Multifactorial Inheritance
Learning objectives
- LO 1Explain how common disorders arise from many genes plus environment, and how twin studies and family risk show it.
- LO 2Describe how neural tube defects and cleft lip/palate present, what causes them, and how they recur.
- LO 3Tell an isolated birth defect from one that is part of a syndrome.
MAPThe liability threshold model
LO 1Complex (multifactorial) inheritance
Q1. How common are multifactorial conditions?
| Category | Frequency |
|---|---|
| Single-gene | 1/500 – 1/100 |
| Chromosomal | 0.5–0.7% |
| Multifactorial | 20–40% (most common) |
| Mitochondrial | Rare |
- Most congenital malformations and common pediatric/adult diseases = genes × environment
- Familial aggregation: occurs more often in some families than in the population
- Highest recurrence in first-degree relatives (share 50% of genes) and MZ twins
| Causes of malformations in affected infants | % |
|---|---|
| Unknown | 43.2 |
| Familial / multifactorial | 37.5 |
| Chromosomal | 10.1 |
| Teratogens | 3.2 |
| Single mutant genes | 3.1 |
| Uterine factors | 2.5 |
| Twinning | 0.4 |
In-house exam
Multifactorial conditions are the most common category (20–40%). Examples: oral clefts (1/500–1/1,500), NTDs (1/2,500), congenital heart defects (1/100), diabetes (1/10), schizophrenia (1/100).
Q2. How do twin studies separate genes from environment?
- Compare concordance (both twins affected) in MZ twins (100% genes + uterus) vs DZ twins (50% genes + uterus)
| Etiology | MZ concordance | DZ concordance |
|---|---|---|
| Purely genetic | 100% | <100%, like ordinary siblings |
| Purely environmental | Equal in MZ and DZ | |
| Multifactorial | MZ > DZ, but MZ <100% | |
- MZ twins reared apart separate genes from shared environment; striking concordance even for behavioral traits (alcoholism)
- Limitations: somatic mutations after the split, epigenetic differences, random X-inactivation in females, few twins reared apart, ethical issues
High yield
Oral cleft: MZ 35–40%, DZ 4–5% → not purely genetic (MZ <100%) and not purely environmental (MZ ≠ DZ) → multifactorial.
Trap
MZ twins aren't perfect genetic copies: post-split somatic mutations, epigenetics and X-inactivation make them differ.
Q3. What makes type 2 diabetes a model adult multifactorial disease?
- Diabetes = type 1, type 2 (80–95%), gestational; insulin secretion defects + insulin resistance
- ≈1 in 10 Americans; many more prediabetic; higher in African American, Alaska Native/American Indian, Asian American, Hispanic, Native Hawaiian/Pacific Islander populations
- Global epidemic → environment matters: BMI, gestational diabetes, diet, BP, stress, lipids, inactivity
- >150 loci; each adds a small risk, and risk adds up; not every carrier gets diabetes. Genes: TCF7L2, ABCC8, CAPN10, GLUT2, GCGR, PTPN1
| Family history | Lifetime DM2 risk |
|---|---|
| One parent affected | 40% |
| Both parents affected | 70% |
| First-degree relative | 3× the population risk |
| Twins | MZ ≈70%, DZ 20–30% |
In-house exam
Both parents with type 2 diabetes → ≈70% lifetime risk; lifestyle modification (exercise, weight loss) is the key intervention.
Analogy — filling a bucket
Each risk allele and each environmental exposure adds a cup of water to a bucket. Disease appears only when the bucket overflows (the threshold). Relatives start with a fuller bucket.
ApplyLO 1 application questions
A1A trait has 100% MZ and 50% DZ concordance. Genetic, environmental or multifactorial?
A2A disorder has MZ and DZ concordance of 20% each. Interpretation?
LO 2Neural tube defects and oral clefts
Q1. What is a neural tube defect, and what types exist?
