Page 2 of 6
AN74.1-4 | Patterns of Inheritance — Part 1
CLINICAL SCENARIO
A 28-year-old woman from Chennai visits the genetics clinic during her first pregnancy. Her husband has a family history of cystic fibrosis — his sister died of the condition at age 12. The woman wants to know: 'What are the chances our baby will be affected?' To answer her question, you need to understand how genes are transmitted from parent to offspring, how recessive alleles can lurk silently for generations, and how pedigree analysis can quantify risk. This is not abstract biology — it is the foundation of genetic counselling, a skill every clinician in India increasingly needs as consanguineous marriages remain common in many communities and genetic testing becomes more accessible.
WHY THIS MATTERS
Patterns of inheritance form the bedrock of clinical genetics. Every time you encounter a patient with a suspected hereditary condition — whether it is a child with unexplained short stature, a young adult with recurrent pulmonary infections, or a family clustering of diabetes and hypertension — your ability to identify the inheritance pattern determines the diagnostic pathway, the counselling you offer, and the preventive strategies you recommend. In Indian clinical practice, autosomal recessive disorders are disproportionately common due to the prevalence of consanguineous marriages in certain communities (estimated at 11% nationally, reaching 20-30% in parts of Tamil Nadu, Karnataka, and Andhra Pradesh). Sickle cell disease affects tribal populations across central India; thalassaemia is a major public health burden with carrier frequencies of 3-17% depending on the community. Understanding inheritance patterns is therefore not merely an academic requirement under AN74.1-74.4 — it is an essential clinical competency for practice in the Indian healthcare landscape.
RECALL
Before diving into inheritance patterns, refresh your understanding of these foundational concepts. Genes are segments of DNA that encode proteins or functional RNA molecules; they occupy specific positions (loci) on chromosomes. Humans have 23 pairs of chromosomes — 22 pairs of autosomes and one pair of sex chromosomes (XX in females, XY in males). Each individual carries two copies (alleles) of every autosomal gene, one inherited from each parent. An individual is homozygous when both alleles are identical and heterozygous when the alleles differ. The genotype refers to the specific allelic combination, while the phenotype is the observable characteristic that results from genotype-environment interaction. A dominant allele expresses its phenotype when present in a single copy (heterozygous state), whereas a recessive allele requires two copies (homozygous state) for phenotypic expression. Penetrance describes the proportion of individuals with a given genotype who actually display the expected phenotype, and expressivity describes the degree to which a penetrant gene manifests its phenotype among affected individuals.
Autosomal Dominant Inheritance
In autosomal dominant (AD) inheritance, a single copy of the mutant allele on an autosome is sufficient to produce the phenotype. The affected individual is typically heterozygous (Aa), possessing one normal allele (a) and one mutant allele (A). Key characteristics of AD pedigrees include: (1) the trait appears in every generation without skipping (vertical transmission); (2) an affected individual has at least one affected parent (unless the mutation arose de novo); (3) males and females are equally affected; (4) an affected heterozygote married to an unaffected individual produces affected and unaffected children in approximately equal proportions (50% risk for each offspring); and (5) unaffected individuals do not transmit the trait to their children.
Figure: Autosomal Dominant Inheritance
Several important modifiers complicate the clean AD pattern. Incomplete penetrance means that some individuals who carry the dominant allele do not show the phenotype — the trait appears to 'skip' a generation, creating diagnostic confusion. For example, retinoblastoma (RB1 gene mutation) has approximately 90% penetrance; about 10% of carriers remain unaffected but can still transmit the allele to their children. Variable expressivity means that affected individuals show different severities of the same condition. Neurofibromatosis type 1 (NF1) is the classic example: within a single family, one member may have only café-au-lait spots while another has extensive neurofibromas, optic gliomas, and skeletal abnormalities — all carrying the identical NF1 mutation. Anticipation refers to the phenomenon where certain AD diseases become more severe or appear at an earlier age in successive generations, classically seen in trinucleotide repeat disorders such as myotonic dystrophy and Huntington disease. The molecular basis is the expansion of unstable trinucleotide repeats (CTG in myotonic dystrophy, CAG in Huntington disease) during meiosis, particularly during transmission through the affected parent.
De novo mutations account for a significant proportion of AD disorders. In achondroplasia, approximately 80% of cases arise from new mutations in the FGFR3 gene, with advanced paternal age being a risk factor. This means that a child with achondroplasia typically has unaffected parents, but the child's own offspring will face a 50% risk of inheriting the condition. Other clinically important AD conditions include Marfan syndrome (FBN1 gene — tall stature, arachnodactyly, aortic root dilatation, lens subluxation), familial hypercholesterolaemia (LDLR gene — elevated LDL cholesterol, premature coronary artery disease, tendon xanthomata), and polycystic kidney disease type 1 (PKD1 gene — bilateral renal cysts, hypertension, progressive renal failure).
