Modes of Inheritance in Orthopaedics
- AUTOSOMAL DOMINANT: a single mutant allele causes disease, so the trait appears in EVERY generation, affects BOTH sexes roughly equally, and male-to-male transmission is possible. Most of the high-yield orthopaedic dysplasias are AD: achondroplasia (FGFR3), osteogenesis imperfecta (COL1A1/COL1A2 in ~85%), Marfan syndrome (FBN1), multiple hereditary exostoses (EXT1/EXT2) and neurofibromatosis type 1 (NF1).
- AUTOSOMAL RECESSIVE: two mutant alleles are needed, so the disease often SKIPS generations, parents are unaffected carriers, and CONSANGUINITY increases risk; many metabolic/storage disorders and some severe OI forms are AR.
- X-LINKED RECESSIVE: predominantly affects MALES (who are hemizygous), is transmitted through carrier females, and shows NO male-to-male transmission; an affected father makes ALL his daughters carriers. Examples: Duchenne muscular dystrophy, haemophilia.
- X-LINKED DOMINANT (e.g. X-linked hypophosphataemic rickets, PHEX gene) affects both sexes; an affected father transmits to ALL daughters and NO sons.
- MITOCHONDRIAL inheritance is MATERNAL only (mitochondria come from the egg): an affected mother passes it to all her children, an affected father to none.
- Modifying concepts to know: PENETRANCE (proportion of carriers who show the phenotype), VARIABLE EXPRESSIVITY (severity varies), ANTICIPATION (earlier/severe in successive generations, e.g. trinucleotide repeats), and MOSAICISM (a new mutation in some cells - relevant to apparently sporadic dominant conditions and recurrence risk).
- “Pedigree read: trait in every generation + male-to-male transmission = autosomal dominant; affected males through carrier mothers with no father-to-son = X-linked recessive.
- “Most classic skeletal dysplasias are AD - but many arise as NEW (de novo) mutations (e.g. ~80% of achondroplasia), so a negative family history does not exclude an AD condition.
- “Pair the gene with the disease: FGFR3-achondroplasia, COL1A1/2-OI, FBN1-Marfan, EXT1/2-MHE, NF1-neurofibromatosis, PHEX-X-linked hypophosphataemic rickets.
Affected individuals in every generation (vertical transmission), both sexes affected, and - decisively - male-to-male (father-to-son) transmission, which excludes X-linkage. Roughly 50% of offspring of an affected heterozygote are affected.
Mostly affected males, connected through unaffected carrier females, with NO male-to-male transmission. An affected man's daughters are all carriers; his sons are unaffected. Skipping through carrier females mimics "skipping generations".
Autosomal Dominant Inheritance
In autosomal dominant (AD) inheritance a single mutant allele produces the phenotype. An affected heterozygote has a 50% chance of passing the allele to each child. The pedigree shows the trait in every generation, both sexes affected, and male-to-male transmission (which rules out X-linkage). Many AD conditions arise from new (de novo) mutations, so a negative family history does not exclude them. AD conditions often result from gain-of-function, dominant-negative (a faulty subunit poisoning a multimer, as in OI), or haploinsufficiency mechanisms.
| 0 | 1 | 2 |
|---|---|---|
| Achondroplasia | FGFR3 | Gain-of-function; suppresses growth-plate chondrocytes; ~80% de novo |
| Osteogenesis imperfecta (classic) | COL1A1 / COL1A2 | ~85% AD; dominant-negative effect on type I collagen |
| Marfan syndrome | FBN1 (fibrillin-1) | Connective tissue; tall, arachnodactyly, scoliosis, aortic root |
| Multiple hereditary exostoses | EXT1 / EXT2 | Multiple osteochondromas; small malignant transformation risk |
| Neurofibromatosis type 1 | NF1 (neurofibromin) | Scoliosis, pseudarthrosis of tibia, café-au-lait spots |
| Hereditary multiple epiphyseal dysplasia | COMP and others | Epiphyseal dysplasia; early osteoarthritis |
Autosomal Recessive Inheritance
In autosomal recessive (AR) inheritance both alleles must be mutated. Affected individuals usually have unaffected carrier parents, the trait commonly skips generations, and the risk rises with consanguinity. Two carrier parents have a 25% chance of an affected child, 50% carriers, 25% unaffected. Many inborn errors of metabolism and storage disorders with skeletal manifestations (e.g. mucopolysaccharidoses), some severe/lethal OI forms, and conditions such as diastrophic dysplasia follow AR inheritance.