- Failure of the neural tube to develop or close → brain and spine defects
- The neural tube forms 18–30 days after conception (before most women know they're pregnant)
- ≈90% isolated; causes = genetics + nutrition + environment; managed by a team (neurosurgery, orthopedics, urology, nursing)
| Type | Defect | Outcome |
|---|---|---|
| Anencephaly | Brain fails to develop (cranial end) | Fatal |
| Encephalocele | Brain herniates through a skull defect | Rare, often fatal; ID and neuro deficits; consider a syndrome |
| Spina bifida occulta | Vertebral arch defect only | Mild (tuft of hair, dimple) |
| Meningocele | Meninges herniate | Milder |
| Myelomeningocele | Meninges + cord/nerves herniate | Paralysis, bladder/bowel, leg deformities |
Anterior vs posterior defects
- Anterior → anencephaly; posterior → spina bifida
Spina bifida spectrum
- Occulta → meningocele → myelomeningocele
Neural folds fusing
- Ridges fold up and fuse into the tube
Step 1
Open NTDs → ↑ maternal serum AFP and ↑ amniotic AChE. Myelomeningocele often comes with Chiari II and hydrocephalus. Anencephaly → polyhydramnios (no swallowing).
Q2. What causes NTDs, and how are they prevented?
Genetic
- MTHFR 677C>T (best studied): ↓ folate processing → ↑ homocysteine; mothers of affected infants often homozygous
- GRHL3, CELSR1 (planar cell polarity), VANGL2 (neurulation)
- Homozygous MTHFR in adults: hearing loss, heart disease, stroke
Environmental
- Maternal obesity, poorly controlled diabetes
- Drugs: phenytoin, carbamazepine, valproate; antifolate methotrexate
- Excess alcohol; hyperthermia
- Low folic acid diet (the "home run")
| Who | Folic acid |
|---|---|
| All women of reproductive age | 0.4–0.8 mg/day (prevents up to 70% of NTDs; half of pregnancies are unplanned) |
| Prior affected pregnancy | 4 mg/day from ≥1 month before conception through the 1st trimester |
- US grain fortification (1998) → 27–35% drop in NTDs; incidence now ≈1/2,500 (was 1/1,000); highest among Hispanic women in the US
- Prenatal: screen with fetal markers; diagnose by ultrasound or amniocentesis
Fortification works
- Prevalence fell after mandatory fortification
In-house exam
Diet low in folic acid, maternal obesity, excess alcohol and anticonvulsants (valproate, phenytoin, carbamazepine) are the tested NTD risk factors. MTHFR 677C>T → ↑ homocysteine.
Memory tool
Folic acid doses: "0.4 for everyone, 4 for the one who's had one."
Q3. How do oral clefts form, and how often do they recur?
- Failure of the frontonasal (medial nasal) process to fuse with the maxillary process; lips close by ≈8 weeks and palate by ≈12 weeks
- Incidence: 1/1,000 Caucasians, 1.7/1,000 Japanese, 0.4/1,000 African Americans
- Concordance: MZ 35–40%, DZ 5%
- Recurrence: 5% after one affected child, 8% after two
- 85% isolated, 15% syndromic; managed by a craniofacial team (plastic surgery, ENT, audiology, speech, dental)
Facial processes
- Upper lip forms by week 6 from medial nasal + maxillary processes
Genes (non-syndromic)
- TGFA, TGFB3, IRF6, VAX1, FOXE1, MYH9, MAFB, ABCA4, BMP4, FGFR2, CRISPLD2, SUMO1, MTHFR, PVRL1, FGF8, PDGFC, GSTT1, MSX1
Prenatal exposures
- Smoking (worse with maternal GSTT1 and infant MSX1 genotypes)
- Alcohol; folic acid deficiency
- Steroids; anticonvulsants (phenytoin, valproate)
High yield
Recurrence rises with each affected child (5% → 8%): a hallmark of multifactorial ("complex") inheritance that single-gene risks don't show.
Step 1
Cleft lip = failed fusion of maxillary + medial nasal processes (primary palate). Cleft palate = failed fusion of the palatine shelves (secondary palate).
ApplyLO 2 application questions
A1A woman whose first child had spina bifida plans another pregnancy. Folic acid dose and timing?
A2A couple has two children with isolated cleft lip. Risk for the next child?
LO 3Isolated vs syndromic defects
Q1. Why does it matter if a defect is part of a syndrome?
