Mendelian Patterns
Autosomal dominant, autosomal recessive, and X-linked inheritance
The first step in analyzing any inheritance pattern is determining whether the gene is on an autosome (chromosomes 1–22) or a sex chromosome (X or Y), and whether the disease allele is dominant or recessive. These distinctions produce characteristic pedigree patterns that allow inheritance mode to be determined even before molecular analysis.
💡 Beyond Mendel — Non-Classical Inheritance Patterns
Many traits and diseases don't follow simple Mendelian patterns:
Mitochondrial inheritance: Mitochondrial DNA (mtDNA) is maternally inherited — all mitochondria in a new embryo come from the egg, not the sperm. Diseases caused by mtDNA mutations show maternal inheritance (ALL children of an affected mother may be affected; children of an affected father are NEVER affected). Heteroplasmy (mix of mutant and normal mtDNA in a cell) causes variable severity. Examples: MELAS, MERRF, Leber's hereditary optic neuropathy (LHON).
Genomic imprinting: Some genes are expressed only from the maternal copy, others only from the paternal copy. Prader-Willi syndrome: deletion of 15q11-13 on the PATERNAL chromosome (the maternal copy is imprinted/silenced). Angelman syndrome: deletion of the SAME region on the MATERNAL chromosome (the paternal copy is imprinted). Same chromosomal deletion → two completely different syndromes depending on parent of origin.
Polygenic inheritance: Traits controlled by multiple genes (height, skin color, intelligence, blood pressure). Show continuous normal distribution. Environmental factors interact with genetic predisposition. No single gene determines the phenotype.
AD
Autosomal dominant — one copy of the mutant allele causes disease
Autosomal dominant (AD) traits are expressed in heterozygotes — one mutant allele is sufficient to cause the phenotype. Pedigree characteristics: affects every generation (vertical pattern), affected individuals have at least one affected parent (except for new mutations), males and females affected equally, approximately 50% of offspring of an affected parent are affected (if parent is heterozygous, the usual case).
Molecular mechanisms of dominance: (1) Haploinsufficiency — one functional copy is not enough for normal phenotype (BRCA1 haploinsufficiency in breast cancer predisposition). (2) Dominant negative — the mutant protein inhibits the normal protein (most collagen disorders — mutant collagen chains disrupt collagen fibers). (3) Gain of function — the mutant protein has a new, abnormal activity (Huntington's disease — polyglutamine huntingtin has toxic gain-of-function).
Examples: Huntington's disease, Marfan syndrome (FBN1 — fibrillin), achondroplasia (FGFR3 gain-of-function), familial hypercholesterolemia (LDL receptor haploinsufficiency), neurofibromatosis type 1 (NF1 haploinsufficiency).
Memory trick: Autosomal dominant = every generation affected, equal in males/females, 50% risk from affected parent. New mutations can appear (no affected parent). Dominant negative = mutant + normal = both broken.
AR
Autosomal recessive — two copies required for disease
Autosomal recessive (AR) traits require two mutant alleles for expression. Heterozygotes are unaffected carriers. Pedigree characteristics: often skips generations (horizontal pattern — two carrier parents have affected child with no affected parents), consanguinity increases risk, males and females affected equally, 25% of offspring of carrier × carrier cross are affected.
Carrier frequency in the population is much higher than disease frequency (by Hardy-Weinberg: if disease frequency q² = 1/10,000, carrier frequency 2pq ≈ 1/50 — much more common than affected individuals).
Examples: Cystic fibrosis (CFTR), sickle cell disease (HBB), PKU (PAH), Tay-Sachs disease (HEXA), galactosemia (GALT), hereditary hemochromatosis (HFE), Wilson disease (ATP7B), spinal muscular atrophy (SMN1).
Memory trick: Autosomal recessive = skips generations, needs two copies, consanguinity increases risk. Carrier parents appear normal (25% affected, 50% carrier, 25% normal offspring). Hardy-Weinberg: carriers = 2pq (much more common than q²).
