Human genetics and genetic counselling

In short

Human genetics studies how traits and diseases are transmitted in families and how that knowledge is used in diagnosis, risk prediction and prevention. Genetic counselling is the communication process that helps a family understand a diagnosis, its mode of inheritance, the numerical recurrence risk, the testing and reproductive options, and the support available — delivered non-directively. It matters in India because carrier frequencies for β-thalassaemia (about 3–4% nationally) and sickle cell disease (high in tribal belts) are large, consanguinity is common in parts of the south and in some communities, and congenital anomalies affect roughly 6–7% of newborns. It is examined in B.Sc/M.Sc human genetics papers, CSIR-UGC NET and GATE BT/XL (genetics and biotech sections), DBT-BET and ICMR/JRF screening. It is practised in hospital genetics departments, IVF and prenatal diagnostic labs, NIDAN Kendras under the DBT UMMID initiative, and in clinical-genomics roles at diagnostic companies.

Explained

Pedigree analysis and modes of Mendelian inheritance

• Pedigree symbols: square = male, circle = female, diamond = unknown sex; filled = affected; a diagonal line = deceased; double horizontal line = consanguineous union; arrow = proband (index case, first ascertained); Roman numerals = generations, Arabic = individuals.
• Autosomal dominant (AD): vertical transmission in every generation, male:female equal, affected parent → 1/2 risk, male-to-male transmission occurs. Examples: Huntington disease, achondroplasia, neurofibromatosis 1, Marfan syndrome, familial hypercholesterolaemia.
• Autosomal recessive (AR): horizontal pattern (sibs affected, parents unaffected carriers), 1/4 risk per pregnancy, consanguinity increases frequency. Examples: β-thalassaemia, sickle cell disease, cystic fibrosis, phenylketonuria, spinal muscular atrophy, congenital deafness.
• X-linked recessive (XLR): affected males, carrier females usually mild/normal, no male-to-male transmission, affected male's daughters are all obligate carriers, maternal uncles affected ("knight's move"). Examples: haemophilia A/B, Duchenne muscular dystrophy, G6PD deficiency, red-green colour blindness.
• X-linked dominant (XLD): affected father → all daughters affected, no sons; females affected about twice as often, often milder. Examples: hypophosphataemic rickets, incontinentia pigmenti (male-lethal), Rett syndrome (de novo).
• Y-linked (holandric): father to all sons only — e.g. some azoospermia-factor deletions.
• Mitochondrial: strictly maternal transmission, affects both sexes, never transmitted by an affected father. Examples: MELAS, LHON, MERRF.

Complications of simple Mendelian rules

• Penetrance: fraction of genotype carriers showing the phenotype; incomplete penetrance makes a pedigree "skip" a generation.
• Variable expressivity: same genotype, different severity (NF1 within one family).
• Pleiotropy: one gene, many unrelated effects (Marfan — eye, skeleton, aorta).
• Genetic heterogeneity: *locus* heterogeneity (retinitis pigmentosa, >80 genes) and *allelic* heterogeneity (many β-globin mutations; IVS1-5 G→C is the commonest in India).
• Anticipation: earlier onset and greater severity down generations, due to expansion of trinucleotide repeats — CAG in Huntington disease and SCAs (polyglutamine), CTG in myotonic dystrophy 1, CGG in *FMR1* (fragile X: normal ≤44, intermediate 45–54, premutation 55–200, full mutation >200 with methylation), GAA in Friedreich ataxia.
• Genomic imprinting: monoallelic parent-of-origin expression — 15q11-13 deletion on the paternal chromosome gives Prader-Willi, on the maternal gives Angelman; the same phenotypes arise from uniparental disomy.
• Mosaicism: somatic (post-zygotic, e.g. mosaic Down syndrome) or germline/gonadal (explains recurrence of an apparently de novo dominant disorder).
• X-inactivation (Lyon hypothesis): one X is randomly inactivated in each female somatic cell → Barr body; skewed inactivation can make a carrier female symptomatic (e.g. DMD carrier cardiomyopathy).
• Multifactorial/polygenic disease: many loci plus environment, a liability-threshold model, recurrence risk empirical (2–5%) not Mendelian — neural tube defects, cleft lip/palate, clubfoot, congenital heart disease, type 2 diabetes.
• Non-classical mechanisms: digenic inheritance, uniparental disomy, dynamic mutations, epigenetic silencing.

