X Linked Disorders

Slide 1 · X-Linked Disorders

Inheritance • mosaicism • disease mechanisms

Core principles

Clinical examples

Pedigree reasoning

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning objectives

Identify characteristic X-linked recessive and dominant pedigrees.

Predict offspring genotypes using parental sex chromosomes.

Explain how X-inactivation creates phenotypic mosaicism.

Relate representative gene defects to their clinical manifestations.

Recognize exceptions to simplified inheritance rules.

Slide 3 · The affected gene lies on the X chromosome

X-chromosomal variant

Altered gene product

Phenotype depends on sex and allele activity

X-linked describes locus; Recessive versus dominant describes expression and transmission.

Slide 4 · The X and Y chromosomes are not equivalent

X chromosome

Y chromosome

Many genes have no functional counterpart on Y.

Its male-specific region encodes fewer genes.

Pathogenic X variants may be unopposed in XY individuals.

Many Y-linked genes relate to spermatogenesis.

The pseudoautosomal regions are an important exception.

Slide 5 · Hemizygosity explains male vulnerability

One X allele at a locus

No second X copy at that locus

Pathogenic allele may be expressed

Hemizygous means one allele copy at a locus, not two identical mutant alleles.

Slide 6 · Sex chromosome origin determines paternal transmission

Father: X / Y

Mother: X / X

Father gives Y to sons

Father gives X to daughters

An X-linked allele is not passed directly from father to son.

Slide 7 · Pseudoautosomal regions: A boundary to the rule

Not every gene on X lacks a corresponding copy on Y

Some genes occur in homologous pseudoautosomal regions of both sex chromosomes.

The classic hemizygous-X model applies most directly outside these regions.

Rare pathogenic variants in pseudoautosomal genes may show different transmission.

Slide 8 · What makes X-linked recessive disease “recessive”?

Carrier XX individual: A normal allele often compensates

XY individual: Pathogenic allele is unopposed

Clinical expression depends on the gene and cellular context

“Recessive” does not mean every heterozygous female is symptom-free.

Slide 9 · Carrier mother: Four equally likely chromosome combinations

Assume XᴺXᵐ mother and XᴺY father

Mother XᴺXᵐ

Father XᴺY

×

XᴺXᴺ daughter

XᴺXᵐ daughter

XᴺY son

XᵐY son

Among sons: 1 in 2 affected; Among daughters: 1 in 2 carriers.

Slide 10 · Risk to a son of a heterozygous mother

A son receives his single X chromosome from his mother

Maternal gametes

Xᴺ

Xᵐ

Unaffected son XᴺY

Affected son XᵐY

Each son independently has a 50% chance of inheriting the maternal variant.

Slide 11 · Risk to a daughter of a heterozygous mother

Each daughter receives paternal Xᴺ plus one maternal X

Maternal Xᴺ

Maternal Xᵐ

XᴺXᴺ noncarrier

XᴺXᵐ carrier

One in two daughters inherit the pathogenic X allele.

Slide 12 · Affected father with an unaffected mother

Assume XᵐY father and XᴺXᴺ mother

Father XᵐY

Mother XᴺXᴺ

×

XᴺXᵐ daughter

XᴺXᵐ daughter

XᴺY son

XᴺY son

All daughters inherit Xᵐ; No sons inherit their father’s Xᵐ.

Slide 13 · Why father-to-son transmission is absent

Affected father: XᵐY

A son inherits paternal Y

Therefore son cannot inherit paternal Xᵐ

This excludes direct paternal transmission of the X variant; A son may still be affected via his mother.

Slide 14 · Why daughters of an affected man are obligate carriers

Father passes Xᵐ to every daughter

Unaffected mother passes Xᴺ

Every daughter is XᴺXᵐ

Assumes a classic X-linked recessive allele and unaffected, noncarrier mother.