XL
X-linked inheritance — genes on the X chromosome
X-linked genes behave differently in males (XY — hemizygous, one copy of each X-linked gene) vs females (XX — two copies, subject to X-inactivation).
X-linked recessive: Pedigree characteristics — mostly males affected (hemizygous males express recessive allele with no normal allele to compensate). Carrier females are usually unaffected (normal X compensates). Affected father CANNOT pass X-linked recessive trait to sons (sons get Y from father, X from mother). Carrier mothers pass the allele to 50% of sons (who are affected) and 50% of daughters (who are carriers). Examples: Hemophilia A (F8), Hemophilia B (F9), Duchenne muscular dystrophy (DMD), red-green color blindness (OPN1LW, OPN1MW), G6PD deficiency, Fabry disease.
X-linked dominant: Affected father passes to ALL daughters (but NO sons). Carrier mothers pass to 50% of sons and 50% of daughters. Often more severe in males (who can't compensate). Examples: Rett syndrome (MECP2 — lethal in males), hypophosphatemia (PHEX), incontinentia pigmenti.
Memory trick: X-linked recessive = mostly males affected, carrier mothers, NO father-to-son transmission. 'From grandfather, through carrier mother, to grandson — skips a generation.' X-linked dominant = affected father passes to ALL daughters.
🔬 Applied Scenario — Pedigree Analysis in Clinical Genetics
Pedigree analysis allows determination of inheritance pattern and calculation of disease risk:
A
Hemophilia A — X-linked recessive pedigree. Queen Victoria of England was a carrier of hemophilia A (F8 mutation). Her son Leopold had hemophilia (affected male). Her daughters Alice and Beatrice were carriers — they passed the allele to several royal families of Europe (explaining the 'royal disease' in multiple European royal houses in the early 20th century). The pedigree shows: no father-to-son transmission (Victoria's husband Albert was unaffected), male-to-female carrier transmission (Leopold's daughters were obligate carriers), and the characteristic skip-generation pattern.
B
Calculating risk for autosomal recessive disease. A couple wants to know their risk of having a child with CF. Both are of Northern European descent and have been tested — both are carriers (Ff). Risk calculation: Ff × Ff → 1/4 FF, 2/4 Ff, 1/4 ff. Each pregnancy has a 1/4 (25%) chance of being affected (ff), 1/2 (50%) chance of being a carrier, 1/4 (25%) chance of being unaffected non-carrier. These risks apply INDEPENDENTLY to each pregnancy — if they have already had one affected child, the next pregnancy STILL has 25% risk.
C
Prader-Willi vs Angelman — imprinting in practice. A child presents with hypotonia, obesity, hypogonadism, intellectual disability, and obsessive eating → Prader-Willi syndrome is suspected. Testing shows deletion of chromosome 15q11-13. Determining whether this is Prader-Willi (paternal deletion) or Angelman (maternal deletion) requires methylation analysis — the same chromosomal region shows different methylation patterns depending on parent of origin. Molecular diagnosis requires knowing which copy is deleted, not just that a deletion exists.
D
New mutations in autosomal dominant disease. A child is born with achondroplasia (autosomal dominant short-limb dwarfism, FGFR3 gain-of-function mutation) to two parents of average height with no family history. This is not a pedigree error — approximately 80% of achondroplasia cases represent new (de novo) mutations in the FGFR3 gene. In autosomal dominant conditions where affected individuals have reduced reproductive fitness, a high proportion of cases are de novo mutations. The recurrence risk for unaffected parents is low (~1/40,000, the de novo mutation rate at this hotspot), but the affected child's children have 50% risk.
📌 Exam Application
Inheritance pattern questions require recognizing pedigree features and calculating risks:
1. Autosomal dominant: Every generation affected, both sexes, 50% risk from affected parent, can be de novo mutations. Mechanisms: haploinsufficiency, dominant negative, gain of function.
2. Autosomal recessive: Skips generations, both sexes, 25% risk when both parents are carriers, consanguinity increases risk.
