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X-Linked Dominant Pedigree

X-linked dominant traits appear when a single copy of the changed allele is present on the X chromosome. Typical clues: affected females can be as common as affected males, and an affected male passes the trait to all of his daughters but none of his sons. This is a fictional, simplified illustration of the pattern.

Inheritance pattern & biological mechanism

X-linked dominant (XD) inheritance occurs when a pathogenic variant in a single allele on the X chromosome produces the phenotype. Both males and females can express the condition, but because females have two X chromosomes, the condition frequently occurs twice as often in females as in males. However, symptoms in males are often substantially more severe or prenatally lethal.

Key transmission rules

Affected males pass the pathogenic X chromosome to 100% of their daughters (all daughters are affected) and to 0% of their sons (no father-to-son transmission).
Affected heterozygous females pass the pathogenic X chromosome to 50% of their children, with equal probability of affecting sons and daughters.
There is no skipping of generations in families where affected individuals reproduce.
In disorders with male lethality in utero, the pedigree shows a marked excess of female offspring and documented miscarriages.

Representative real-world conditions

X-Linked Hypophosphatemic Rickets (XLH)

PHEX gene

Phosphate-wasting renal tubular disorder causing rickets, bowed legs, short stature, and dental abscesses resistant to vitamin D.

Incontinentia Pigmenti (IP)

IKBKG (NEMO) gene

Neurocutaneous syndrome causing skin whorls and dental anomalies; generally lethal in hemizygous male fetuses.

Rett Syndrome

MECP2 gene

Neurodevelopmental disorder characterized by progressive loss of motor/speech milestones and stereotypic hand movements; almost exclusively female-viable.

Alport Syndrome (X-Linked)

COL4A5 gene

Hereditary nephropathy causing hematuria, progressive renal failure, and sensorineural hearing loss.

How to analyze this chart

1
Generation I: Affected father Arthur (I-1, X^A Y) is partnered with unaffected mother Beatrice (I-2, X^a X^a).
2
Generation II: Arthur transmits his single X^A chromosome to all three of his daughters: Diana (II-1), Fiona (II-2), and Grace (II-3), who are all affected. His sons are unaffected.
3
Generation III: Affected daughter Diana (II-1) partners with unaffected male and transmits the condition to 50% of her offspring (one affected son III-1 and one affected daughter III-3).
4
The dramatic pattern of an affected father having 100% affected daughters and 0% affected sons is the diagnostic gold standard.

Diagnostic checklist: confirming vs ruling out

Confirming clues

✓ Affected fathers transmit the trait to all daughters and never to sons.
✓ Condition appears in every generation without skipping.
✓ Females typically outnumber males among affected individuals roughly 2:1.
✓ Affected mothers transmit the condition to 50% of sons and 50% of daughters.

Caveats & ruling out

⚠ Any instance of father-to-son transmission excludes X-linked dominant.
⚠ If daughters of an affected father remain unaffected, the condition is not X-linked dominant.

Frequently asked questions

How do you differentiate X-linked dominant from autosomal dominant?

Examine the offspring of affected males. If an affected male passes the trait to a son, it is autosomal. If an affected male passes the trait to all daughters and no sons, it is X-linked dominant.

Why are females often more mildly affected than males in X-linked dominant conditions?

Females undergo random X-inactivation in somatic cells, creating a mosaic tissue population where cells with the active healthy X chromosome compensate for cells expressing the mutant allele.

What indicates male lethality in an X-linked dominant pedigree?

Pedigrees showing only affected females, a high ratio of surviving female to male offspring (2:1), and a history of spontaneous early miscarriages in affected mothers.

Three-Generation Pedigree

Grandparents, parents, and children with a proband.

Autosomal Dominant Pedigree

Trait appears in every generation; one affected parent usually has affected children.

Autosomal Recessive Pedigree

Unaffected carrier parents can have an affected child; the trait may skip generations.