, and HbAA genotypes; malaria exposure differs by place and time",
        "exposureContext": "Clinical associations are population-level; polymerization and parasite-growth experiments model specified oxygen and cell conditions."
      },
      "references": [
        {
          "referenceId": "sickle-hb-polymerization",
          "relation": "supports",
          "sourceRecordId": "PMID:6933568",
          "sourceVersion": "Published 1981"
        },
        {
          "referenceId": "medlineplus-scd",
          "relation": "context",
          "sourceRecordId": "MedlinePlus Genetics: sickle cell disease",
          "sourceVersion": "Page version not recorded in this snapshot"
        }
      ]
    },
    {
      "id": "sickle-genotype-shapes-clinical-category",
      "version": 1,
      "subjectId": "gene-hbb",
      "predicate": "PHENOTYPE_DEPENDS_ON_GENOTYPE",
      "objectValue": "HbSS can cause sickle cell disease; HbAS is usually sickle cell trait, with outcomes still varying among individuals",
      "statement": "The combination of HBB alleles changes the expected phenotype: two HbS alleles can cause sickle cell disease, while HbAS is generally classified as sickle cell trait.",
      "qualifier": "Other disease-associated HBB alleles, co-inherited modifiers, health context, and care affect phenotype; this is not a personal diagnosis.",
      "evidenceKind": "educational_resource",
      "context": {
        "scope": "Human red cells and Plasmodium falciparum; genotype- and oxygen-dependent effects",
        "organism": "Homo sapiens",
        "taxonId": "9606",
        "cellTypeId": "cell-type-erythrocyte",
        "cellStage": "Mature erythrocyte; oxygenation varies by tissue and assay",
        "conditionId": "disease-sickle-cell",
        "population": "HbAS, HbSS, and HbAA genotypes; malaria exposure differs by place and time",
        "exposureContext": "Clinical associations are population-level; polymerization and parasite-growth experiments model specified oxygen and cell conditions."
      },
      "references": [
        {
          "referenceId": "medlineplus-scd",
          "relation": "supports",
          "sourceRecordId": "MedlinePlus Genetics: sickle cell disease",
          "sourceVersion": "Page version not recorded in this snapshot"
        },
        {
          "referenceId": "hhmi-sickle-storyline",
          "relation": "supports",
          "sourceRecordId": "HHMI BioInteractive: Sickle Cell Disease Storyline",
          "sourceVersion": "Educator resource; version not recorded"
        }
      ]
    },
    {
      "id": "sickle-hbas-reduces-malaria-severity-in-cohorts",
      "version": 1,
      "subjectId": "variant-hbb-hbs",
      "predicate": "ASSOCIATED_WITH_REDUCED_SEVERE_MALARIA_RISK",
      "objectId": "taxon-plasmodium-falciparum",
      "statement": "In two Kenyan child cohorts, HbAS was associated with lower risk of severe falciparum malaria and lower parasite densities during clinical episodes.",
      "qualifier": "These are cohort-specific estimates, not universal effect sizes or immunity. Exposure, age, parasite, host background, and study design matter; HbSS disease is not a malaria-protection strategy.",
      "evidenceKind": "primary_study",
      "context": {
        "scope": "Human red cells and Plasmodium falciparum; genotype- and oxygen-dependent effects",
        "organism": "Homo sapiens",
        "taxonId": "9606",
        "cellTypeId": "cell-type-erythrocyte",
        "cellStage": "Mature erythrocyte; oxygenation varies by tissue and assay",
        "conditionId": "disease-sickle-cell",
        "population": "HbAS, HbSS, and HbAA genotypes; malaria exposure differs by place and time",
        "exposureContext": "Clinical associations are population-level; polymerization and parasite-growth experiments model specified oxygen and cell conditions."
      },
      "references": [
        {
          "referenceId": "sickle-malaria-cohort",
          "relation": "supports",
          "sourceRecordId": "PMID:15942909",
          "sourceVersion": "Published 2005; two cohorts in coastal Kenya"
        },
        {
          "referenceId": "hhmi-sickle-storyline",
          "relation": "context",
          "sourceRecordId": "HHMI BioInteractive: Sickle Cell Disease Storyline",
          "sourceVersion": "Educator resource; version not recorded"
        }
      ]
    },
    {
      "id": "sickle-hypoxia-can-restrict-parasite-growth",
      "version": 1,
      "subjectId": "variant-hbb-hbs",
      "predicate": "CAN_RESTRICT_GROWTH_OF",
      "objectId": "taxon-plasmodium-falciparum",
      "statement": "In experimental infected HbAS red cells, low oxygen promoted HbS polymerization and stalled parasite growth at a defined intracellular stage.",
      "qualifier": "This is one experimentally supported mechanism under specified in-vitro conditions. Malaria protection is multifactorial, and not every proposed mechanism has equal or settled support.",
      "evidenceKind": "primary_study",
      "context": {
        "scope": "Human red cells and Plasmodium falciparum; genotype- and oxygen-dependent effects",
        "organism": "Homo sapiens",
        "taxonId": "9606",
        "cellTypeId": "cell-type-erythrocyte",
        "cellStage": "Mature erythrocyte; oxygenation varies by tissue and assay",
        "conditionId": "disease-sickle-cell",
        "population": "HbAS, HbSS, and HbAA genotypes; malaria exposure differs by place and time",
        "exposureContext": "Clinical associations are population-level; polymerization and parasite-growth experiments model specified oxygen and cell conditions."