Slide 15 · An X-linked recessive pedigree can skip generations

XᵐY

XᴺXᴺ

XᴺXᵐ

XᴺY

XᵐY

Filled square = affected male Slashed circle = carrier female Open symbol = unaffected

Slide 16 · X-inactivation: One X is transcriptionally silenced

Two X chromosomes in an early female cell

One X largely inactivated in each cell

Descendant cells retain the selected active X

This dosage-compensation process creates cellular mosaicism in heterozygotes.

Slide 17 · Random X-inactivation produces two cell populations

Heterozygous XᴺXᵐ individual

Xᴺ active

Xᵐ active

Xᴺ active

Xᵐ active

Xᴺ active

Xᵐ active

A tissue may contain mixtures of cells expressing different X alleles.

Slide 18 · Skewed X-inactivation changes phenotypic expression

Relatively balanced activation

Skewed activation

Cells expressing Xᴺ and Xᵐ coexist.

One X is active in more cells.

Clinical effect may be attenuated.

Phenotype shifts toward the active allele.

Skewing can modify heterozygous female manifestations.

Slide 19 · A carrier state does not guarantee an absent phenotype

Heterozygous females often retain enough normal gene product to be unaffected.

X-inactivation may leave a functionally important fraction of cells expressing the mutant allele.

Skewing can increase or decrease disease expression.

Consequently, family history alone cannot define clinical severity.

Slide 20 · Red-cell mosaicism gives a measurable example

Normal-G6PD erythroid clones

Deficient-G6PD erythroid clones

Mixed population of circulating erythrocytes

Only the enzyme-deficient fraction is especially vulnerable to oxidant stress.

Slide 21 · G6PD: The biochemical defense against oxidants

G6PD activity

NADPH generation

Reduced glutathione

Protection from oxidants

Enzyme deficiency weakens erythrocyte protection from oxidative injury.

Slide 22 · G6PD deficiency is often unmasked by exposure

Inherited enzyme deficiency

Infection, certain drugs, or foods

Oxidant stress

Acute hemolysis

A molecular defect can remain clinically silent until an environmental trigger.

Slide 23 · Why older G6PD-deficient erythrocytes are vulnerable

Mature red cells cannot synthesize new proteins.

Some G6PD variants yield enzyme with reduced stability.

With erythrocyte aging, protective enzyme activity may decline.

Oxidative challenge preferentially injures insufficiently protected cells.

Slide 24 · G6PD heterozygotes: Disease risk is not all-or-none

Hemizygous affected male

Heterozygous female

All erythroid precursors carry the pathogenic X allele.

Two erythrocyte populations may coexist.

Many red cells share the enzymatic vulnerability.

Clinical hemolysis remains possible.

The proportion of susceptible red cells reflects X-inactivation.

Slide 25 · Robbins groups X-linked disorders across organ systems

System

Examples in Table 5.3

Musculoskeletal

Duchenne muscular dystrophy

Hematopoietic

Hemophilia A/B; CGD; G6PD deficiency

Immune

Agammaglobulinemia; Wiskott–Aldrich

Metabolic

Diabetes insipidus; Lesch–Nyhan

Nervous

Fragile X syndrome (inheritance is nonclassic)

Table groupings follow Robbins; They are not substitutes for gene-specific classification.

Slide 26 · Dystrophinopathy: X-linked structural-protein disease

DMD gene variant

Loss or reduced function of dystrophin

Fragile sarcolemmal link

Progressive muscle injury

Slide 27 · Dystrophin links muscle cytoskeleton to matrix

Simplified membrane-support schematic

Contractile cytoskeleton

Dystrophin complex

Basement membrane

Membrane fragility → calcium influx → necrosis

Slide 28 · Muscle pathology evolves from damage to replacement

Myofiber necrosis and regeneration

Myofiber size variability

Fatty and fibrous tissue replacement

Morphologic progression is Ch. 27; This is a schematic, not a histology image.

Slide 29 · Clinical consequences of Duchenne muscular dystrophy

Early childhood proximal weakness, initially affecting pelvic-girdle muscles.

Calf pseudohypertrophy reflects apparent enlargement despite weakness.

