Genetic Principles

Mendelian Inheritance;

Mendelian inheritance describes transmission of pathogenic variants at a single nuclear locus according to segregation and independent assortment. For MRCP Part 1, the key task is to infer mode of inheritance from a pedigree, calculate recurrence risks, and recognise exceptions that mimic or obscure classic patterns. The phenotype results from genotype at a locus: homozygous denotes two identical alleles, heterozygous two different alleles, and compound heterozygous two different pathogenic variants in the same gene, a common mechanism in autosomal recessive disease.

Core Mendelian mechanisms

Mendel’s first law, segregation, states that the two alleles at a locus separate during meiosis so each gamete receives one allele. Mendel’s second law, independent assortment, applies to loci on different chromosomes, or sufficiently far apart on the same chromosome; linked loci violate this principle and recombine according to genetic distance, where 1 centimorgan approximates a 1% recombination frequency. Mendelian disorders usually show a high genotype–phenotype correlation, but observed pedigrees may be modified by new mutation, mosaicism, variable expressivity, penetrance, lyonisation, imprinting, or phenocopies.

Autosomal dominant inheritance

Autosomal dominant disorders require one pathogenic allele for disease expression. They typically show vertical transmission, affect both sexes equally, and allow male-to-male transmission, which excludes X-linked inheritance. An affected heterozygous individual has a 50% risk of transmitting the pathogenic allele to each child, irrespective of sex. Examples include familial hypercholesterolaemia, Marfan syndrome, autosomal dominant polycystic kidney disease, hereditary haemorrhagic telangiectasia, and Huntington disease.

Pathogenic mechanisms include haploinsufficiency, where one normal allele is insufficient; dominant negative effects, where mutant protein impairs the normal protein complex, as in some collagen disorders; and gain-of-function, as in activating receptor or signalling pathway mutations. Severe dominant disorders may be sporadic because affected individuals have reduced reproductive fitness; achondroplasia is a classic example, most cases arising from de novo FGFR3 variants, with paternal age effect.

Autosomal recessive inheritance

Autosomal recessive disease requires biallelic pathogenic variants. Pedigrees often show horizontal clustering among siblings with unaffected parents. Both sexes are affected equally, and consanguinity increases likelihood by raising the chance that both parents carry the same ancestral allele. For two carrier parents, each pregnancy carries a 25% risk of an affected child, 50% risk of a carrier child, and 25% chance of a child inheriting neither familial pathogenic allele. The chance that an unaffected sibling of an affected child is a carrier is 2/3, not 1/2, because the affected genotype has been excluded.

Autosomal recessive disorders commonly reflect loss-of-function in enzymes, transporters, or structural proteins. Examples relevant to adult medicine include cystic fibrosis, haemochromatosis due to HFE variants, Wilson disease, alpha-1 antitrypsin deficiency, phenylketonuria, and many inborn errors of metabolism. Carrier frequency estimation often uses Hardy–Weinberg equilibrium: if disease prevalence is q², mutant allele frequency is q, and carrier frequency is approximately 2q when q is small. For cystic fibrosis in northern European populations, incidence is approximately 1 in 2,500, so q ≈ 1/50 and carrier frequency ≈ 1 in 25.

X-linked inheritance

Pattern	Pedigree clue	Transmission risk	Examples
X-linked recessive	Predominantly affected males; no male-to-male transmission; affected males often related through maternal line	Carrier mother: 50% of sons affected, 50% of daughters carriers. Affected father: all daughters carriers, no sons affected	Haemophilia A/B, Duchenne/Becker muscular dystrophy, G6PD deficiency, X-linked agammaglobulinaemia
X-linked dominant	Affected females often less severe; no male-to-male transmission; affected father transmits to all daughters and no sons	Heterozygous affected mother: 50% of all children affected. Affected father: 100% daughters affected, 0% sons affected	Rett syndrome, incontinentia pigmenti, hypophosphataemic rickets

X-linked recessive disease may affect females if they are homozygous, have Turner syndrome, have skewed X-inactivation, or carry X-autosome translocations disrupting the normal allele. X-linked dominant disorders may be male-lethal, producing an excess of affected females and recurrent miscarriages; incontinentia pigmenti is a classic example. In contrast, affected males with non-lethal X-linked dominant disease transmit the condition to all daughters and no sons.