      },
      "references": [
        {
          "referenceId": "sickle-hbas-mechanism",
          "relation": "supports",
          "sourceRecordId": "PMID:29946035",
          "sourceVersion": "Published 2018; erythrocyte infection experiments"
        },
        {
          "referenceId": "sickle-malaria-cohort",
          "relation": "context",
          "sourceRecordId": "PMID:15942909",
          "sourceVersion": "Published 2005; epidemiological cohorts"
        }
      ]
    },
    {
      "id": "sickle-selection-is-malaria-context-dependent",
      "version": 1,
      "subjectId": "variant-hbb-hbs",
      "predicate": "CAN_BE_MAINTAINED_BY_CONTEXT_DEPENDENT_SELECTION",
      "objectId": "concept-genetic-variation",
      "statement": "Where falciparum malaria imposes substantial mortality, a relative advantage for HbAS can help maintain the HbS allele across generations despite the costs of disease-associated genotypes.",
      "qualifier": "This is a population-genetic explanation for allele frequencies, not evidence that sickle cell disease is beneficial; selection depends on local malaria transmission and reproductive outcomes.",
      "evidenceKind": "mechanistic_review",
      "context": {
        "scope": "Human red cells and Plasmodium falciparum; genotype- and oxygen-dependent effects",
        "organism": "Homo sapiens",
        "taxonId": "9606",
        "cellTypeId": "cell-type-erythrocyte",
        "cellStage": "Mature erythrocyte; oxygenation varies by tissue and assay",
        "conditionId": "disease-sickle-cell",
        "population": "HbAS, HbSS, and HbAA genotypes; malaria exposure differs by place and time",
        "exposureContext": "Clinical associations are population-level; polymerization and parasite-growth experiments model specified oxygen and cell conditions."
      },
      "references": [
        {
          "referenceId": "sickle-malaria-cohort",
          "relation": "supports",
          "sourceRecordId": "PMID:15942909",
          "sourceVersion": "Published 2005; two cohorts in coastal Kenya"
        },
        {
          "referenceId": "hhmi-sickle-storyline",
          "relation": "context",
          "sourceRecordId": "HHMI BioInteractive: Sickle Cell Disease Storyline",
          "sourceVersion": "Educator resource; version not recorded"
        }
      ]
    }
  ],
  "limitation": "HbS polymerization, sickle cell disease, and malaria protection are related but distinct outcomes. The case combines molecular experiments, cohort associations, and population-level selection; none predicts an individual’s clinical course.",
  "release": {
    "snapshotSchemaVersion": "BASE-case-snapshot/1.0.0",
    "exportedAt": "2026-10-11T01:51:26.538Z",
    "caseContentRevision": 1,
    "diagramRevision": 1,
    "diagramId": "sickle-cell-case-map",
    "sourceCommitSha": "e97dc5327f9d807ecaf44b77e87ddf6f3f6d2bc2",
    "snapshotSha256": "2302137373a72f1becfbd609ae3de668ff8a37d4af1fff0f275ce237db296f30"
  },
  "snapshot": {
    "releaseInputs": {
      "caseContentRevision": 1,
      "diagramRevision": 1,
      "diagramId": "sickle-cell-case-map",
      "sourceCommitSha": "e97dc5327f9d807ecaf44b77e87ddf6f3f6d2bc2"
    },
    "caseStudy": {
      "slug": "sickle-cell-malaria",
      "title": "Sickle Cell, Inheritance, and Malaria",
      "area": "Genetics · Proteins · Evolution",
      "question": "How can one hemoglobin variant affect both health and survival in a malaria region?",
      "overview": "A DNA variant in HBB changes beta-globin, a subunit of hemoglobin. People with two disease-causing copies can develop sickle cell disease; some people with one copy have partial protection from severe malaria. The relationship depends on genotype and environment.",
      "mechanism": [
        "The HbS allele changes beta-globin: glutamate is replaced by valine at residue 6 of the mature chain (often numbered p.Glu7Val in precursor nomenclature).",
        "When oxygen saturation falls, HbS molecules can polymerize. Polymer formation depends on oxygen, concentration, cell hydration, pH, and the other hemoglobins in the red cell.",
        "Polymerization can stiffen and deform red cells. Repeated sickling and membrane injury contribute to hemolysis and vaso-occlusion in sickle cell disease, but an individual cell’s shape is not a complete prediction of a clinical event.",
        "The inherited allele combination matters: HbSS can cause sickle cell disease, while HbAS is generally sickle cell trait. These genotypes do not have equivalent health effects.",
        "In malaria-endemic settings, studies of child cohorts found HbAS associated with lower risk of severe Plasmodium falciparum malaria. This is partial protection, not immunity, and the effect varies with population and parasite context.",
        "Experiments with infected HbAS cells under low oxygen support a mechanism in which HbS polymerization can restrict parasite growth. Several mechanisms may contribute; no single laboratory result explains every epidemiological observation.",
        "Across generations, a relative heterozygote advantage where malaria is common can help maintain the HbS allele despite the costs of disease-associated genotypes. This is population-level selection, not a health benefit of sickle cell disease."