Progressive respiratory and cardiac involvement follows muscle disease.

Clinical severity correlates with near-total dystrophin loss.

Slide 30 · Duchenne and Becker differ in residual dystrophin

Duchenne phenotype

Becker phenotype

Mutations typically prevent effective dystrophin production.

Some truncated dystrophin retains partial function.

Usually early onset and more severe progression.

Usually later onset and milder progression.

Both are X-linked dystrophinopathies, but protein consequences differ.

Slide 31 · Hemophilia A and B disrupt the same coagulation step

Factor IXa + factor VIIIa

Activation of factor X

Thrombin generation and fibrin formation

Hemophilia A: Factor VIII deficiency; Hemophilia B: Factor IX deficiency.

Slide 32 · Bleeding phenotype reflects impaired secondary hemostasis

Deep tissue hemorrhage and bleeding after trauma or surgery.

Hemarthroses are particularly characteristic.

Recurrent joint hemorrhage may produce progressive deformity.

Petechiae are not a typical defining feature.

Slide 33 · Distinguishing hemophilia A from B

Feature

Hemophilia A

Hemophilia B

Affected factor

VIII

IX

Inheritance

X-linked recessive

X-linked recessive

Coagulation screen

Prolonged aPTT; normal PT

Prolonged aPTT; normal PT

Confirmation

Factor VIII activity assay

Factor IX activity assay

PT/aPTT alone do not distinguish the factor deficiency.

Slide 34 · Chronic granulomatous disease: Defective oxidative killing

Phagocyte microbicidal failure

Phagocyte oxidase defect

Reduced reactive oxygen generation

Recurrent microbial infections

Slide 35 · Not all chronic granulomatous disease is X-linked

X-linked forms

Autosomal recessive forms

Often involve gp91phox, a membrane component of oxidase.

Can involve cytoplasmic oxidase components.

Classic X-linked transmission can occur.

Both sexes can be similarly affected.

A clinical syndrome may have multiple molecular inheritance patterns.

Slide 36 · BTK deficiency blocks B-lymphocyte maturation

BTK pathogenic variant

Deficient pre–B-cell receptor signaling

Arrested B-cell maturation

Marked antibody deficiency

Slide 37 · X-linked agammaglobulinemia: Clinical phenotype

B-cell maturation fails, impairing immunoglobulin production.

Maternal antibodies transiently protect young infants.

Recurrent bacterial respiratory infections usually emerge after this protection wanes.

Typical inheritance is X-linked; Distinguish phenocopies due to other genes.

Slide 38 · Wiskott–Aldrich: Disturbed cytoskeletal responses

WAS gene pathogenic variant

Abnormal WASP function

Impaired cytoskeletal responses

Combined hematologic and immune phenotype

Slide 39 · Wiskott–Aldrich: Three high-yield clinical features

One X-linked disorder affects platelets and immune function.

Thrombocytopenia

Eczema

Recurrent infection

The classic triad is a recognition aid, not a replacement for molecular diagnosis.

Slide 40 · Lesch–Nyhan: Defective purine salvage

HPRT deficiency

Impaired purine salvage

Increased uric acid production

Neurologic disease dominates complete deficiency

Slide 41 · Hyperuricemia alone does not capture Lesch–Nyhan

Partial HPRT deficiency

Complete HPRT deficiency

Increased uric acid production.

Marked neurologic phenotype characterizes Lesch–Nyhan.

May contribute to gout through hyperuricemia.

Hyperuricemia is an associated biochemical feature.

Slide 42 · “Diabetes insipidus” in the X-linked disorder list

Interpret disease labels within a specific molecular context

Robbins Table 5.3 lists diabetes insipidus among X-linked recessive conditions.

The label alone does not identify the pathogenic gene or distinguish all DI etiologies.

A phenotype-based name cannot establish inheritance without its molecular subtype.

Do not treat every case of diabetes insipidus as X-linked.