Less common Mendelian patterns and examination traps

• Y-linked inheritance: father-to-son transmission only; rare and usually related to male infertility genes on the Y chromosome.
• Pseudoautosomal inheritance: genes in pseudoautosomal regions of X and Y recombine and may show apparent autosomal transmission despite sex chromosome location.
• De novo mutation: produces an affected child with unaffected parents; recurrence risk is low but not zero because of possible germline mosaicism.
• Germline mosaicism: clinically unaffected parent carries pathogenic variant in a proportion of gametes; explains recurrence of dominant disorders such as osteogenesis imperfecta or Duchenne muscular dystrophy in siblings.
• Compound heterozygosity: two different pathogenic alleles at the same locus; important in cystic fibrosis and many metabolic disorders.
• Consanguinity: increases autosomal recessive disease risk; first cousins share approximately 1/8 of genes, and offspring have an inbreeding coefficient of 1/16.

In pedigree interpretation, first establish whether transmission is vertical or horizontal, whether male-to-male transmission exists, whether both sexes are equally affected, and whether unaffected individuals transmit disease. Recurrence calculations should be pregnancy-specific and independent unless parental genotype or carrier probability is revised by test results or birth outcome.

Penetrance

Penetrance is the probability that an individual carrying a specified genotype manifests the associated phenotype: formally, P(phenotype | genotype, age, sex, environment). It is a property of a genotype in a defined population and context, not of an individual patient. In postgraduate exam genetics, penetrance is central because it explains why a pathogenic variant may appear to “skip” generations, particularly in autosomal dominant disease, and why genotype-positive relatives may be clinically unaffected at assessment.

Core definitions and classifications

Term	Meaning	Clinical example
Complete penetrance	Virtually all individuals with the genotype develop the phenotype, usually allowing for survival to the relevant age.	Classic familial adenomatous polyposis due to APC pathogenic variants: colorectal adenomas develop in >95% by age 35–40 years if untreated.
Incomplete/reduced penetrance	Some genotype-positive individuals never manifest the phenotype.	BRCA1/BRCA2, HFE C282Y homozygosity, long QT syndrome, retinoblastoma.
Age-dependent penetrance	Cumulative probability of disease rises with age; a young unaffected carrier may still be pre-symptomatic.	Huntington disease, MEN1, Lynch syndrome, hereditary breast/ovarian cancer.
Sex-influenced or sex-limited penetrance	Expression differs by biological sex because of hormonal, anatomical or modifier effects.	BRCA-associated breast cancer risk is substantially lower in males; haemochromatosis has lower clinical penetrance in premenopausal women.

Penetrance must be distinguished from variable expressivity. Penetrance asks whether any phenotype occurs; expressivity describes its severity, organ distribution or age at onset among affected carriers. Neurofibromatosis type 1 is highly penetrant by adulthood, but expressivity ranges from café-au-lait macules alone to plexiform neurofibromas, optic pathway glioma and skeletal dysplasia.

Mechanisms underlying reduced penetrance

Reduced penetrance arises because pathogenic variants rarely act in isolation. Important mechanisms include modifier genes, epigenetic regulation, stochastic developmental effects, mosaicism, sex hormones, environmental exposure and competing mortality. For example, HFE C282Y homozygosity is common in Northern European populations, but clinical haemochromatosis is much less frequent than the genotype: biochemical iron loading occurs in a minority, and clinically significant disease develops in roughly 10–30% of male homozygotes and substantially fewer female homozygotes, reflecting menstruation, pregnancy, diet, alcohol intake and other modifiers.

Variant class also matters. In long QT syndrome, penetrance depends on the gene and variant, QTc threshold used, age, sex and provoking drugs. A genotype-positive individual with a normal resting QTc may still have arrhythmic risk, especially with QT-prolonging drugs, hypokalaemia or fever in specific channelopathies. Thus “unaffected” is not equivalent to “risk-free”.