Chromosomal disorders and cytogenetic nomenclature

• Numerical (aneuploidy) from meiotic non-disjunction: trisomy 21 (Down syndrome, 47,XY,+21 — about 1 in 800 births, risk rises steeply with maternal age), trisomy 18 (Edwards), trisomy 13 (Patau), 45,X (Turner — short stature, streak gonads), 47,XXY (Klinefelter — tall, azoospermia, gynaecomastia), 47,XYY, 47,XXX. Triploidy (69,XXX) is usually lethal.
• Structural: deletion (5p− cri-du-chat; 22q11.2 deletion/DiGeorge), duplication, inversion (paracentric/pericentric), ring, isochromosome, translocation — reciprocal (balanced carrier normal but at risk of unbalanced gametes) and Robertsonian (acrocentric fusion, e.g. t(14;21)).
• Translocation Down syndrome (about 3–4% of cases): an unaffected carrier parent gives an empirical recurrence of roughly 10–15% if the mother carries and 1–5% if the father carries; a 21q21q carrier gives essentially 100% — so karyotyping the *child* is mandatory before any recurrence risk is quoted, and the parents are karyotyped when the child's result shows a translocation (not needed for free trisomy 21).
• ISCN format: total chromosome number, sex chromosomes, then the abnormality — 46,XX,del(5)(p15.2); 46,XY,t(9;22)(q34;q11); 47,XX,+21; mosaic as mos 47,XY,+21[12]/46,XY[18].
• Microdeletion/microduplication syndromes are below karyotype resolution (~5–10 Mb) and need FISH, MLPA or microarray: Williams 7q11.23, Prader-Willi/Angelman 15q11-13, Smith-Magenis 17p11.2, 1p36 deletion.

Laboratory methods used in counselling

• Karyotyping (GTG-banded, 550-band, from PHA-stimulated lymphocytes or amniocytes): detects aneuploidy, balanced rearrangements and mosaicism; resolution 5–10 Mb; turnaround 2–3 weeks.
• FISH / QF-PCR: rapid targeted aneuploidy or microdeletion detection in 1–2 days; cannot find balanced rearrangements.
• Chromosomal microarray (CMA/SNP array): first-tier test for developmental delay, intellectual disability, autism and multiple congenital anomalies; finds copy-number variants and loss of heterozygosity; blind to balanced translocations and triplet repeats.
• Targeted mutation tests: ARMS-PCR or reverse dot blot for the common Indian β-thalassaemia mutations; HPLC/electrophoresis for HbE, HbS, HbD and raised HbA2 as the carrier screen; MLPA for *DMD* exon deletions/duplications and SMN1 copy number; triplet-repeat PCR plus Southern/TP-PCR for fragile X; methylation-specific PCR for PWS/AS.
• Sequencing: Sanger for a known single gene; NGS panels, clinical exome, WES and WGS for heterogeneous phenotypes; mitochondrial genome sequencing for suspected mtDNA disease.
• Variant interpretation (ACMG/AMP 2015, five tiers): pathogenic, likely pathogenic, variant of uncertain significance (VUS), likely benign, benign — evidence codes combine population frequency, computational, functional and segregation data. A VUS must never be used to make a reproductive decision.
• Biochemical and metabolic tests: tandem mass spectrometry for aminoacidopathies/organic acidaemias, enzyme assays for lysosomal storage disorders, urine GAGs for mucopolysaccharidoses.

