Genetics

In short

Genetics is the study of how traits are stored in genes, transmitted from parents to offspring and altered by mutation and recombination. It supplies the logic behind plant and animal breeding, diagnostics, pharmacogenomics, GMO development and population screening, so every biotechnology degree treats it as a core paper. It is examined heavily in GATE BT (Mendelian ratios, linkage mapping, Hardy-Weinberg, pedigrees), CSIR-UGC NET Life Sciences, ICMR/DBT-BET and university semester papers, usually as numerical problems. Employers who test it include Dr Reddy's, Biocon, Serum Institute, Syngene, Thermo Fisher, MedGenome, Strand Life Sciences, Mahyco, Rasi Seeds, Nuziveedu and public labs such as CCMB, NIPGR, ICAR institutes and NIBMG.

Explained

Mendelian basis of inheritance

• Gene - a heritable unit of DNA specifying a product; allele - one variant form of a gene; locus - its fixed position on a chromosome.
• Genotype is the allele pair carried (e.g. Tt); phenotype is the expressed trait (tall). Homozygous = identical alleles; heterozygous = different alleles; hemizygous = single copy (X-linked genes in males).
• Law of dominance: in a heterozygote one allele (dominant) masks the other (recessive). Mendel's F1 tall × dwarf pea cross gave all tall.
• Law of segregation (purity of gametes): the two alleles of a gene separate at anaphase I of meiosis, so each gamete carries only one. Monohybrid F2 = 3:1 phenotypic, 1:2:1 genotypic.
• Law of independent assortment: alleles of different genes on different chromosome pairs assort independently. Dihybrid F2 = 9:3:3:1; genotypic ratio has 9 classes.
• Test cross: cross the unknown with the homozygous recessive. All dominant offspring → unknown was homozygous; ~1:1 → heterozygous. Back cross is a cross with either parent; a test cross is the special case with the recessive parent.
• Chromosomal theory of inheritance (Sutton and Boveri, 1902): genes sit on chromosomes, whose meiotic behaviour explains Mendel's ratios. Morgan's *Drosophila* work gave experimental proof.
• Mendel's success factors: pea has clear contrasting characters, true-breeding lines, self-pollination with easy artificial cross-pollination, and he counted large numbers.

Deviations from Mendelian ratios (allelic and gene interactions)

• Incomplete dominance: heterozygote is intermediate; *Antirrhinum/Mirabilis* red × white → pink; F2 = 1:2:1 for both phenotype and genotype.
• Codominance: both alleles expressed fully; human AB blood group, roan cattle coat. F2 = 1:2:1.
• Multiple alleles: more than two alleles in the population at one locus; ABO blood group (I^A, I^B, i) — I^A and I^B codominant, both dominant over i. Any diploid still carries only two.
• Lethal alleles: homozygous lethal gives 2:1 instead of 3:1 (yellow lethal in mice, *Agouti* A^Y).
• Pleiotropy: one gene affecting many traits — sickle-cell allele (HbS) alters haemoglobin, RBC shape, anaemia, infarcts, malaria resistance.
• Epistasis: one gene masks another non-allelic gene. Dihybrid F2 modifications: recessive epistasis 9:3:4 (coat colour in mice, Bombay blood group h/h), dominant epistasis 12:3:1 (summer squash fruit colour), duplicate recessive/complementary 9:7 (flower colour in sweet pea), duplicate dominant 15:1 (seed coat in wheat), dominant and recessive (inhibitory) 13:3.
• Polygenic (quantitative) inheritance: many genes with additive small effects plus environment give continuous variation — human skin colour, height, grain yield. Nilsson-Ehle's wheat kernel colour with 2 genes gave 1:4:6:4:1.
• Penetrance - the fraction of individuals with a genotype showing the phenotype; expressivity - the degree to which it is shown.
• Phenocopy - an environmentally produced trait mimicking a genotype. Norm of reaction - the range of phenotypes one genotype gives across environments.