3. X-linked recessive: Mostly males affected, no father-to-son transmission, carrier mothers, ~50% of sons affected, ~50% of daughters are carriers.
4. Mitochondrial: Maternal inheritance — all children of affected mother may be affected; children of affected father never affected.
5. Imprinting: Same region deleted → different syndrome depending on parent of origin. Prader-Willi = paternal 15q11-13 deletion. Angelman = maternal 15q11-13 deletion.
⚠️ The Most Common Inheritance Pattern Mistakes
No father-to-son transmission is SPECIFIC to X-linked inheritance. The single most diagnostic pedigree feature of X-linked recessive inheritance is that affected fathers NEVER pass the trait to sons (sons receive Y from father, X from mother). If you see an affected son whose father is affected → cannot be X-linked recessive (must be autosomal). This is the single most important pedigree clue to identify X-linked inheritance.
Carrier females can be AFFECTED in X-linked recessive disease (manifesting carriers). Due to skewed X-inactivation, some carrier females inactivate the normal X in a disproportionate number of cells → express the mutant allele predominantly → partial or full disease phenotype. DMD carrier females can develop cardiomyopathy. Hemophilia carrier females can have reduced clotting factor levels. 'Carrier females are unaffected' is an oversimplification.
Mitochondrial inheritance: ALL children of affected mothers may be affected — not 50%. Unlike nuclear gene inheritance where only 50% of children get a dominant allele, mitochondrial inheritance is maternal — ALL children of an affected mother inherit her mitochondria. However, due to heteroplasmy (variable proportions of mutant and normal mtDNA in different cells/tissues), not all affected mothers' children will show disease — severity varies. The key point: children of affected FATHERS are never affected by mtDNA mutations.
✓ Quick Self-Test
1. What are the pedigree characteristics of autosomal dominant inheritance?
2. Why are X-linked recessive traits more common in males than females?
3. What is the expected ratio of affected, carrier, and unaffected offspring from two carrier parents (Aa × Aa) for an autosomal recessive trait?
4. What is genomic imprinting and how does it explain Prader-Willi and Angelman syndromes?
5. What distinguishes mitochondrial inheritance from autosomal dominant inheritance?
Answers:
1. Autosomal dominant pedigree features: (1) trait appears in every generation (vertical pattern), (2) both males and females are equally affected, (3) an affected individual has at least one affected parent (unless it is a new mutation), (4) approximately 50% of offspring of an affected parent are affected (assuming the parent is heterozygous), (5) unaffected individuals do not pass the trait to their children.
2. X-linked recessive genes are on the X chromosome. Males have only one X chromosome (XY — hemizygous) — any recessive allele on the X is expressed because there is no second X to provide a dominant allele. Females have two X chromosomes (XX) — the recessive allele is usually masked by the normal dominant allele on the second X. Males only need one copy of the mutant allele to be affected; females need two copies.
3. From Aa × Aa: 1/4 AA (unaffected, non-carrier), 2/4 Aa (unaffected carriers), 1/4 aa (affected). As phenotype ratios: 3/4 unaffected (including 1/3 non-carriers and 2/3 carriers among the unaffected) and 1/4 affected.
4. Genomic imprinting is the epigenetic silencing of one parental copy of specific genes — some genes are only expressed from the maternal copy, others only from the paternal copy. Prader-Willi syndrome and Angelman syndrome are both caused by deletion of the same chromosomal region (15q11-13) but differ in which parent's copy is deleted: Prader-Willi = paternal deletion (maternal copy is imprinted/silenced, so no functional copy remains). Angelman = maternal deletion (paternal copy is imprinted, so no functional copy remains). Same deletion, completely different syndromes.
5. Mitochondrial inheritance: (1) strictly maternal — all children of an affected mother may inherit the trait; children of an affected father are never affected (mitochondria come from the egg, not sperm). (2) All children of an affected mother are at risk (vs 50% for autosomal dominant). (3) Variable expressivity due to heteroplasmy (variable proportions of mutant mtDNA). Autosomal dominant: affects both sexes equally, 50% risk from affected parent, inheritance from either parent.