Slide 43 · Fragile X syndrome is associated with FMR1 CGG expansion

Expanded CGG repeat in FMR1

Altered FMR1 gene regulation

Reduced FMRP function

Neurodevelopmental phenotype

Triplet-repeat expansion follows distinct mechanisms from a simple recessive point mutation.

Slide 44 · Fragile X: Clinical and molecular clues

Intellectual disability and developmental involvement predominate.

Clinical features can include long face, large ears, and macroorchidism.

Physical features are variable; Some may be subtle or absent.

DNA-based repeat analysis replaced historical cytogenetic fragile-site testing.

Slide 45 · Why fragile X is not a simple Mendelian pedigree

Robbins Table 5.3

Detailed Chapter 5 discussion

Places fragile X in its historical X-linked recessive list.

Explains unstable repeat expansions and nonclassic inheritance.

Male predominance is a useful initial clue.

Premutation transmission can differ from fully affected states.

Use gene- and repeat-specific counseling; Avoid oversimplified 50% “affected” labels.

Slide 46 · X-linked dominant: One altered X may express disease

Pathogenic X-linked variant

Phenotype in heterozygous XX individual

Inheritance can occur through either parent

X-linked dominant expression differs from the typical carrier phenotype.

Slide 47 · X-linked dominant: Affected father

Assume affected XᴰY father and unaffected XᴺXᴺ mother

Father XᴰY

Mother XᴺXᴺ

×

XᴺXᴰ daughter

XᴺXᴰ daughter

XᴺY son

XᴺY son

All daughters inherit the variant; None of the sons receive the paternal X.

Slide 48 · X-linked dominant: Affected heterozygous mother

Assume affected XᴰXᴺ mother and unaffected XᴺY father

Mother XᴰXᴺ

Father XᴺY

×

XᴺXᴺ daughter

XᴰXᴺ daughter

XᴺY son

XᴰY son

50% risk of inheriting the variant within either offspring sex.

Slide 49 · Rett syndrome: MECP2-related neurodevelopmental disease

MECP2 loss of function

Altered methyl-CpG–dependent gene regulation

Neurodevelopmental stagnation or regression

Slide 50 · Rett: Characteristic clinical evolution

Often recognized after apparently normal early development.

Progressive loss of acquired speech and purposeful hand movements.

Stereotyped hand movements, ataxia, seizures, and breathing disturbances may appear.

Phenotypic variability reflects broader effects of MECP2 dysfunction.

Slide 51 · MECP2 disorders in males: Avoid an absolute rule

Earlier simplified description

Expanded Robbins discussion

Rett-associated variants were described as lethal in hemizygous males.

Male survivors with MECP2 variants are recognized.

A classic teaching heuristic.

Outcomes range from severe encephalopathy to intellectual disability.

“Always male-lethal” is not a scientifically defensible blanket statement.

Slide 52 · Other X-linked dominant examples require subtype precision

Vitamin D–resistant rickets

Alport syndrome

Some forms as X-linked dominant.

X-linked disease occurs in a major subset.

The inherited molecular subtype determines its pedigree.

Autosomal recessive and dominant forms also exist.

A disease name may refer to more than one genetic mechanism.

Slide 53 · Alport syndrome: Basement-membrane collagen defect

Type IV collagen chain defect

Defective basement membrane network

Glomerular, cochlear and ocular abnormalities

COL4A5 is implicated in X-linked Alport; Other collagen IV genes yield autosomal forms.

Slide 54 · Alport: Morphology predicts progressive renal disease

GBM thinning and thickening

Splitting and lamination on EM

Progressive glomerulosclerosis

Schematic of pathological progression; The “basket-weave” appearance is an actual described EM finding, not pictured here.