Achondroplasia deserves detailed attention as it is specifically named in the NMC competencies (AN74.4). This is the most common form of short-limbed dwarfism, with an incidence of approximately 1 in 15,000-40,000 live births. It results from a gain-of-function mutation in the fibroblast growth factor receptor 3 (FGFR3) gene on chromosome 4p16.3. Over 97% of cases are caused by the same point mutation (G380R — glycine to arginine substitution at position 380). The constitutively activated FGFR3 receptor inhibits endochondral ossification at the growth plates, leading to the characteristic features: rhizomelic (proximal) limb shortening, macrocephaly with frontal bossing and midface hypoplasia, trident hand configuration, exaggerated lumbar lordosis, and a narrowed foramen magnum (which can cause cervicomedullary compression in infancy). Intelligence is normal. Homozygous achondroplasia (inheriting the mutation from both parents) is lethal, a critical counselling point when both parents are affected.
Autosomal Recessive Inheritance and Pedigree Analysis
In autosomal recessive (AR) inheritance, the phenotype manifests only when both alleles at a locus are mutant (homozygous aa). Heterozygous carriers (Aa) are phenotypically normal but can transmit the mutant allele to their offspring. The hallmarks of AR pedigrees are: (1) the trait often appears in siblings but not in parents (horizontal pattern); (2) both parents of an affected child are typically unaffected carriers; (3) when two carriers marry, each pregnancy carries a 25% risk of an affected child, a 50% chance of a carrier, and a 25% chance of a homozygous normal individual; (4) males and females are equally affected; (5) consanguinity (marriage between blood relatives) increases the probability of producing affected offspring because related individuals are more likely to share the same rare recessive allele inherited from a common ancestor.
Figure: Autosomal Recessive Inheritance and Pedigree Analysis
Consanguinity and AR disease in India: India has one of the highest rates of consanguineous marriage globally. Uncle-niece marriages are traditional in parts of southern India (particularly among Hindu communities in Andhra Pradesh, Tamil Nadu, and Karnataka), while first-cousin marriages are common in Muslim communities across the country. The coefficient of inbreeding (F) for first-cousin marriages is 1/16 (0.0625) and for uncle-niece marriages is 1/8 (0.125). This means that for any rare recessive allele with a carrier frequency of q, the risk of a consanguineous couple having an affected child is substantially higher than for an unrelated couple. This explains the higher incidence of rare AR disorders in consanguineous communities, including certain metabolic disorders, deafness, and congenital malformations.
Cystic fibrosis (CF) is the most common lethal AR disorder in Caucasian populations (carrier frequency ~1 in 25, incidence ~1 in 2,500 live births). While less common in India, CF is increasingly recognised in Indian children, with estimates suggesting a carrier frequency of approximately 1 in 40-100 in certain populations. The disease results from mutations in the CFTR (cystic fibrosis transmembrane conductance regulator) gene on chromosome 7q31.2, which encodes a chloride channel protein. Over 2,000 mutations have been identified; the most common worldwide is ΔF508 (deletion of phenylalanine at position 508), though Indian patients show a different mutation spectrum with relatively lower ΔF508 frequency. The defective CFTR protein leads to thick, viscid secretions in multiple organ systems: recurrent pulmonary infections progressing to bronchiectasis (the major cause of morbidity and mortality), pancreatic insufficiency with malabsorption and failure to thrive, meconium ileus in neonates (present in 10-15% of CF newborns), male infertility due to congenital bilateral absence of the vas deferens (CBAVD), and elevated sweat chloride (the basis of the diagnostic sweat test, with values >60 mmol/L being diagnostic).
Pedigree chart construction (AN74.2) is a fundamental skill. Standard symbols include: squares for males, circles for females, filled symbols for affected individuals, half-filled for carriers (when known), horizontal lines connecting mated pairs, vertical lines descending to offspring, and double horizontal lines indicating consanguinity. Roman numerals designate generations (I, II, III) and Arabic numerals identify individuals within each generation. The proband (index case) is indicated by an arrow. When analysing a pedigree, systematically check: (1) Is the trait more common in males or females? (2) Does it appear in every generation or skip generations? (3) Are all children of affected individuals affected? (4) Is there consanguinity? These observations allow you to determine the most likely mode of inheritance and calculate recurrence risks for genetic counselling.
Vitamin D-resistant rickets (X-linked hypophosphataemia, XLH) is specifically named in AN74.4 but note that it follows X-linked dominant inheritance, not AR. It results from mutations in the PHEX gene on Xp22.1, leading to renal phosphate wasting and defective mineralisation of bone. Affected children present with bowed legs, short stature, and radiographic features of rickets that do not respond to standard vitamin D supplementation. An affected father transmits the condition to all daughters but no sons; an affected mother transmits it to 50% of sons and 50% of daughters.
