# Multifactorial Inheritance > Multifactorial inheritance in MBBS Pathology: polygenic traits, the threshold model, empirical recurrence risks and folic acid prevention of NTD. - Canonical URL: https://prepelephant.com/topics/mbbs/pathology/multifactorial-inheritance - Exam / course: MBBS · Subject: Pathology - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Multifactorial Inheritance", PrepElephant, https://prepelephant.com/topics/mbbs/pathology/multifactorial-inheritance ## 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.