      ],
      "evidence": "This case combines variant annotation, polymerization experiments, malaria cohorts, and population-genetic interpretation. Their conclusions have different scopes: HbS polymerization is oxygen- and concentration-dependent, HbAS protection is partial and population-specific, and neither predicts an individual's clinical outcome.",
      "entityIds": [
        "gene-hbb",
        "variant-hbb-hbs",
        "protein-beta-globin",
        "protein-hemoglobin-a",
        "molecular-complex-hemoglobin-s",
        "disease-sickle-cell",
        "taxon-plasmodium-falciparum",
        "process-meiosis",
        "concept-genotype",
        "concept-phenotype",
        "concept-genetic-variation",
        "cell-type-erythrocyte"
      ],
      "references": [
        "hhmi-sickle-storyline",
        "openstax-inheritance",
        "ncbi-hbb",
        "uniprot-beta-globin",
        "medlineplus-scd",
        "sickle-hb-polymerization",
        "sickle-malaria-cohort",
        "sickle-hbas-mechanism"
      ],
      "assertionIds": [
        "sickle.hbb-encodes-beta-globin",
        "sickle.hbs-is-gluval-substitution",
        "sickle.hbs-forms-deoxy-polymers",
        "sickle-genotype-shapes-clinical-category",
        "sickle-hbas-reduces-malaria-severity-in-cohorts",
        "sickle-hypoxia-can-restrict-parasite-growth",
        "sickle-selection-is-malaria-context-dependent"
      ],
      "check": [
        {
          "question": "Which gene encodes the beta-globin chain discussed here?",
          "answer": "HBB encodes beta-globin, a subunit of hemoglobin."
        },
        {
          "question": "What is the difference between HbAS and HbSS in this case?",
          "answer": "HbAS is usually sickle cell trait and is associated with partial protection against severe malaria in some populations. HbSS can cause sickle cell disease. They are distinct genotypes with distinct health effects."
        },
        {
          "question": "What does the malaria evidence show—and not show?",
          "answer": "Cohort studies found lower severe-malaria risk among HbAS children in particular malaria-endemic settings, and lab studies support specific mechanisms. This does not mean immunity, universal protection, or that sickle cell disease is beneficial."
        }
      ]
    },
    "diagram": {
      "revision": 1,
      "id": "sickle-cell-case-map",
      "title": "One allele, different outcomes by genotype and environment",
      "subtitle": "Connect a beta-globin variant to red-cell biology and population-level selection.",
      "note": "The classic HbS substitution is β6 Glu→Val in mature beta-globin (HGVS precursor numbering is often p.Glu7Val). Two HbS alleles can cause sickle cell disease; one HbA and one HbS allele is usually sickle cell trait, though individual outcomes vary. In malaria-endemic settings, trait is associated with partial protection against severe malaria, not immunity. HHMI describes mechanisms supported by research, including reduced parasite replication under some infected-cell conditions; the population diagram is not a promise about any individual.",
      "sources": [
        {
          "label": "HHMI BioInteractive · Sickle Cell Disease Storyline",
          "href": "https://www.biointeractive.org/classroom-resources/sickle-cell-disease-storyline"
        },
        {
          "label": "HHMI · Educator narrative and mechanism",
          "href": "https://www.biointeractive.org/sites/default/files/media/file/2025-10/SCD-Educator-Narrative.pdf"
        }
      ],
      "lanes": [
        {
          "label": "Gene to protein",
          "note": "The gene and the HbS variant are distinct records.",
          "links": [
            "encodes"
          ],
          "assertionIds": [
            "sickle.hbb-encodes-beta-globin"
          ],
          "nodes": [
            {
              "label": "HBB gene",
              "detail": "HBB encodes one of the globin subunits in adult hemoglobin.",
              "href": "/gene/hbb"
            },
            {
              "label": "Beta-globin",
              "detail": "The protein chain encoded by HBB.",
              "href": "/protein/beta-globin"
            }
          ]
        },
        {
          "label": "Variant annotation",
          "note": "Protein numbering differs between the mature chain and the precursor sequence.",
          "links": [
            "changes the sequence of"
          ],
          "assertionIds": [
            "sickle.hbs-is-gluval-substitution"
          ],
          "nodes": [
            {
              "label": "HbS HBB variant",
              "detail": "The HbS allele changes one amino acid in beta-globin.",
              "href": "/variant/hbb-hbs"
            },
            {
              "label": "Beta-globin",
              "detail": "Glutamate is replaced by valine at residue 6 in mature-chain numbering (often p.Glu7Val in precursor numbering).",
              "href": "/protein/beta-globin"
            }
          ]
        },
        {
          "label": "Oxygen-dependent mechanism",
          "note": "Polymer formation depends on oxygen saturation and intracellular hemoglobin concentration.",
          "links": [
            "can form intracellular polymers when sufficiently deoxygenated"
          ],
          "assertionIds": [
            "sickle.hbs-forms-deoxy-polymers"