Genetic counselling: definition, process and types

• Definition (NSGC 2006, widely used): the process of helping people understand and adapt to the medical, psychological and familial implications of the genetic contribution to disease — comprising interpretation of family and medical history, education about inheritance, testing, management and prevention, and counselling to promote informed choice and adaptation.
• Core principles: autonomy, non-directiveness (information and options, not advice on what to choose), informed consent, confidentiality, truthfulness, equity, and the right not to know.
• Steps: (1) referral and reason; (2) three-generation pedigree with ethnicity, consanguinity, stillbirths and abortions; (3) clinical examination, dysmorphology, previous records and photographs; (4) confirm the diagnosis — *counselling without a confirmed diagnosis is guesswork*; (5) establish the mode of inheritance; (6) compute the recurrence risk; (7) explain risk in several formats (fraction, percentage, comparison with the population risk); (8) present options — testing, prenatal diagnosis, PGT, gamete donation, adoption, no further pregnancy, acceptance; (9) non-directive support and a written summary letter; (10) follow-up and cascade screening of at-risk relatives.
• Timing and setting: *prospective* (before an affected child, e.g. preconception carrier screening) is preventively superior to *retrospective* (after an affected child).
• Types: preconception/premarital, prenatal, paediatric/dysmorphology, carrier screening, presymptomatic (Huntington disease, with a mandatory protocol and psychological assessment), predictive/familial cancer (*BRCA1/2*, Lynch syndrome), pharmacogenetic, and counselling for infertility and recurrent pregnancy loss.
• Who counsels: clinical geneticists (MD/DM Medical Genetics), paediatricians and obstetricians, and genetic counsellors. In India there is no statutory licence for genetic counsellors; certification by the Board of Genetic Counseling, India is voluntary, while the *facility* must be registered under the PCPNDT Act.

Screening, prenatal diagnosis and newborn screening

• First-trimester combined test (11–13⁺⁶ weeks): nuchal translucency + PAPP-A + free β-hCG with maternal age; detection of trisomy 21 roughly 85–90%.
• Quadruple marker (15–20 weeks): AFP, hCG, uE3, inhibin A; raised AFP suggests open neural tube defect. The anomaly scan is done at 18–20 weeks.
• NIPT / cfDNA screening (from 10 weeks): very high detection for trisomy 21 (>99%) with a low false-positive rate, but it is a screening test — a positive result must be confirmed by CVS or amniocentesis; performance is poorer for trisomy 13 and sex-chromosome aneuploidy and depends on fetal fraction. Under the PCPNDT Act the fetal sex found incidentally must not be disclosed.
• Diagnostic sampling: CVS at 11–14 weeks (chorionic villi; fetal loss risk about 0.2% above background; beware confined placental mosaicism and maternal cell contamination); amniocentesis at 15–18 weeks (loss risk about 0.1–0.3%); cordocentesis/fetal blood sampling after 18 weeks for fetal haematology; both must be ultrasound-guided and consented in writing.
• Pre-implantation genetic testing (PGT) on an IVF embryo biopsy: PGT-M (monogenic), PGT-SR (structural rearrangement), PGT-A (aneuploidy) — avoids termination but needs IVF and carries mosaicism and no-result risks.
• Newborn screening: heel-prick dried blood spot (Guthrie card) at 48–72 h for congenital hypothyroidism, congenital adrenal hyperplasia, G6PD deficiency, galactosaemia and (where offered) aminoacidopathies by MS/MS. There is no single national mandate; programmes run in some states and institutions and the panel varies — state the local panel rather than assume one.
• Carrier screening in India: HPLC-based β-thalassaemia carrier screening of antenatal women and college students is run under the National Health Mission; the National Sickle Cell Anaemia Elimination Mission (2023) screens the 0–40 age group in tribal districts and issues genetic-status cards, with elimination targeted by 2047.