Linkage, crossing over and mapping

• Linkage: genes on the same chromosome tend to be inherited together, giving parental combinations in excess of the 1:1:1:1 test-cross expectation. Complete linkage gives only parental types (Morgan's male *Drosophila*).
• Crossing over: reciprocal exchange between non-sister chromatids of homologues at pachytene of prophase I, at the chiasma; it produces recombinant gametes.
• Recombination frequency (RF) = recombinants/total × 100, expressed in map units (centiMorgan, cM); 1 cM = 1% recombination. RF saturates at 50% for genes far apart or on different chromosomes.
• Two-point cross: RF between two loci from test-cross progeny. Three-point test cross (heterozygote × triple recessive) gives 8 classes: largest two = parental, smallest two = double crossovers, which identify the middle gene (the gene whose allele is switched relative to the parentals).
• Interference I = 1 − coefficient of coincidence, where CoC = observed double crossovers/expected (expected = product of the two interval recombination frequencies × total). Positive interference (I > 0) is usual — one crossover suppresses a nearby second.
• Map distances are additive over short intervals; over long ones they understate true distance because double crossovers go undetected, so mapping function corrections are applied.
• Cis (coupling) arrangement AB/ab versus trans (repulsion) Ab/aB — determines which progeny classes are parental.
• Linkage group = all genes on one chromosome; their number equals the haploid chromosome number (*Drosophila* 4, human 23, pea 7).
• Other mapping routes: somatic cell hybridisation, in situ hybridisation (FISH), RFLP/SSR/SNP molecular maps, and whole-genome sequence maps now standard in crop breeding at ICAR and seed companies.

Sex determination, sex linkage and pedigrees

• Sex-determination systems: XX-XY (human, *Drosophila*), XX-XO (grasshopper), ZZ-ZW (birds), haplodiploidy (honeybee), temperature-dependent (turtles, crocodiles); environment and dosage, not a single gene, in many plants.
• In humans the SRY gene on Yp triggers testis development; X-inactivation (Lyon hypothesis) silences one X in female somatic cells as the Barr body, giving dosage compensation and mosaics (tortoiseshell cat).
• X-linked recessive (haemophilia A, red-green colour blindness, Duchenne muscular dystrophy, G6PD deficiency): affected males far outnumber females; no male-to-male transmission; criss-cross inheritance — affected father → carrier daughters, carrier mother → affected sons.
• X-linked dominant (vitamin D-resistant rickets): affected male passes to all daughters, no sons. Y-linked (holandric): father to all sons only.
• Autosomal recessive (thalassaemia, sickle-cell anaemia, cystic fibrosis, albinism, PKU): skips generations, consanguinity raises risk, 1/4 of a carrier × carrier cross affected.
• Autosomal dominant (Huntington disease, achondroplasia, Marfan syndrome, myotonic dystrophy): appears in every generation, affected have an affected parent, ~1/2 risk per child.
• Mitochondrial (maternal) inheritance (MELAS, LHON): passed only by the mother to all children; heteroplasmy and threshold effect make expression variable.
• Pedigree symbols: square = male, circle = female, filled = affected, horizontal line = mating, double line = consanguineous, diamond = unspecified sex, arrow = proband, dot inside circle = carrier.

Mutation and chromosomal aberrations

• Mutation - a heritable change in DNA sequence. Point mutations: transition (purine↔purine or pyrimidine↔pyrimidine) and transversion (purine↔pyrimidine); substitution gives silent, missense (sickle-cell: GAG→GTG, Glu6Val in β-globin) or nonsense mutations; frameshift from indels not in multiples of three.
• By origin: spontaneous (replication error, depurination, deamination, tautomeric shift) and induced. By cell type: somatic (not inherited; cancer) and germline (inherited). Forward versus reverse (back) mutation; suppressor mutation at a second site.
• Mutagens: physical (UV → thymine dimers, X-rays and gamma rays → breaks), chemical (base analogues 5-bromouracil, 2-aminopurine; alkylating agents EMS, mustard gas; deaminating nitrous acid; intercalating acridine orange, ethidium bromide), biological (transposons, some viruses).
• Detection: Ames test (*Salmonella* his− reversion) for mutagenicity, CLB/ClB and attached-X methods in *Drosophila*, Luria-Delbrück fluctuation test for spontaneity, replica plating (Lederbergs).
• Structural aberrations: deletion (cri-du-chat, 5p−), duplication (*Bar* eye), inversion (paracentric, pericentric — crossover suppressors, inversion loops in heterozygotes), translocation (reciprocal; Robertsonian in familial Down syndrome; Philadelphia t(9;22) BCR-ABL in CML).
• Numerical aberrations — aneuploidy: monosomy (2n−1, Turner 45,X), trisomy (2n+1, Down 47,+21, Edwards +18, Patau +13, Klinefelter 47,XXY), usually from non-disjunction in meiosis.
• Euploidy/polyploidy: autopolyploidy (triploid banana, seedless watermelon), allopolyploidy (bread wheat *T. aestivum* hexaploid AABBDD; *Brassica* triangle of U), induced by colchicine (spindle inhibitor); polyploidy is widespread and useful in crops but usually lethal in animals.
• Karyotyping by G-banding, plus FISH, chromosomal microarray and NGS-based tests in Indian diagnostic labs; prenatal options are amniocentesis, CVS and NIPT (cell-free fetal DNA).
• Indian regulation: the Pre-Conception and Pre-Natal Diagnostic Techniques (PCPNDT) Act, 1994 permits prenatal diagnosis only for listed medical indications and bans sex determination and its disclosure. The Assisted Reproductive Technology (Regulation) Act, 2021 and Surrogacy (Regulation) Act, 2021 govern ART clinics and PGT practice. Human genetic research follows the ICMR National Ethical Guidelines for Biomedical and Health Research (2017); GM organisms fall under the Rules, 1989 of the Environment (Protection) Act, 1986, administered by GEAC/RCGM under DBT and MoEFCC.