          ],
          "nodes": [
            {
              "label": "Hemoglobin S",
              "detail": "A hemoglobin complex containing sickle beta-globin subunits.",
              "href": "/molecular_complex/hemoglobin-s"
            },
            {
              "label": "Deoxygenated HbS polymers",
              "detail": "Polymerization can contribute to red-cell deformation; it is not identical to every clinical event.",
              "href": "/molecular_complex/hemoglobin-s"
            }
          ]
        },
        {
          "label": "Possible genotypes",
          "note": "These are alternative inherited combinations, not steps in a sequence",
          "links": [],
          "assertionIds": [],
          "branch": true,
          "branchLabel": "Genotype",
          "nodes": [
            {
              "label": "HbA / HbA",
              "detail": "Two typical beta-globin alleles; malaria risk still depends on exposure and other factors.",
              "href": "/concept/genotype"
            },
            {
              "label": "HbA / HbS",
              "detail": "Sickle cell trait; usually no sickle cell disease, with partial protection against severe malaria in some settings.",
              "href": "/concept/phenotype"
            },
            {
              "label": "HbS / HbS",
              "detail": "Sickle cell disease, with severity and experience varying among people.",
              "href": "/disease/sickle-cell-disease"
            }
          ]
        },
        {
          "label": "Genotype and clinical category",
          "note": "Genotype changes expected clinical categories but does not predict an individual's course.",
          "links": [
            "shapes the expected clinical category"
          ],
          "assertionIds": [
            "sickle-genotype-shapes-clinical-category"
          ],
          "nodes": [
            {
              "label": "HBB allele combination",
              "detail": "The two inherited alleles matter; other disease-associated alleles can also affect phenotype.",
              "href": "/gene/hbb"
            },
            {
              "label": "HbSS can cause disease; HbAS is usually trait",
              "detail": "HbAS and HbSS are not equivalent; individual outcomes and disease severity vary.",
              "href": "/disease/sickle-cell-disease"
            }
          ]
        },
        {
          "label": "Cohort evidence",
          "note": "A population association in two Kenyan child cohorts—not immunity or a prediction for one person.",
          "links": [
            "was associated with lower risk of severe"
          ],
          "assertionIds": [
            "sickle-hbas-reduces-malaria-severity-in-cohorts"
          ],
          "nodes": [
            {
              "label": "HbAS genotype",
              "detail": "One HbA and one HbS allele.",
              "href": "/variant/hbb-hbs"
            },
            {
              "label": "Falciparum malaria",
              "detail": "Cohorts found lower severe-malaria risk and parasite densities during clinical episodes.",
              "href": "/taxon/plasmodium-falciparum"
            }
          ]
        },
        {
          "label": "Experimental mechanism",
          "note": "An in-vitro result under a defined infected-cell and oxygen condition.",
          "links": [
            "under low oxygen can restrict growth of"
          ],
          "assertionIds": [
            "sickle-hypoxia-can-restrict-parasite-growth"
          ],
          "nodes": [
            {
              "label": "Infected HbAS red cells",
              "detail": "Low oxygen promoted HbS polymerization in the experimental system.",
              "href": "/variant/hbb-hbs"
            },
            {
              "label": "Plasmodium falciparum",
              "detail": "Parasite growth stalled at a defined intracellular stage in that experiment.",
              "href": "/taxon/plasmodium-falciparum"
            }
          ]
        },
        {
          "label": "Population-level selection",
          "note": "An evolutionary explanation that applies where malaria has imposed substantial mortality.",
          "links": [
            "can be maintained by context-dependent selection affecting"
          ],
          "assertionIds": [
            "sickle-selection-is-malaria-context-dependent"
          ],
          "nodes": [
            {
              "label": "HbS allele",
              "detail": "A relative heterozygote advantage in some malaria-endemic settings can help maintain the allele despite disease costs.",
              "href": "/variant/hbb-hbs"
            },
            {
              "label": "Population genetic variation",
              "detail": "Allele frequencies reflect relative reproductive outcomes across generations.",
              "href": "/concept/genetic-variation"
            }
          ]
        }
      ]
    },
    "connectedRecords": [
      {
        "id": "gene-hbb",
        "type": "gene",
        "slug": "hbb",
        "name": "HBB",
        "canonicalName": "hemoglobin subunit beta",
        "summary": "HBB provides instructions for beta-globin, one of the protein subunits in adult hemoglobin. Particular HBB variants produce hemoglobin S and can cause sickle cell disease when inherited in disease-associated combinations."