X-linked and Mitochondrial Inheritance
Comparison of X-linked Recessive, X-linked Dominant, and Mitochondrial Inheritance
| Feature | X-linked Recessive | X-linked Dominant | Mitochondrial |
|---|---|---|---|
| Males affected | Yes (hemizygous) | Yes | Yes (if mother affected) |
| Females affected | Rarely (homozygous or skewed X-inactivation) | Yes (heterozygous) | Yes (if mother affected) |
| Male-to-male transmission | Never | Never | Never (paternal mtDNA eliminated) |
| Carrier females | Yes (heterozygous) | Not applicable | Not applicable |
| Affected mother to children | All sons affected, all daughters carriers | 50% of all children affected | All children affected |
| Examples | Haemophilia A, Duchenne MD, G6PD deficiency | Rett syndrome, vitamin D-resistant rickets | LHON, MELAS, MERRF |
X-linked recessive (XLR) inheritance involves genes on the X chromosome. Because males have only one X chromosome (hemizygous), a single copy of the recessive allele produces the phenotype. Females, with two X chromosomes, are typically carriers — they possess one mutant and one normal allele. Key pedigree features include: (1) the condition predominantly affects males; (2) affected males receive the mutant allele from their carrier mothers; (3) carrier females are usually unaffected (though they may show mild manifestations due to unfavourable X-inactivation patterns); (4) no male-to-male transmission (fathers pass their Y chromosome, not their X, to sons); (5) all daughters of affected males are obligate carriers. Classic XLR conditions include haemophilia A (Factor VIII deficiency), haemophilia B (Factor IX deficiency), Duchenne muscular dystrophy (dystrophin gene mutations), colour blindness (red-green), and glucose-6-phosphate dehydrogenase (G6PD) deficiency. G6PD deficiency is particularly relevant in India, where it affects 2-15% of the male population depending on the community, and can cause neonatal jaundice, chronic haemolytic anaemia, and acute haemolytic crises triggered by oxidant drugs (primaquine, sulphonamides), fava beans, or infections.
Figure: X-linked and Mitochondrial Inheritance
X-linked dominant (XLD) inheritance is less common. In XLD conditions, a single copy of the mutant allele on the X chromosome produces disease in both males and females. However, affected males are often more severely affected than females (who have a second, normal X chromosome providing partial compensation through X-inactivation). Some XLD conditions are lethal in hemizygous males — for example, Rett syndrome (MECP2 mutations) and incontinentia pigmenti (IKBKG mutations) are seen almost exclusively in females because affected male embryos do not survive to birth.
Mitochondrial (maternal) inheritance follows a distinctive pattern because mitochondria are inherited exclusively from the mother via the oocyte cytoplasm. Sperm contribute virtually no mitochondria to the zygote. Therefore: (1) an affected mother transmits the condition to all her children (both sons and daughters); (2) an affected father does not transmit mitochondrial disorders to any children; (3) the severity of mitochondrial disease varies between siblings and even between tissues within the same individual due to heteroplasmy — the coexistence of normal and mutant mitochondrial DNA (mtDNA) within cells. The proportion of mutant mtDNA molecules determines disease severity (threshold effect). During cell division, mitochondria are distributed randomly to daughter cells (replicative segregation), which can lead to different tissues having different mutation loads. Important mitochondrial disorders include Leber hereditary optic neuropathy (LHON — sudden bilateral visual loss in young adults, predominantly males), MELAS (mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes), MERRF (myoclonic epilepsy with ragged red fibres), and Kearns-Sayre syndrome (progressive external ophthalmoplegia, pigmentary retinopathy, cardiac conduction defects).
Non-Mendelian inheritance mechanisms beyond mitochondrial inheritance include: genomic imprinting, where gene expression depends on the parent of origin (e.g., Prader-Willi syndrome when the paternal copy of 15q11-q13 is deleted vs. Angelman syndrome when the maternal copy is deleted); uniparental disomy, where both copies of a chromosome come from one parent; and trinucleotide repeat expansion, where unstable DNA repeats expand across generations, causing anticipation in diseases like Huntington disease, fragile X syndrome, and myotonic dystrophy.
SELF-CHECK
A couple seeks genetic counselling. The husband is healthy but his mother had haemophilia A (a carrier). The wife has no family history of bleeding disorders. What is the probability that their first son will have haemophilia A?
A. A. 0% — haemophilia skips a generation
B. B. 25% — the father has a 50% chance of being a carrier, and if so, 50% of sons would be affected
C. C. 50% — the father is definitely a carrier
D. D. The father cannot be a carrier because haemophilia A is X-linked recessive
Reveal Answer
Answer: D. D. The father cannot be a carrier because haemophilia A is X-linked recessive
Haemophilia A is X-linked recessive. The husband's mother was a carrier (X^H X^h). The husband received his X chromosome from his mother and his Y from his father. Since the husband is healthy and male (XY), he either received the normal X (X^H Y — unaffected, not a carrier) or the mutant X (X^h Y — would be affected). Since he is healthy, he has X^H Y and CANNOT be a carrier of an X-linked condition. Males cannot be carriers of X-linked recessive traits — they are either affected or unaffected. Therefore, there is 0% risk to his sons. The correct answer is D. This illustrates the critical concept that there is NO male-to-male transmission in X-linked inheritance — the father passes his Y chromosome to sons.