Multifactorial Inheritance
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Direct answer
Diabetes, essential hypertension, schizophrenia, coronary artery disease, neural tube defects and cleft lip all share a genetic logic Mendel never described: multifactorial inheritance, in which many genes of small effect plus environment generate a continuous distribution of liability that crosses into disease only when enough factors accumulate — the threshold model. Inheritance is familial but not Mendelian: recurrence risks are empirical, usually quoted near the square root of the population prevalence and falling typically in the 2-5 per cent band for first-degree relatives. Risk rises with the number of affected relatives, the severity in the proband, and — a favourite discriminator — the sex least often affected, because that sex must carry more liability to cross the threshold.
What you must remember
- Threshold model in one sentence: liability (genetic plus environmental) is normally distributed in the population, and disease appears only beyond a threshold; relatives of affected persons have shifted curves, so more of them cross.
- Recurrence risk rules: empirical, about 2-5 per cent for most multifactorial disorders; rises with each additional affected first-degree relative; rises with severity in the proband (a bilateral cleft confers higher risk than unilateral); rises with consanguinity — relevant in Indian communities where cousin marriage is practised in some groups.
- The pyloric stenosis lesson: males affected about five times more often than females; relatives of an affected female carry the higher risk, because she needed more liability to cross the threshold — the cleanest exam demonstration of the sex effect.
- Hirschsprung disease behaves the same way: risk to relatives falls as the aganglionic segment lengthens, and is higher for relatives of affected girls.
- Twin evidence: monozygotic concordance well below 100 per cent (typically 20-50 per cent) with dizygotic concordance far lower — the signature that separates multifactorial disease from single-gene (near 100 per cent) and environmental disease (near 0 per cent).
- Neural tube defects — the preventable example: periconceptional folic acid lowers recurrence substantially; the MRC vitamin trial of 1991 found high-dose folic acid reduced recurrent NTD by about 70 per cent — a number worth memorising.
- Indian context: NTD burden in India is among the world's highest, and periconceptional folic acid (400 micrograms for all planning pregnancy; a higher 4-5 mg dose after an affected child, per guidance) is the counselling anchor of every genetics viva.
Counselling after a neural tube defect
A couple whose first child had a lumbosacral myelomeningocele asks about the next pregnancy. The counselling writes itself from the threshold model. First, quantify: recurrence risk for NTD is a few per cent — well above the population baseline but far below Mendelian quarters — and higher still if any relative is affected or if the mother has diabetes or takes valproate, both multifactorial contributors. Second, prevent: the mother should start folic acid at least one month before conception and continue through the first trimester, at the higher dose used for recurrence prevention; the MRC trial's roughly 70 per cent reduction in recurrence is the evidence to cite. Third, monitor: maternal serum alpha-fetoprotein at 15-20 weeks with a detailed anomaly scan, since screening detects open defects efficiently. The same three-step frame — empirical risk, modifiable contributors, targeted antenatal detection — serves every multifactorial counselling station, from cleft lip to congenital heart disease.
Where students slip
Multifactorial and polygenic are used loosely; strictly, polygenic describes the many-gene architecture while multifactorial insists on environmental interaction — say both precisely and you earn the distinction. Students also misapply the square-root rule as if it were a law rather than an approximation for rare conditions. Isolated cleft palate follows a different threshold from cleft lip with or without cleft palate — they are distinct multifactorial entities and are not counselled together. Finally, do not quote 25 or 50 per cent risks here; Mendelian arithmetic applied to multifactorial disease is the single most penalised error in the genetics viva.
Frequently asked questions
What is the threshold model of multifactorial inheritance?
Liability from many genes plus environment is continuously distributed; disease manifests only in individuals whose total liability crosses a threshold.
What recurrence risk is quoted for a multifactorial disorder?
An empirical risk, roughly 2-5 per cent for first-degree relatives and approximating the square root of population incidence for rare conditions.
Why do relatives of affected females run higher risk in pyloric stenosis?
Females are less often affected, so an affected female carries a higher liability load — and transmits more of it to relatives.
How effectively does folic acid prevent recurrent neural tube defects?
High-dose periconceptional folic acid reduces recurrence by about 70 per cent, per the landmark MRC vitamin trial.
What twin concordance pattern suggests multifactorial inheritance?
Monozygotic concordance of roughly 20-50 per cent with much lower dizygotic concordance — above environmental, below monogenic expectations.