      },
      {
        "id": "variant-hbb-hbs",
        "type": "variant",
        "slug": "hbb-hbs",
        "name": "HbS β6 Glu→Val Variant",
        "canonicalName": "HbS β6 Glu→Val Variant",
        "summary": "The HBB missense variant that produces sickle hemoglobin: glutamate is replaced by valine at mature beta-globin residue 6 (often described as p.Glu7Val using precursor numbering)."
      },
      {
        "id": "protein-beta-globin",
        "type": "protein",
        "slug": "beta-globin",
        "name": "Beta-globin",
        "canonicalName": "hemoglobin subunit beta",
        "summary": "Beta-globin pairs with alpha-globin and heme to form hemoglobin, the oxygen-binding protein in red blood cells."
      },
      {
        "id": "protein-hemoglobin-a",
        "type": "molecular_complex",
        "slug": "hemoglobin-a",
        "name": "Adult Hemoglobin A",
        "canonicalName": "hemoglobin alpha2-beta2 tetramer",
        "summary": "Hemoglobin A is the main adult hemoglobin, assembled from two alpha-globin and two beta-globin subunits, each bound to heme."
      },
      {
        "id": "molecular-complex-hemoglobin-s",
        "type": "molecular_complex",
        "slug": "hemoglobin-s",
        "name": "Hemoglobin S",
        "canonicalName": "Hemoglobin S",
        "summary": "A hemoglobin tetramer containing sickle beta-globin subunits; deoxygenated molecules can polymerize under susceptible intracellular conditions."
      },
      {
        "id": "disease-sickle-cell",
        "type": "disease",
        "slug": "sickle-cell-disease",
        "name": "Sickle Cell disease",
        "canonicalName": "inherited hemoglobin disorders involving hemoglobin S",
        "summary": "A group of inherited blood disorders in which hemoglobin S is present with another disease-associated HBB allele. Low-oxygen HbS can form fibers, making red blood cells rigid and contributing to anemia and blocked blood flow."
      },
      {
        "id": "taxon-plasmodium-falciparum",
        "type": "taxon",
        "slug": "plasmodium-falciparum",
        "name": "Plasmodium falciparum",
        "canonicalName": "Plasmodium falciparum",
        "summary": "A protozoan parasite and a major cause of severe malaria in humans."
      },
      {
        "id": "process-meiosis",
        "type": "process",
        "slug": "meiosis",
        "name": "Meiosis",
        "canonicalName": "Meiosis",
        "summary": "Two coordinated divisions that reduce chromosome-set number and generate genetically varied haploid cells in sexual life cycles."
      },
      {
        "id": "concept-genotype",
        "type": "concept",
        "slug": "genotype",
        "name": "Genotype",
        "canonicalName": "Genotype",
        "summary": "The genetic-variant or allele combination considered for an individual or locus."
      },
      {
        "id": "concept-phenotype",
        "type": "concept",
        "slug": "phenotype",
        "name": "Phenotype",
        "canonicalName": "Phenotype",
        "summary": "An observable or measurable characteristic shaped by genetic, developmental, and environmental factors."
      },
      {
        "id": "concept-genetic-variation",
        "type": "concept",
        "slug": "genetic-variation",
        "name": "Genetic Variation",
        "canonicalName": "Genetic variation",
        "summary": "Differences in genetic material among individuals or populations, generated and reshaped by multiple processes."
      },
      {
        "id": "cell-type-erythrocyte",
        "type": "cell_type",
        "slug": "erythrocyte",
        "name": "Erythrocyte",
        "canonicalName": "Erythrocyte",
        "summary": "An erythrocyte, or red blood cell, carries oxygen through the blood using hemoglobin."