Indian law, policy and ethics

• PC&PNDT Act, 1994 (amended 2003): registration of every genetic counselling centre, genetic laboratory, genetic clinic and imaging centre; prohibits sex selection and any communication of fetal sex; prescribes written consent, Form F records, display of the "sex determination not done here" notice, Appropriate Authorities and Advisory Committees; imprisonment and fine on conviction, and suspension/cancellation of registration and of medical registration.
• MTP (Amendment) Act, 2021: termination up to 20 weeks on the opinion of one registered medical practitioner; 20–24 weeks for specified categories (including survivors of rape, minors, and change of marital status) on the opinion of two RMPs; beyond 24 weeks only for substantial fetal abnormality, on the approval of a State-constituted Medical Board; the woman's identity is protected.
• ART (Regulation) Act, 2021 and Surrogacy (Regulation) Act, 2021 (in force from 25 January 2022): registration of ART clinics and banks on a National Registry, National and State Boards, donor and consent conditions; PGT is permitted for genetic disease, never for sex selection; commercial surrogacy is prohibited.
• Rights of Persons with Disabilities Act, 2016: 21 specified disabilities include thalassaemia, haemophilia, sickle cell disease, muscular dystrophy, intellectual disability and multiple sclerosis — so counselling should also cover certification, reservation and benefits.
• National Policy for Rare Diseases, 2021: Centres of Excellence; financial assistance of up to ₹50 lakh per patient under the Rashtriya Arogya Nidhi umbrella for the listed groups (amounts and lists are revised by MoHFW — quote the current order).
• Research ethics: ICMR *National Ethical Guidelines for Biomedical and Health Research Involving Human Participants*, 2017 — a dedicated chapter on biobanking and genetic research, requiring specific consent, re-consent for new use, family implications and handling of incidental findings; Institutional Ethics Committee approval and registration.
• Data and discrimination: genetic data is personal (and in practice sensitive) data — under the DPDP Rules, 2025 the Data Fiduciary duties of notice, consent, breach intimation and erasure apply only from 13 May 2027; until then IT Act section 43A and the SPDI Rules, 2011 govern. The Delhi High Court in *United India Insurance v. Jai Prakash Tayal* (2018) held exclusion from health insurance purely on genetic disorder to be unsustainable; India has no dedicated genetic non-discrimination statute (nothing equivalent to the US GINA), and the DNA Technology Regulation Bill was withdrawn in 2023.
• Genomic resources: DBT UMMID initiative and NIDAN Kendras for district-level diagnosis and counselling; CSIR IndiGen (1,029 genomes) and the GenomeIndia project (about 10,000 whole genomes, first phase announced in January 2025) are building Indian allele-frequency references needed for correct variant classification.

Key points

• Pedigree — a standardised family diagram; the mode of inheritance is read from it before any risk is quoted.
• AR 1/4, AD 1/2, XLR 1/2 of sons — the three recurrence risks most often asked.
• Carrier risk of an unaffected sib of an AR patient = 2/3, not 1/2, because the affected genotype is excluded.
• Obligate carrier — an individual whose carrier status is certain from the pedigree (every daughter of an affected XLR male).
• Consanguinity — raises AR disease risk by increasing homozygosity; first cousins give offspring F = 1/16.
• Penetrance vs expressivity — whether the phenotype appears at all vs how severely.
• Anticipation — earlier/severer disease across generations from repeat expansion.
• Imprinting — parent-of-origin expression; PWS (paternal 15q loss) vs Angelman (maternal).
• Heteroplasmy and threshold — mtDNA disease appears only above a critical mutant load, which explains intrafamilial variability.
• Mosaicism — explains mild phenotypes and recurrence of "de novo" dominant mutations.
• CMA is the first-tier test for unexplained developmental delay/ID/ASD; karyotype remains first choice for suspected balanced rearrangement or recurrent miscarriage.
• VUS — a variant of uncertain significance is not actionable; do not base prenatal decisions on it.
• NIPT screens, CVS/amniocentesis diagnose — never report cfDNA as a diagnosis.
• PC&PNDT Act, 1994 — registration of the centre, Form F, and an absolute bar on disclosing fetal sex.
• MTP Amendment Act, 2021 — 20 weeks (one RMP), 20–24 weeks (two RMPs, specified categories), beyond 24 weeks only via a State Medical Board for substantial fetal abnormality.
• Non-directive counselling — give risks and options; the decision is the family's.
• Cascade screening — once a familial variant is known, test the at-risk relatives; this is the highest-yield prevention step.
• β-thalassaemia carrier rate about 3–4% in India — HPLC with raised HbA2 is the screen; molecular confirmation follows.