X-linked & Mitochondrial Inheritance
Males are hemizygous (one X), so a single mutant X-allele causes disease - hence X-linked recessive conditions predominantly affect males, transmitted through carrier females. There is no male-to-male transmission (a father gives his Y to sons), and an affected father's daughters are all carriers. Orthopaedic-relevant examples: Duchenne/Becker muscular dystrophy (dystrophin) and haemophilia A/B (factor VIII/IX - relevant to haemophilic arthropathy).
Key Genetic Concepts
| 0 | 1 | 2 |
|---|---|---|
| Penetrance | Proportion of mutation carriers who show ANY phenotype | Reduced penetrance can make AD conditions appear to skip a generation |
| Variable expressivity | Severity/features vary between carriers of the same mutation | e.g. wide severity range in NF1, Marfan |
| De novo mutation | New mutation absent in parents | ~80% of achondroplasia; negative family history does not exclude AD |
| Anticipation | Earlier onset / worse in successive generations | Trinucleotide-repeat disorders (e.g. myotonic dystrophy) |
| Mosaicism | Mutation present in only some cells (somatic/germline) | Explains some sporadic cases and recurrence risk despite unaffected parents |
| Pleiotropy | One gene -> multiple organ effects | Marfan: skeletal + ocular + cardiovascular |
These concepts drive counselling and recurrence-risk estimates. A patient with an apparently sporadic AD condition may have a de novo mutation (low sibling-recurrence risk but 50% for the patient's own offspring) or germline mosaicism in a parent (raising sibling-recurrence risk). Reduced penetrance and variable expressivity mean an "unaffected" relative may still carry the gene - important before reassuring a family.
Evidence & Key Studies
Osteogenesis imperfecta
- About 85% of osteogenesis imperfecta is caused by DOMINANT autosomal mutations in the type I collagen genes COL1A1 and COL1A2, affecting collagen quantity or structure.
- Newly recognised recessive, dominant and X-linked defects in many other genes (collagen processing, secretion, post-translational modification, osteoblast regulation) also cause OI, complicating classification.
- Manifestations extend beyond bone (cardiovascular, pulmonary, skin, hearing, dentinogenesis imperfecta) - an example of pleiotropy.
Achondroplasia: development, pathogenesis, and therapy
- Autosomal dominant GAIN-OF-FUNCTION mutations in FGFR3 cause achondroplasia and related chondrodysplasias (hypochondroplasia, SADDAN, thanatophoric dysplasia).
- Increased FGFR3 signalling paradoxically SUPPRESSES growth-plate chondrocyte proliferation and maturation, reducing endochondral bone elongation.
- Illustrates how a single recurrent dominant mutation produces a defined skeletal phenotype and how therapies aim to counter excess FGFR3 signalling.
According to PubMed, the OI inheritance/classification facts come from the cited Nature Reviews Disease Primers article and the achondroplasia FGFR3 gain-of-function mechanism from the cited review. The other gene-condition pairings (Marfan-FBN1, MHE-EXT1/2, NF1, X-linked hypophosphataemic rickets-PHEX) are standard, well-established medical genetics used throughout the literature.
Clinical Decision Scenarios
Practise clinical reasoning and management decisions out loud
“You are shown a pedigree in which an orthopaedic condition appears in every generation, affects men and women about equally, and includes a clearly affected father with an affected son. What is the mode of inheritance, what does that tell you, and name some orthopaedic examples.”
“How would you recognise X-linked recessive inheritance on a pedigree, and how does it differ from mitochondrial inheritance? Give an orthopaedic example of each.”
Mnemonics & Memory Aids
DOMINANT
Hook:DOMINANT: One allele, every generation, male-to-male - the AD signature.
X-LINK
Hook:X-LINK: affected males via carrier mothers, never father to son.
Pattern recognition
- AD: every generation, both sexes, male-to-male transmission, 50% risk
- AR: skips generations, carrier parents, consanguinity, 25% risk
- X-linked recessive: mainly males via carrier females, no male-to-male transmission
- Mitochondrial: maternal only, all children of an affected mother
AD conditions (gene)
- Achondroplasia (FGFR3, gain-of-function, ~80% de novo)
- Osteogenesis imperfecta (COL1A1/2, dominant-negative, ~85%)
- Marfan (FBN1); MHE (EXT1/2); NF1 (neurofibromin)
X-linked / other
- Duchenne (dystrophin), haemophilia (FVIII/IX) - X-linked recessive
- X-linked hypophosphataemic rickets (PHEX) - X-linked dominant
- Affected father: all daughters carriers, no affected sons
Modifying concepts
- Penetrance (shows phenotype) vs expressivity (severity)
- De novo mutation + germline mosaicism affect recurrence risk
- Anticipation (trinucleotide repeats); pleiotropy (Marfan)