Quantifying penetrance

Penetrance is often reported as a cumulative incidence by a specified age, commonly using Kaplan–Meier methods in genotype-positive cohorts. This is superior to crude proportions because unaffected young carriers have not yet passed through the risk period. Ascertainment is a major pitfall: families referred because of multiple affected members overestimate penetrance compared with population-based sequencing cohorts.

Condition/genotype	Approximate penetrance/risk figures	Exam relevance
BRCA1	Female breast cancer lifetime risk commonly quoted around 65–80%; ovarian cancer around 35–45% by age 70–80 years.	Autosomal dominant susceptibility with incomplete, age-dependent and sex-influenced penetrance.
BRCA2	Female breast cancer risk around 45–70%; ovarian cancer around 10–20%; male breast cancer risk increased, approximately 5–10% lifetime.	Unaffected male transmission may obscure pedigree recognition.
Lynch syndrome	Colorectal cancer cumulative risk varies by gene: highest for MLH1/MSH2, lower for MSH6/PMS2; broad estimates range from about 10–50% by age 70–80 depending on gene and sex.	Gene-specific penetrance affects surveillance counselling.
RB1 germline pathogenic variant	Retinoblastoma penetrance often around 90%, but low-penetrance alleles occur.	Unaffected carrier parent can transmit severe disease.
MEN1	Clinical/biochemical manifestations in >90–95% by age 40–50 years.	Age-related penetrance justifies early biochemical surveillance.

Pedigree interpretation and recurrence risk

Reduced penetrance modifies Mendelian risk calculations. In a classic autosomal dominant disorder, an affected heterozygous parent transmits the variant to 50% of offspring. If penetrance is 80%, the probability that a child both inherits the variant and manifests disease is 0.5 × 0.8 = 40%. Conversely, an unaffected individual with an affected parent is not automatically genotype-negative. Bayesian reasoning is required: if penetrance is 80%, a child of an affected heterozygote has a prior 50% chance of inheriting the variant; being unaffected lowers, but does not eliminate, this probability, particularly if below the usual age of onset.

In pedigrees, incomplete penetrance may produce an apparent skipped generation and can mimic autosomal recessive inheritance, especially in small families. It also explains discordance between monozygotic twins in some multifactorial or epigenetically influenced conditions, although monozygotic discordance is not synonymous with Mendelian non-penetrance.

Clinical and examination implications

• Testing strategy: predictive testing should be offered according to the familial pathogenic variant, not solely according to current phenotype.
• Surveillance: genotype-positive/phenotype-negative individuals may require screening because penetrance is age-dependent; examples include colonoscopy in Lynch syndrome and endocrine surveillance in MEN1.
• Counselling language: avoid saying a patient “has the disease gene and therefore will get the disease” unless penetrance is effectively complete; instead provide age-, sex- and gene-specific risk.
• Exam trap: penetrance is about the proportion of mutation carriers affected; expressivity is about phenotypic severity among those affected.

Anticipation

Anticipation is the phenomenon whereby a genetic disorder presents at an earlier age, and often with greater severity, in successive generations. In modern clinical genetics it is most characteristically explained by unstable repeat expansion, particularly trinucleotide repeat disorders. For MRCP Part 1, anticipation should immediately suggest Huntington disease, myotonic dystrophy type 1, fragile X syndrome and selected spinocerebellar ataxias.

Mechanistic basis

The molecular substrate is a repetitive DNA tract that is unstable during meiosis, and sometimes mitosis. Repeat sequences may expand because of slipped-strand mispairing during replication, abnormal repair of DNA hairpin structures, and mismatch repair pathway involvement, particularly proteins such as MSH2, MSH3 and MLH1. Once a repeat exceeds a disorder-specific instability threshold, further expansion becomes more likely in gametogenesis; this produces progressively larger alleles in descendants and hence earlier or more severe disease.