Formulas and examples

Example 1 — AR carrier couple, β-thalassaemia. Both parents are HbA2-raised carriers; they have one unaffected daughter and want the risk for the next pregnancy and their daughter's status.

• Risk of an affected child = 1/4 = 25% every pregnancy; past children do not change it.
• Risk of a carrier child = 1/2 = 50%; of a completely normal child = 1/4.
• The unaffected daughter: genotypes AA : Aa : aA remain possible in ratio 1 : 1 : 1 → carrier probability = 2/3 (66.7%).
• Options: prenatal diagnosis by CVS at 11–12 weeks with ARMS-PCR for the parental mutations, or PGT-M.

Example 2 — Bayesian carrier risk, Duchenne muscular dystrophy. A woman's mother is a proven *DMD* carrier; the woman herself has two unaffected sons and no genetic test result.

• Posterior carrier probability = 0.125 / (0.125 + 0.500) = 0.125/0.625 = 0.20 = 1/5.
• Risk that her next child is an affected boy = 1/5 × 1/4 = 1/20 = 5%.
• Risk that her next son is affected = 1/5 × 1/2 = 10%.

Example 3 — Consanguinity. A man is a known carrier of an AR disorder and marries his first cousin; no other family history.

• First cousins: r = 1/8, so the probability his cousin carries the same allele = 1/8.
• Risk of an affected child = 1/8 × 1/4 = 1/32 ≈ 3.1%, against a population risk that may be 1 in several thousand.
• The child's inbreeding coefficient F = 1/16 = 0.0625; for an uncle-niece union r = 1/4 and F = 1/8.

Example 4 — Hardy-Weinberg carrier estimate. Sickle cell disease affects 1 in 2,500 newborns in a community.

• q = √(1/2,500) = 1/50 = 0.02; p = 0.98.
• Carrier frequency 2pq = 2 × 0.98 × 0.02 = 0.0392 = 3.92%, about 1 in 26.
• Risk for a random couple from this community = 0.0392 × 0.0392 × 1/4 = 0.000384 ≈ 1 in 2,600, which reproduces the stated incidence — a consistency check worth doing in the exam.

Example 5 — Why a positive screen is not a diagnosis. NIPT with 99% detection and 99.9% false-positive-free specificity, applied to 100,000 pregnancies where trisomy 21 prevalence is 1 in 500 (200 affected).

• True positives = 200 × 0.99 = 198; false positives = 99,800 × 0.001 = 99.8.
• PPV = 198 / (198 + 99.8) = 198/297.8 = 66.5% — one in three positives is a false alarm, so confirmation by amniocentesis is mandatory.
• Negative predictive value = 99,700.2 / (99,700.2 + 2) ≈ 99.998%.

Where it is used

• Hospital genetics and prenatal diagnosis units — AIIMS, PGIMER, SGPGI, CMC Vellore, NIMHANS, Sanjay Gandhi and state medical colleges: pedigree-based counselling, fetal medicine, dysmorphology clinics.
• Diagnostic genomics industry — MedGenome, Strand Life Sciences, Neuberg/Centogene India, Lal PathLabs, Metropolis and Thyrocare: roles as variant scientist/curator, bioinformatics analyst, cytogeneticist, genetic counsellor, NGS wet-lab associate.
• IVF and fertility chains — PGT-A/M workflows, karyotype and Y-microdeletion testing for male infertility, counselling for recurrent pregnancy loss.
• Public-health programmes — NHM thalassaemia control and antenatal HPLC screening, the National Sickle Cell Anaemia Elimination Mission, RBSK child-health screening for birth defects, DBT UMMID NIDAN Kendras.
• Haemophilia, thalassaemia and muscular dystrophy societies and NGOs — carrier screening camps, cascade testing of relatives, RPwD certification support.
• Pharma and research — rare-disease and gene-therapy programmes, biobanks, GenomeIndia/IndiGen variant-frequency curation, pharmacogenomics (e.g. *TPMT*, *CYP2C19*, *HLA-B*57:01*).