      }
    ],
    "references": [
      {
        "id": "hhmi-sickle-storyline",
        "title": "Sickle Cell Disease Storyline",
        "publisher": "HHMI BioInteractive",
        "url": "https://www.biointeractive.org/classroom-resources/sickle-cell-disease-storyline",
        "kind": "guide"
      },
      {
        "id": "openstax-inheritance",
        "title": "Laws of Inheritance",
        "publisher": "OpenStax Biology 2e",
        "url": "https://openstax.org/books/biology-2e/pages/12-3-laws-of-inheritance",
        "kind": "textbook"
      },
      {
        "id": "ncbi-hbb",
        "title": "HBB hemoglobin subunit beta (Gene ID 3043)",
        "publisher": "NCBI Gene",
        "url": "https://www.ncbi.nlm.nih.gov/gene/3043",
        "kind": "database"
      },
      {
        "id": "uniprot-beta-globin",
        "title": "Hemoglobin subunit beta (P68871)",
        "publisher": "UniProtKB",
        "url": "https://www.uniprot.org/uniprotkb/P68871/entry",
        "kind": "database"
      },
      {
        "id": "medlineplus-scd",
        "title": "Sickle cell disease",
        "publisher": "MedlinePlus Genetics",
        "url": "https://medlineplus.gov/genetics/condition/sickle-cell-disease/",
        "kind": "database"
      },
      {
        "id": "sickle-hb-polymerization",
        "title": "Determination of deoxyhemoglobin S polymer in sickle erythrocytes upon deoxygenation",
        "publisher": "Noble et al. · PMID 6933568",
        "url": "https://pubmed.ncbi.nlm.nih.gov/6933568/",
        "kind": "article"
      },
      {
        "id": "sickle-malaria-cohort",
        "title": "Sickle cell trait and the risk of Plasmodium falciparum malaria and other childhood diseases",
        "publisher": "Williams et al. · PMID 15942909",
        "url": "https://pubmed.ncbi.nlm.nih.gov/15942909/",
        "kind": "article"
      },
      {
        "id": "sickle-hbas-mechanism",
        "title": "Resistance to Plasmodium falciparum in sickle cell trait erythrocytes is driven by oxygen-dependent growth inhibition",
        "publisher": "LaMonte et al. · PMID 29946035",
        "url": "https://pubmed.ncbi.nlm.nih.gov/29946035/",
        "kind": "article"
      }
    ],
    "mechanismEvidence": {
      "schemaVersion": "BASE-biological-assertion/1.1.0",
      "context": {
        "scope": "Human red cells and Plasmodium falciparum; genotype- and oxygen-dependent effects",
        "organism": "Homo sapiens",
        "taxonId": "9606",
        "cellTypeId": "cell-type-erythrocyte",
        "cellStage": "Mature erythrocyte; oxygenation varies by tissue and assay",
        "conditionId": "disease-sickle-cell",
        "population": "HbAS, HbSS, and HbAA genotypes; malaria exposure differs by place and time",
        "exposureContext": "Clinical associations are population-level; polymerization and parasite-growth experiments model specified oxygen and cell conditions."
      },
      "groups": [
        {
          "id": "variant",
          "title": "From Allele to Hemoglobin",
          "description": "Connect the HBB sequence change to the altered protein complex.",
          "assertionIds": [
            "sickle.hbb-encodes-beta-globin",
            "sickle.hbs-is-gluval-substitution"
          ]
        },
        {
          "id": "red-cell",
          "title": "Oxygen, Polymerization & Genotype",
          "description": "Separate the molecular mechanism from the clinical phenotype.",
          "assertionIds": [
            "sickle.hbs-forms-deoxy-polymers",
            "sickle-genotype-shapes-clinical-category"
          ]
        },
        {
          "id": "malaria-selection",
          "title": "Malaria Evidence & Population Selection",
          "description": "Keep experiments, cohort associations, and evolutionary inference in their proper scopes.",
          "assertionIds": [
            "sickle-hbas-reduces-malaria-severity-in-cohorts",
            "sickle-hypoxia-can-restrict-parasite-growth",
            "sickle-selection-is-malaria-context-dependent"
          ]
        }
      ],
      "assertions": [
        {
          "id": "sickle.hbb-encodes-beta-globin",
          "version": 1,
          "subjectId": "gene-hbb",
          "predicate": "ENCODES",
          "objectId": "protein-beta-globin",
          "statement": "HBB encodes beta-globin, one of the globin subunits in adult hemoglobin.",
          "qualifier": "The same gene has multiple alleles; a gene-to-protein link alone does not specify hemoglobin composition or clinical outcome.",
          "evidenceKind": "database_annotation",
          "context": {
            "scope": "Human red cells and Plasmodium falciparum; genotype- and oxygen-dependent effects",
            "organism": "Homo sapiens",
            "taxonId": "9606",
            "cellTypeId": "cell-type-erythrocyte",
            "cellStage": "Mature erythrocyte; oxygenation varies by tissue and assay",
            "conditionId": "disease-sickle-cell",
            "population": "HbAS, HbSS, and HbAA genotypes; malaria exposure differs by place and time",
            "exposureContext": "Clinical associations are population-level; polymerization and parasite-growth experiments model specified oxygen and cell conditions."
          },
          "references": [
            {
              "referenceId": "ncbi-hbb",
              "relation": "supports",
              "sourceRecordId": "NCBI Gene:3043",
              "sourceVersion": "Record accessed 2026-09-27"
            },
            {
              "referenceId": "uniprot-beta-globin",
              "relation": "supports",
              "sourceRecordId": "UniProtKB:P68871",
              "sourceVersion": "Reviewed entry; release not recorded in this snapshot"
            }
          ]
        },
        {
          "id": "sickle.hbs-is-gluval-substitution",
          "version": 1,
          "subjectId": "variant-hbb-hbs",
          "predicate": "CHANGES_SEQUENCE_OF",
          "objectId": "protein-beta-globin",
          "statement": "The canonical HbS allele changes glutamate to valine at beta-globin residue 6 in mature-chain numbering (often written p.Glu7Val with precursor numbering).",
          "qualifier": "The residue-number difference reflects mature-chain versus precursor-protein conventions; it is one specific variant, not a complete description of HBB variation.",
          "evidenceKind": "database_annotation",
          "context": {
            "scope": "Human red cells and Plasmodium falciparum; genotype- and oxygen-dependent effects",
            "organism": "Homo sapiens",
            "taxonId": "9606",
            "cellTypeId": "cell-type-erythrocyte",
            "cellStage": "Mature erythrocyte; oxygenation varies by tissue and assay",
            "conditionId": "disease-sickle-cell",
            "population": "HbAS, HbSS, and HbAA genotypes; malaria exposure differs by place and time",
            "exposureContext": "Clinical associations are population-level; polymerization and parasite-growth experiments model specified oxygen and cell conditions."