The pathogenic consequence depends on repeat location:

• Coding CAG expansions encode polyglutamine tracts, producing toxic gain-of-function proteins. Examples include Huntington disease and several spinocerebellar ataxias. In these disorders, repeat length correlates inversely with age at onset, although imperfectly.
• Non-coding expansions may cause transcriptional silencing, abnormal methylation, RNA toxicity or altered splicing. Examples include fragile X syndrome and myotonic dystrophy type 1.
• Intronic or untranslated repeat expansions often show large intergenerational changes and marked phenotypic variability, making them especially relevant to examination questions on anticipation.

Parent-of-origin effects

Anticipation is not synonymous with imprinting, but several repeat disorders show parent-of-origin bias because repeat instability differs between spermatogenesis and oogenesis. This is a common exam discriminator.

Condition	Repeat and gene	Typical repeat ranges	Anticipation pattern	Key examination point
Huntington disease	CAG expansion in HTT	Normal ≤26; intermediate 27–35; reduced penetrance 36–39; full penetrance usually ≥40; juvenile disease often >60	More marked with paternal transmission	Autosomal dominant; chorea, psychiatric disease and dementia; paternal expansion explains juvenile cases
Myotonic dystrophy type 1	CTG expansion in DMPK	Normal approximately 5–34; premutation 35–49; disease usually ≥50; congenital disease often >1000	Often severe with maternal transmission	Congenital myotonic dystrophy is classically maternally inherited
Fragile X syndrome	CGG expansion in FMR1	Normal 5–44; intermediate 45–54; premutation 55–200; full mutation >200 with methylation	Expansion to full mutation occurs almost exclusively through maternal transmission	Sherman paradox: risk and severity appear to increase in later generations
Spinocerebellar ataxias, e.g. SCA1, SCA2, SCA3	Usually CAG expansions	Thresholds vary by subtype; many pathogenic alleles are in the high 30s to >60 range	Often paternal bias for expansion	Autosomal dominant progressive cerebellar ataxia with variable pyramidal, extrapyramidal or neuropathic features

Clinical interpretation and pedigree recognition

In a pedigree, anticipation is suspected when affected children develop disease earlier than an affected parent, or when a later generation has a more severe phenotype. It is most credible when the disorder is autosomal dominant or X-linked with repeat instability, and when molecular confirmation demonstrates an expanded repeat. Apparent anticipation may be artefactual: families with mild late-onset disease in earlier generations may be under-recognised, whereas severe paediatric cases prompt diagnostic investigation. This ascertainment bias historically complicated interpretation before molecular testing became available.

Anticipation should be distinguished from variable expressivity, where disease manifestations differ among individuals with the same genotype, and from reduced penetrance, where a mutation carrier is clinically unaffected. Huntington disease illustrates the overlap: alleles with 36–39 CAG repeats have reduced penetrance, while larger expansions show high penetrance and earlier onset. Age at onset is influenced by repeat length, but not determined solely by it; modifier genes and environmental factors also contribute.

Diagnostic and counselling implications

The diagnostic test is targeted molecular sizing of the repeat expansion, using PCR-based methods for smaller expansions and Southern blot or triplet-primed PCR for large or methylated expansions, depending on the disorder. Conventional karyotyping is inadequate except historically for fragile site demonstration in fragile X syndrome. Predictive testing for adult-onset disorders such as Huntington disease requires formal genetic counselling, assessment of psychological readiness, and informed consent; testing asymptomatic minors for adult-onset untreatable disease is generally avoided unless there is direct medical benefit.

Risk counselling must incorporate both Mendelian transmission and expansion dynamics. Huntington disease remains a 50% autosomal dominant transmission risk, but paternal transmission increases the probability of a larger CAG repeat and juvenile onset. In fragile X syndrome, a male premutation carrier transmits the premutation to all daughters and no sons, whereas a female premutation carrier may transmit a full mutation, with expansion risk increasing with premutation size. In myotonic dystrophy type 1, an affected mother carries particular risk of having an infant with congenital myotonic dystrophy, characterised by neonatal hypotonia, respiratory failure and feeding difficulty.

For examinations, the high-yield formulation is: anticipation equals earlier onset or increasing severity in successive generations due to unstable repeat expansion, with paternal expansion in Huntington disease and maternal severe congenital disease in myotonic dystrophy type 1.