Population and quantitative genetics

• Gene pool - all alleles in a population; allele frequency p and q, with p + q = 1 for two alleles.
• Hardy-Weinberg principle: in a large, randomly mating population with no selection, mutation, migration or drift, allele frequencies stay constant and genotype frequencies are p² : 2pq : q². Used to estimate carrier frequency from disease incidence.
• Forces that disturb equilibrium: selection (directional, stabilising, disruptive), mutation, migration (gene flow), genetic drift (founder effect, bottleneck), non-random mating (inbreeding, assortative mating). Inbreeding raises homozygosity, measured by the inbreeding coefficient F.
• Quantitative traits: phenotypic variance V_P = V_G + V_E + V_GE; broad-sense heritability H² = V_G/V_P, narrow-sense h² = V_A/V_P. h² predicts response to selection.
• Breeder's equation R = h² S, where S is the selection differential — the basis of yield selection in ICAR crop programmes and dairy breeding at NDDB.
• Heterosis (hybrid vigour) and inbreeding depression underpin hybrid seed (Bt cotton hybrids, hybrid maize, rice); explained by dominance and overdominance hypotheses.
• QTL mapping and marker-assisted selection (MAS): e.g. *Sub1* for submergence tolerance in Swarna-Sub1 rice, *Xa21*/bacterial blight genes in Improved Samba Mahsuri, *Pusa Basmati 1718* with blight resistance.
• Indian population genetics facts examined: β-thalassaemia carrier rate roughly 3-4% nationally with higher rates in some communities, high sickle-cell allele frequency in central Indian tribal belts (Madhya Pradesh, Maharashtra, Chhattisgarh, Odisha, Gujarat), and high consanguinity in parts of south India raising recessive disease risk. Exact figures vary by survey — quote the source.
• Testing of significance for ratios uses the chi-square test; degrees of freedom = (number of classes − 1).

Key points

• Allele - one of the alternative forms of a gene at a locus; a diploid carries two.
• Segregation - the two alleles separate in meiosis I so each gamete gets one; gives 3:1 in F2.
• Independent assortment - unlinked genes assort separately; dihybrid F2 = 9:3:3:1.
• Test cross - cross to the homozygous recessive to reveal a hidden genotype; 1:1 means heterozygous.
• Incomplete dominance vs codominance - intermediate phenotype versus both phenotypes shown; both give 1:2:1 in F2.
• Multiple alleles - more than two alleles in the population (ABO); still two per individual.
• Epistasis ratios - 9:3:4 recessive, 12:3:1 dominant, 9:7 complementary, 15:1 duplicate dominant, 13:3 inhibitory.
• Linkage - excess of parental types; measured as recombination frequency in cM (1 cM = 1% recombinants).
• Three-point cross - the rarest (double-crossover) class identifies the gene in the middle.
• Interference - 1 − (observed/expected double crossovers); usually positive.
• Criss-cross inheritance - X-linked recessive traits pass father → daughter → grandson; no male-to-male.
• Barr body - the inactivated X in female somatic cells; basis of dosage compensation and mosaicism.
• Mitochondrial inheritance - strictly maternal, affects both sexes, shows heteroplasmy.
• Mutation types - silent, missense, nonsense, frameshift; transition versus transversion.
• Aneuploidy versus polyploidy - loss/gain of single chromosomes (Down 47,+21) versus whole extra sets (hexaploid wheat).
• Hardy-Weinberg - p² : 2pq : q² under no evolutionary force; gives carrier frequency from disease incidence.
• Heritability h² - the additive-genetic share of variance; R = h² S predicts selection gain.
• PCPNDT Act, 1994 - prenatal diagnosis only for medical indications; sex determination is a criminal offence in India.