          },
          "references": [
            {
              "referenceId": "ncbi-hbb",
              "relation": "supports",
              "sourceRecordId": "NCBI Gene:3043",
              "sourceVersion": "Record accessed 2026-09-27"
            },
            {
              "referenceId": "uniprot-beta-globin",
              "relation": "supports",
              "sourceRecordId": "UniProtKB:P68871; hemoglobin S substitution",
              "sourceVersion": "Reviewed entry; release not recorded in this snapshot"
            }
          ]
        },
        {
          "id": "sickle.hbs-forms-deoxy-polymers",
          "version": 1,
          "subjectId": "molecular-complex-hemoglobin-s",
          "predicate": "CAN_FORM",
          "objectValue": "Intracellular HbS polymers when sufficiently deoxygenated",
          "statement": "Deoxygenated hemoglobin S can assemble into intracellular polymers; the amount of polymer depends strongly on oxygen saturation and hemoglobin concentration.",
          "qualifier": "Polymer formation is not identical to visible sickling. HbF and other hemoglobin species, pH, cell hydration, and the duration of deoxygenation also modify the process.",
          "evidenceKind": "primary_study",
          "context": {
            "scope": "Human red cells and Plasmodium falciparum; genotype- and oxygen-dependent effects",
            "organism": "Homo sapiens",
            "taxonId": "9606",
            "cellTypeId": "cell-type-erythrocyte",
            "cellStage": "Mature erythrocyte; oxygenation varies by tissue and assay",
            "conditionId": "disease-sickle-cell",
            "population": "HbAS, HbSS, and HbAA genotypes; malaria exposure differs by place and time",
            "exposureContext": "Clinical associations are population-level; polymerization and parasite-growth experiments model specified oxygen and cell conditions."
          },
          "references": [
            {
              "referenceId": "sickle-hb-polymerization",
              "relation": "supports",
              "sourceRecordId": "PMID:6933568",
              "sourceVersion": "Published 1981"
            },
            {
              "referenceId": "medlineplus-scd",
              "relation": "context",
              "sourceRecordId": "MedlinePlus Genetics: sickle cell disease",
              "sourceVersion": "Page version not recorded in this snapshot"
            }
          ]
        },
        {
          "id": "sickle-genotype-shapes-clinical-category",
          "version": 1,
          "subjectId": "gene-hbb",
          "predicate": "PHENOTYPE_DEPENDS_ON_GENOTYPE",
          "objectValue": "HbSS can cause sickle cell disease; HbAS is usually sickle cell trait, with outcomes still varying among individuals",
          "statement": "The combination of HBB alleles changes the expected phenotype: two HbS alleles can cause sickle cell disease, while HbAS is generally classified as sickle cell trait.",
          "qualifier": "Other disease-associated HBB alleles, co-inherited modifiers, health context, and care affect phenotype; this is not a personal diagnosis.",
          "evidenceKind": "educational_resource",
          "context": {
            "scope": "Human red cells and Plasmodium falciparum; genotype- and oxygen-dependent effects",
            "organism": "Homo sapiens",
            "taxonId": "9606",
            "cellTypeId": "cell-type-erythrocyte",
            "cellStage": "Mature erythrocyte; oxygenation varies by tissue and assay",
            "conditionId": "disease-sickle-cell",
            "population": "HbAS, HbSS, and HbAA genotypes; malaria exposure differs by place and time",
            "exposureContext": "Clinical associations are population-level; polymerization and parasite-growth experiments model specified oxygen and cell conditions."
          },
          "references": [
            {
              "referenceId": "medlineplus-scd",
              "relation": "supports",
              "sourceRecordId": "MedlinePlus Genetics: sickle cell disease",
              "sourceVersion": "Page version not recorded in this snapshot"
            },
            {
              "referenceId": "hhmi-sickle-storyline",
              "relation": "supports",
              "sourceRecordId": "HHMI BioInteractive: Sickle Cell Disease Storyline",
              "sourceVersion": "Educator resource; version not recorded"
            }
          ]
        },
        {
          "id": "sickle-hbas-reduces-malaria-severity-in-cohorts",
          "version": 1,
          "subjectId": "variant-hbb-hbs",
          "predicate": "ASSOCIATED_WITH_REDUCED_SEVERE_MALARIA_RISK",
          "objectId": "taxon-plasmodium-falciparum",
          "statement": "In two Kenyan child cohorts, HbAS was associated with lower risk of severe falciparum malaria and lower parasite densities during clinical episodes.",
          "qualifier": "These are cohort-specific estimates, not universal effect sizes or immunity. Exposure, age, parasite, host background, and study design matter; HbSS disease is not a malaria-protection strategy.",
          "evidenceKind": "primary_study",
          "context": {
            "scope": "Human red cells and Plasmodium falciparum; genotype- and oxygen-dependent effects",
            "organism": "Homo sapiens",
            "taxonId": "9606",
            "cellTypeId": "cell-type-erythrocyte",
            "cellStage": "Mature erythrocyte; oxygenation varies by tissue and assay",
            "conditionId": "disease-sickle-cell",
            "population": "HbAS, HbSS, and HbAA genotypes; malaria exposure differs by place and time",
            "exposureContext": "Clinical associations are population-level; polymerization and parasite-growth experiments model specified oxygen and cell conditions."