Interview questions

• Q: How do you tell X-linked recessive from autosomal recessive inheritance in a pedigree? A: XLR affects almost only males, is transmitted through unaffected carrier mothers, shows no male-to-male transmission, and all daughters of an affected male are carriers; AR affects both sexes equally and often appears in consanguineous families.
• Q: Why is the carrier risk of a healthy sibling of a cystic-fibrosis patient 2/3 and not 1/2? A: Conditioning on being unaffected removes the aa class, leaving AA : Aa : aA in a 1 : 1 : 1 ratio, so two of the three remaining genotypes are carriers.
• Q: A couple already has three children with an AR disorder. What is the risk for the fourth? A: Still 1/4 — each conception is an independent event; "chance has no memory".
• Q: Parents of a child with trisomy 21 ask about recurrence. What do you need first? A: The child's karyotype — free trisomy 21 carries about a 1% recurrence (higher with advanced maternal age), whereas a familial Robertsonian translocation needs parental karyotypes and can carry 10–15% if the mother is the carrier.
• Q: Why is a mitochondrial disorder never transmitted by an affected father? A: Mitochondria in the zygote are essentially all from the oocyte; sperm mitochondria are excluded or degraded, so transmission is exclusively maternal.
• Q: What is anticipation and what causes it? A: Progressively earlier onset and greater severity in successive generations, caused by expansion of an unstable trinucleotide repeat during gametogenesis — as in myotonic dystrophy 1 and Huntington disease.
• Q: How do Prader-Willi and Angelman syndromes arise from the same deletion? A: The 15q11-13 region is imprinted; loss of the paternally expressed genes gives Prader-Willi and loss of the maternally expressed *UBE3A* gives Angelman, and uniparental disomy produces the same imbalance.
• Q: NIPT reports a high risk for trisomy 21. What next? A: Counsel that NIPT is a screen with imperfect positive predictive value, and offer confirmatory CVS or amniocentesis with karyotype or QF-PCR/CMA before any decision.
• Q: What does the PC&PNDT Act prohibit, and how does it affect a genetics laboratory? A: It prohibits sex selection and any communication of fetal sex; the laboratory and clinic must be registered, maintain Form F records and display the prescribed notice, with imprisonment, fine and loss of registration on conviction.
• Q: Up to what gestation can a pregnancy be terminated in India for a fetal abnormality? A: Up to 20 weeks on one RMP's opinion and 20–24 weeks on two RMPs' opinion for specified categories; beyond 24 weeks only for substantial fetal abnormality with the approval of a State Medical Board under the MTP (Amendment) Act, 2021.
• Q: What would you report if a prenatal exome finds a VUS? A: Report it as uncertain, do not use it to guide termination, and offer parental testing, segregation analysis and reanalysis later as evidence accumulates.
• Q: Why is non-directive counselling emphasised? A: Reproductive decisions involve the family's values and circumstances; the counsellor's duty is accurate information and support, and directing a choice risks coercion and eugenic practice.

Common mistakes

• Quoting a recurrence risk before the diagnosis and the mode of inheritance are confirmed — the commonest real-world counselling error.
• Writing 1/2 instead of 2/3 for the carrier probability of an unaffected sib in an AR pedigree.
• Believing previous affected children reduce (or increase) the risk for the next pregnancy.
• Setting up Bayes' table wrongly — using the prior as the conditional, or forgetting to normalise the joint probabilities by their sum.
• Treating NIPT, the quadruple marker or an HPLC screen as diagnostic, and ignoring positive predictive value.
• Applying a Mendelian fraction to a multifactorial condition such as a neural tube defect or clubfoot instead of the empirical risk.
• Omitting the legal frame — unregistered premises, missing Form F, or disclosing fetal sex "because the parents insisted" — each is an offence under the PC&PNDT Act, 1994.
• Ordering a karyotype where a microarray, MLPA or repeat-sizing test is needed (karyotyping cannot see microdeletions, single-gene mutations or repeat expansions).

Related topics

Population and quantitative genetics · Molecular genetics and gene expression · Cytogenetics and chromosome banding · Medical biotechnology and gene therapy · Bioinformatics: variant calling and annotation · Biostatistics and epidemiology of screening tests