Formulas and examples

Example 1 — Dihybrid test cross and chi-square. A pea plant round-yellow (RrYy) is test crossed to wrinkled-green (rryy). Progeny observed: 55 round-yellow, 51 round-green, 49 wrinkled-yellow, 45 wrinkled-green; total 200.

• Expected on independent assortment = 1:1:1:1 → 50 each.
• χ² = (55−50)²/50 + (51−50)²/50 + (49−50)²/50 + (45−50)²/50 = 25/50 + 1/50 + 1/50 + 25/50 = 0.5 + 0.02 + 0.02 + 0.5 = 1.04.
• df = 4 − 1 = 3; critical χ² at p = 0.05 is 7.81. Since 1.04 < 7.81, the deviation is due to chance — genes are unlinked and assort independently.

Example 2 — Recombination frequency. In a *Drosophila* test cross for body colour and wing size, 1,000 progeny give 415 wild type, 405 black-vestigial, 92 black-normal, 88 wild-vestigial.

• Parental classes (the two largest) = 415 + 405 = 820; recombinants = 92 + 88 = 180.
• RF = 180/1,000 × 100 = 18% → the two loci are 18 cM apart, and the parent was in cis (coupling) arrangement.

Example 3 — Three-point cross, gene order and interference. A trihybrid is test crossed; 1,000 progeny:

• Rarest classes are + b + and a + c; compared with parentals + + + / a b c, the middle allele b is switched, so the order is a - b - c.
• RF(a-b) = (72 + 68 + 8 + 6)/1,000 = 154/1,000 = 15.4 cM.
• RF(b-c) = (80 + 76 + 8 + 6)/1,000 = 170/1,000 = 17.0 cM.
• Expected DCO = 0.154 × 0.170 × 1,000 = 26.18; observed = 14.
• CoC = 14/26.18 = 0.535; interference I = 1 − 0.535 = 0.465 (46.5%).
• Total map length a-c = 15.4 + 17.0 = 32.4 cM.

Example 4 — Hardy-Weinberg carrier frequency. β-thalassaemia major (autosomal recessive) affects about 1 in 2,500 newborns in a population.

• q² = 1/2,500 = 0.0004 → q = 0.02; p = 1 − 0.02 = 0.98.
• Carrier frequency 2pq = 2 × 0.98 × 0.02 = 0.0392 = 3.92%, i.e. about 1 in 26 people — which is why premarital and antenatal carrier screening is run in Gujarat, Maharashtra and West Bengal.
• Risk that two random individuals from this population produce an affected child = 0.0392 × 0.0392 × 1/4 = 0.000384, i.e. about 1 in 2,604 (matching the incidence, as expected at equilibrium).

Example 5 — Heritability and response to selection. In a rice population, mean grain yield is 4.0 t/ha with V_P = 0.50, V_A = 0.20. Plants selected for the next generation average 4.8 t/ha.

• h² = V_A/V_P = 0.20/0.50 = 0.40.
• S = 4.8 − 4.0 = 0.8 t/ha.
• R = h² × S = 0.40 × 0.8 = 0.32 t/ha, so the next generation's mean ≈ 4.0 + 0.32 = 4.32 t/ha.

Where it is used

• Seed and agri-biotech (Mahyco, Nuziveedu, Rasi, Kaveri, Bayer-Monsanto India, ICAR institutes): hybrid development, marker-assisted selection, QTL mapping, trait introgression; roles as breeder, molecular breeding associate, seed-quality geneticist.
• Clinical diagnostics and genetic counselling (MedGenome, Strand, Metropolis, Dr Lal PathLabs, SRL, AIIMS and CMC Vellore genetics units): karyotyping, FISH, carrier screening for thalassaemia and SMA, NIPT, exome interpretation — all within PCPNDT and ICMR ethics rules.
• Pharma and biologics (Biocon, Dr Reddy's, Sun Pharma, Serum Institute, Syngene): cell-line genetics and clonal stability, pharmacogenomics (CYP2C19, TPMT/NUDT15 before thiopurines), genetic toxicology including the Ames test for regulatory dossiers.
• Animal and dairy breeding (NDDB, NBAGR, BAIF): progeny testing, estimated breeding values, genomic selection in cattle and buffalo; conservation genetics of indigenous breeds.
• Forensics and identity (CFSL, state FSLs, under the DNA-based technology framework): STR profiling, parentage and disaster-victim identification; relationship testing relies on Mendelian exclusion logic.
• Research and public health (CCMB, NIBMG, IGIB, NIPGR, DBT programmes): GenomeIndia-type population variation studies, rare-disease gene discovery, state sickle-cell and thalassaemia control missions.