Slide 55 · Alport demonstrates inheritance heterogeneity

Inheritance form

Typical genetic context

Key interpretive point

X-linked

COL4A5

Males often more severely affected; females may have hematuria

Autosomal recessive

Other type IV collagen genes

Both sexes can express full disease

Autosomal dominant

Other type IV collagen genes

Vertical transmission may occur in either sex

Slide 56 · X-linked recessive versus X-linked dominant

Feature

X-linked recessive

X-linked dominant

Typical affected group

Hemizygous XY individuals

Either XX or XY individuals

Heterozygous XX phenotype

Often unaffected; may manifest

Often affected; severity variable

Affected father → sons

No paternal X transmission

No paternal X transmission

Affected father → daughters

Obligate carriers if mother unaffected

All inherit paternal variant

Heterozygous mother

Half sons may be affected

Half children of either sex inherit variant

Slide 57 · Compare X-linked with autosomal inheritance

Pedigree feature

X-linked

Autosomal

Father-to-son transmission

Absent for paternal X variants

Possible

Sex distribution

Often asymmetric

Frequently similar by sex

Carrier females

Common in X-linked recessive

Common in autosomal recessive

Affected father → daughters

All inherit his X variant

Not obligatorily all daughters

Slide 58 · Pedigree analysis: A reproducible sequence

Confirm phenotype and molecular diagnosis

Look for father–son transmission

Map affected relatives by sex

Account for carriers and variability

A pedigree suggests a model; The gene-level diagnosis determines the final classification.

Slide 59 · Consider female manifestations explicitly

Symptoms in a heterozygous woman do not exclude X-linked recessive inheritance.

Mosaic X-inactivation can produce tissue-dependent clinical effects.

X-linked dominant disorders may manifest in heterozygous women by definition.

Phenocopies and alternative genetic mechanisms should remain in the differential.

Slide 60 · Clinical pattern recognition before molecular confirmation

Clinical presentation

Illustrative X-linked mechanism

Proximal weakness with muscle replacement

Dystrophin deficiency

Hemarthroses, prolonged aPTT

Factor VIII or IX deficiency

Oxidant-triggered acute hemolysis

G6PD deficiency

Early recurrent bacterial infections

BTK-associated B-cell maturation arrest

Eczema, thrombocytopenia, infections

WASP dysfunction

Hematuria, deafness, renal injury

Type IV collagen abnormality

Slide 61 · Worked case: Maternal transmission of hemophilia

An unaffected woman has an affected brother and an affected son.

What mode of inheritance is suggested?

Why is her normal clinical examination insufficient to exclude carriage?

Would the affected son pass the pathogenic X to his sons?

Slide 62 · Case analysis: What the pedigree establishes

Affected males through maternal line

X-linked recessive is a strong hypothesis

Carrier status requires appropriate assessment

An affected father passes Y—not his mutant X—to every son.

Slide 63 · Worked case: Episodic hemolysis in a woman

A heterozygous woman develops hemolysis after an oxidant exposure.

Does symptomatic disease exclude a classic X-linked trait?

Which erythrocyte population is susceptible to oxidant injury?

What mechanism creates different susceptibility among her red cells?

Slide 64 · Case analysis: Mosaicism explains the hemolysis

Heterozygous X-linked G6PD variant

Different active X alleles among erythroid clones

Variable oxidant-induced hemolysis

Carrier females are not necessarily protected from an X-linked phenotype.

Slide 65 · Five interpretive traps in X-linked genetics

Equating X-linked with exclusively male disease.

Calling every heterozygous woman completely asymptomatic.

Ignoring de novo variants or genes with nonclassic repeat expansion.

Equating a syndrome name with only one inheritance pattern.

Using a pedigree as a substitute for gene-level diagnosis.

Slide 66 · Integrative synthesis: Locus, transmission, expression

Locus: Variant resides on X

Transmission: Parental X/Y segregation

Expression: Gene effect and X-inactivation

These three variables must be considered separately in every case.

Slide 67 · Primary sources and cross-chapter reading

Primary textbook

Robbins, Cotran & Kumar: Pathologic Basis of Disease, 11th edition (2026).

Source sections

Chapter 5: Genes and Human Diseases — X-Linked Disorders, pp. 126–127.

Chapter 5: Epigenetic machinery and nonclassic repeat disorders.

Chapters 6, 14, 20, 26, and 27: selected disease mechanisms.