          },
          "references": [
            {
              "referenceId": "sickle-malaria-cohort",
              "relation": "supports",
              "sourceRecordId": "PMID:15942909",
              "sourceVersion": "Published 2005; two cohorts in coastal Kenya"
            },
            {
              "referenceId": "hhmi-sickle-storyline",
              "relation": "context",
              "sourceRecordId": "HHMI BioInteractive: Sickle Cell Disease Storyline",
              "sourceVersion": "Educator resource; version not recorded"
            }
          ]
        },
        {
          "id": "sickle-hypoxia-can-restrict-parasite-growth",
          "version": 1,
          "subjectId": "variant-hbb-hbs",
          "predicate": "CAN_RESTRICT_GROWTH_OF",
          "objectId": "taxon-plasmodium-falciparum",
          "statement": "In experimental infected HbAS red cells, low oxygen promoted HbS polymerization and stalled parasite growth at a defined intracellular stage.",
          "qualifier": "This is one experimentally supported mechanism under specified in-vitro conditions. Malaria protection is multifactorial, and not every proposed mechanism has equal or settled support.",
          "evidenceKind": "primary_study",
          "context": {
            "scope": "Human red cells and Plasmodium falciparum; genotype- and oxygen-dependent effects",
            "organism": "Homo sapiens",
            "taxonId": "9606",
            "cellTypeId": "cell-type-erythrocyte",
            "cellStage": "Mature erythrocyte; oxygenation varies by tissue and assay",
            "conditionId": "disease-sickle-cell",
            "population": "HbAS, HbSS, and HbAA genotypes; malaria exposure differs by place and time",
            "exposureContext": "Clinical associations are population-level; polymerization and parasite-growth experiments model specified oxygen and cell conditions."
          },
          "references": [
            {
              "referenceId": "sickle-hbas-mechanism",
              "relation": "supports",
              "sourceRecordId": "PMID:29946035",
              "sourceVersion": "Published 2018; erythrocyte infection experiments"
            },
            {
              "referenceId": "sickle-malaria-cohort",
              "relation": "context",
              "sourceRecordId": "PMID:15942909",
              "sourceVersion": "Published 2005; epidemiological cohorts"
            }
          ]
        },
        {
          "id": "sickle-selection-is-malaria-context-dependent",
          "version": 1,
          "subjectId": "variant-hbb-hbs",
          "predicate": "CAN_BE_MAINTAINED_BY_CONTEXT_DEPENDENT_SELECTION",
          "objectId": "concept-genetic-variation",
          "statement": "Where falciparum malaria imposes substantial mortality, a relative advantage for HbAS can help maintain the HbS allele across generations despite the costs of disease-associated genotypes.",
          "qualifier": "This is a population-genetic explanation for allele frequencies, not evidence that sickle cell disease is beneficial; selection depends on local malaria transmission and reproductive outcomes.",
          "evidenceKind": "mechanistic_review",
          "context": {
            "scope": "Human red cells and Plasmodium falciparum; genotype- and oxygen-dependent effects",
            "organism": "Homo sapiens",
            "taxonId": "9606",
            "cellTypeId": "cell-type-erythrocyte",
            "cellStage": "Mature erythrocyte; oxygenation varies by tissue and assay",
            "conditionId": "disease-sickle-cell",
            "population": "HbAS, HbSS, and HbAA genotypes; malaria exposure differs by place and time",
            "exposureContext": "Clinical associations are population-level; polymerization and parasite-growth experiments model specified oxygen and cell conditions."
          },
          "references": [
            {
              "referenceId": "sickle-malaria-cohort",
              "relation": "supports",
              "sourceRecordId": "PMID:15942909",
              "sourceVersion": "Published 2005; two cohorts in coastal Kenya"
            },
            {
              "referenceId": "hhmi-sickle-storyline",
              "relation": "context",
              "sourceRecordId": "HHMI BioInteractive: Sickle Cell Disease Storyline",
              "sourceVersion": "Educator resource; version not recorded"
            }
          ]
        }
      ],
      "limitation": "HbS polymerization, sickle cell disease, and malaria protection are related but distinct outcomes. The case combines molecular experiments, cohort associations, and population-level selection; none predicts an individual’s clinical course."
    }
  }
}