Interview questions

• Q: Why did Mendel get a 9:3:3:1 ratio, and when would he not? A: Because the two genes were on different chromosome pairs and assorted independently; linked genes, epistasis or lethal alleles distort the ratio.
• Q: Difference between incomplete dominance and codominance? A: In incomplete dominance the heterozygote is an intermediate blend (pink); in codominance both alleles are fully and separately expressed (AB blood group, roan coat).
• Q: How do you tell whether two genes are linked? A: Do a test cross and compare progeny with 1:1:1:1 using chi-square; a significant excess of parental types with RF below 50% means linkage.
• Q: In a three-point cross, how do you find the middle gene? A: Take the rarest (double-crossover) classes and compare them with the parental classes; the gene whose allele has swapped is the middle one.
• Q: Why can recombination frequency never exceed 50%? A: Even with multiple crossovers in an interval, at most half the resulting gametes are recombinant, so RF plateaus at 50% — the same as for unlinked genes.
• Q: A colour-blind man marries a normal non-carrier woman. What about the children? A: All sons are normal (they get his Y), all daughters are carriers; grandsons through those daughters have a 50% risk — criss-cross inheritance.
• Q: What is a Barr body and what does it prove? A: The condensed, inactivated X chromosome in female somatic cells; it demonstrates dosage compensation by random X inactivation (Lyon hypothesis) and explains female mosaics.
• Q: Disease incidence is 1 in 10,000 for a recessive trait — carrier frequency? A: q = √0.0001 = 0.01, so 2pq = 2 × 0.99 × 0.01 ≈ 0.0198, about 2% or 1 in 50.
• Q: Why is polyploidy common in crops but rare in animals? A: Plants tolerate extra genomes, self-fertilise and often gain vigour; in animals extra sets disrupt sex determination, dosage balance and development.
• Q: What is the difference between a mutation and a polymorphism? A: Both are sequence variants; a variant present in more than about 1% of the population is called a polymorphism, while "mutation" usually implies a rarer, often disease-associated change — current clinical practice prefers the neutral term "variant" with ACMG-style classification.
• Q: What is the Ames test used for in industry? A: Detecting mutagenicity of chemicals and drug candidates by reversion of histidine-requiring *Salmonella typhimurium* strains, with and without liver S9 activation, as part of regulatory genotoxicity packages.
• Q: Why is sickle-cell allele frequency high in parts of central India? A: Heterozygote advantage — carriers resist falciparum malaria, so balancing selection maintains the allele in historically malaria-endemic tribal regions.
• Q: What does heritability of 0.4 mean for a breeding programme? A: Forty per cent of the phenotypic variance is additive-genetic, so selection transmits 40% of the selection differential as genetic gain (R = h²S) — it is a population- and environment-specific figure, not a property of the trait.

Common mistakes

• Treating multiple alleles as more than two alleles in one individual — a diploid has only two per locus.
• Adding all crossover classes but forgetting that double crossovers must be counted in both flanking intervals when computing RF.
• Reporting map distance as the number of recombinants instead of the percentage, or mixing cM with physical base pairs (the cM/Mb ratio varies along a chromosome).
• Using the wrong degrees of freedom in chi-square (df = classes − 1, not the number of progeny), or running chi-square on percentages instead of counts.
• Applying Hardy-Weinberg to X-linked traits with the autosomal formula, or to small, inbred or selected populations where its assumptions fail.
• Confusing dominance with frequency — a dominant allele is not necessarily the common one in the population.
• Saying X-linked recessive disorders never appear in females; a homozygous daughter of an affected father and carrier mother is affected, and skewed X-inactivation can cause partial expression.
• Calling incomplete dominance "mixing of genes" — alleles stay discrete and segregate unchanged, as the 1:2:1 F2 shows.

Related topics

Cell biology · Molecular biology: replication, transcription and translation · Biochemistry: biomolecules and enzymes · Microbiology basics · Plant and animal biotechnology · Bioinformatics and genomics