- Isolated defects follow multifactorial recurrence (≈2–5%); syndromic defects follow the syndrome's rule (AR 25%, AD 50%, teratogen avoidable)
- Clues: growth retardation, developmental delay, a cluster of minor anomalies, other major anomalies, family history across generations, a known exposure
- Approach: listen to the family, take a full history, examine for minor anomalies (inner lower lip, palpebral fissures, philtrum, nails), get old records and photos, and reexamine when stuck
Memory tool
"Cleft + something else" or "NTD + something else" = look for a syndrome before quoting a multifactorial risk.
Q2. Which syndromes appeared in the lecture?
| Syndrome | Cause / inheritance | Key features | Recurrence |
|---|---|---|---|
| Fetal alcohol syndrome | Teratogen: alcohol | Growth retardation; microcephaly, short palpebral fissures, smooth philtrum, thin upper lip, hypertelorism; small 5th nails; cardiac/renal/eye; ID, ADHD, speech problems; can have spina bifida | Preventable; clinical diagnosis (history + exam, no genetic test) |
| Meckel-Gruber | AR (MKS1, TMEM67, CEP290, others; ciliopathy) | Encephalocele + CNS anomalies, polydactyly, polycystic kidneys, small eyes, oral cleft, poor survival | 25%; carrier testing both parents |
| Fetal hydantoin (Dilantin) | Teratogen: phenytoin | Growth retardation; hypertelorism, depressed nasal bridge, short nose, oral cleft; hypoplastic distal phalanges, small nails; learning/attention problems | Avoidable (drug choice) |
| van der Woude | AD, IRF6 (chr 1) | Lower lip pits + cleft lip ± palate + hypodontia; CHD, finger webbing; variable expressivity | 50% |
In-house exam
Cleft lip + mother with epilepsy on phenytoin → fetal hydantoin syndrome. Cleft + lower-lip pits across generations → van der Woude (IRF6, AD). Toddler with spina bifida, short palpebral fissures and thin philtrum → FAS (clinical diagnosis).
Trap — penetrance vs expressivity
van der Woude shows variable expressivity (lip pits only vs full cleft) with high penetrance. Pits can hide on the inner lower lip; a "normal" relative may be affected.
Step 1
Valproate → NTDs (spina bifida); carbamazepine → NTDs; phenytoin → fetal hydantoin; methotrexate → NTDs (antifolate). All anticonvulsant pregnancies get high-dose folic acid.
ApplyLO 3 application questions
A1A newborn has an encephalocele, polydactyly and enlarged cystic kidneys and dies days later. Recurrence?
A2A child has bilateral cleft lip; his father has two depressions on his lower lip and missing teeth. Diagnosis and risk to siblings?
IDImage ID
Image 1 — spine
Image 2 — chart
Image 3 — embryo
Image 4 — diagram
MCQExam-style questions
1Which category of genetic disease is most common in the population?
2A condition shows 38% concordance in MZ twins and 5% in DZ twins. Best interpretation?
3Why might monozygotic twins differ for a genetic trait?
4A woman with epilepsy on valproate wants to conceive. Which birth defect is her baby at highest risk of, and what reduces it?
5The MTHFR 677C>T variant raises NTD risk mainly by:
6A woman's previous pregnancy had a myelomeningocele. Recommended folic acid?
7When does the neural tube normally close?
8A couple has had two children with isolated cleft lip and palate. Recurrence risk for the next child?
9A child has cleft lip, hypertelorism, a short nose and small, hypoplastic fingernails. His mother takes an anticonvulsant for epilepsy. Most likely diagnosis?
10A child with cleft lip has a father with lower-lip pits and missing teeth. Gene and inheritance?
11A newborn has an encephalocele, polydactyly and polycystic kidneys. Recurrence risk for the parents?
12Both parents of a 30-year-old have type 2 diabetes. Her approximate lifetime risk?
!Where sources disagree
| Item | In-house | Correct / Step 1 |
|---|---|---|
| Encephalocele | Slide: "herniation of the brain through the spine" | Herniation through a skull defect |
| DM2 gene | Slide: "CF7L2" | TCF7L2 |
| van der Woude | "Variable expressivity / penetrance 100%" | Variable expressivity with high (incomplete, ≈90%+) penetrance |
| Single-gene frequency | Slide 1/500–1/100; class notes 0.2–1% | Same range, written two ways |
| DM2 share | 80–90% (one slide) vs 90–95% (CDC slide) | ≈90–95% |