Principles of Inheritance and Variation

Long before anyone had seen a chromosome or a gene, a monk growing pea plants in a monastery garden worked out the rules by which traits pass from parent to offspring. NCERT's genetics chapter builds outward from that single set of experiments: first how one trait behaves across generations, then how two traits behave together, then how the physical chromosome carrying those traits was tracked down, and finally the cases where inheritance breaks the ordinary rules altogether, sex-linked conditions and the genetic material that lives outside the nucleus. This is the classical, organism-level layer of genetics, distinct from the DNA-level mechanism covered in Cell Biology, Molecular Genetics and Evolution, which handles the double helix, replication and the central dogma; here the focus is on crosses, ratios and who inherits what from whom.

Why Mendel chose the garden pea

Gregor Mendel ran his hybridisation trials on garden pea plants (Pisum sativum) between 1856 and 1863, and the choice of organism did much of the experimental work for him. Pea plants self-pollinate by default because their reproductive parts sit enclosed within the petals, which let Mendel maintain pure, true-breeding lines for generations undisturbed; when he wanted a deliberate cross instead, he could remove the immature anthers from one flower (a step called emasculation) and dust its stigma with pollen from a chosen second plant. The plant also completes a generation quickly and sets a large number of seeds per pod, so a single season's crosses produced enough offspring for the statistical patterns to show up clearly, something Mendel was unusually attentive to for a nineteenth-century biologist. Crucially, he did not study the pea plant as a whole; he picked seven traits that each came in two sharply distinguishable forms with nothing in between, among them stem height (tall or dwarf), seed shape (round or wrinkled), seed colour (yellow or green) and flower colour (violet or white). Working with either-or traits, rather than traits that blend or vary continuously, is what let him count offspring cleanly and spot the ratios that carried his name into every biology textbook since.

The law of dominance and the law of segregation

Mendel's first move was to cross two pure-breeding plants that differed in a single trait, a monohybrid cross. Crossing a true-breeding tall plant with a true-breeding dwarf one produced a first (F1) generation that was uniformly tall, with no trace of the dwarf trait and no intermediate height. Mendel then let the F1 plants self-pollinate, and in the second (F2) generation the dwarf trait reappeared, in a ratio of roughly three tall plants to every one dwarf plant. From this he drew two separate conclusions, later formalised as his first two laws.

The law of dominance holds that each trait is governed by a pair of hereditary units (what we now call alleles of a gene), and where the two members of a pair differ, one masks the other's effect entirely rather than the two blending. In the standard notation, a capital letter denotes the version expressed in the F1 (T for tall) and the lower-case letter its recessive counterpart (t for dwarf); a plant of genotype Tt looks identical to one of genotype TT because T's effect dominates.

The law of segregation explains why the recessive trait comes back in F2 at all. Because each parent carries two alleles for a trait but passes only one into each gamete, the pair separates cleanly during gamete formation; there is no mixing of the two alleles into a hybrid allele that a gamete could carry. A heterozygous Tt plant therefore produces T-bearing and t-bearing gametes in equal numbers, and random fertilisation between two such gametes gives a genotypic split of one TT to two Tt to one tt, which shows up as the visible 3:1 phenotypic ratio because both TT and Tt look tall. To pin down whether a tall-looking plant was genotype TT or Tt, Mendel crossed it back to a homozygous recessive plant, a test cross: a TT parent gives only tall offspring, while a Tt parent gives tall and dwarf offspring in equal numbers, exposing the hidden genotype through the ratio of its progeny.

Not every trait obeys strict either-or dominance. When true-breeding red-flowered and white-flowered snapdragons are crossed, the F1 comes out pink rather than red, a case of incomplete dominance where neither allele fully masks the other and the F2 splits into red, pink and white in the same 1:2:1 ratio as the underlying genotypes. A related but distinct pattern is codominance, where the F1 shows both parental traits at once rather than a blend. The clearest textbook case is the human ABO blood group system, governed by a single gene with three alleles, IA, IB and i. Both IA and IB sit fully dominant over i, but when a person carries one of each, both are expressed together on the red blood cell surface, producing type AB blood rather than a compromise between A and B. Because three alleles exist in the population even though any one person carries only two, ABO blood grouping is also the standard example of a gene with multiple alleles, giving six possible genotypes that collapse into four visible blood types (A, B, AB and O).

The law of independent assortment

Mendel's second major set of trials tracked two traits at once, a dihybrid cross, crossing plants with round yellow seeds against plants with wrinkled green seeds. The F1 offspring were uniformly round and yellow, confirming those traits as dominant, but self-pollinating the F1 produced an F2 generation with four distinct combinations, round yellow, round green, wrinkled yellow and wrinkled green, in a ratio close to 9:3:3:1. That ratio matters because it is exactly what you get by multiplying two independent 3:1 ratios together (3 round : 1 wrinkled, multiplied by 3 yellow : 1 green), which told Mendel that the seed-shape gene and the seed-colour gene were not travelling together as a package; a gamete's chance of carrying the round allele had nothing to do with whether that same gamete carried the yellow allele or the green one. This became the law of independent assortment: when two gene pairs are considered together, each pair segregates into gametes independently of the other pair, so a heterozygous plant for both traits produces four kinds of gametes in equal proportion rather than only the two parental combinations.

The law holds cleanly for genes sitting on separate chromosome pairs, which is exactly what later work on chromosome behaviour during meiosis would confirm; it needed a further discovery, examined next, to explain why some gene pairs stubbornly refuse to assort independently at all.

The chromosomal theory and Morgan's fruit flies

Mendel published his results in 1865, but they went largely unread for 35 years until three separate researchers, Hugo de Vries, Carl Correns and Erich von Tschermak, rediscovered the same patterns independently around 1900. By then microscopy had advanced enough for biologists to watch thread-like structures inside a dividing nucleus double and separate, and two scientists, Walter Sutton and Theodor Boveri, noticed that these newly named chromosomes behaved exactly the way Mendel's abstract "factors" were supposed to: they occur in pairs, each pair separates during gamete formation so that only one member reaches a given gamete, and different pairs separate independently of one another. Sutton put the parallel into words as the chromosomal theory of inheritance: genes are physically carried on chromosomes, and the mechanical behaviour of chromosomes during meiosis is what produces Mendel's segregation and independent-assortment ratios.

The theory needed experimental proof, and Thomas Hunt Morgan supplied it working with the fruit fly Drosophila melanogaster, a species that breeds in about two weeks, produces large broods, and shows easily visible eye-colour and body-colour variation under a simple microscope. Running dihybrid crosses the way Mendel had in peas, Morgan found that certain trait pairs did not assort independently at all; the parental combinations turned up far more often than a fresh 9:3:3:1 ratio would allow. He traced this to both genes sitting on the same chromosome, physically tied together during gamete formation, a phenomenon he named linkage; the occasional offspring breaking from the parental combination, produced when homologous chromosomes exchange segments during meiosis, he called recombination, or crossing over. Linkage strength varied: gene pairs sitting close together on a chromosome recombined only rarely, while pairs further apart recombined more often. His student Alfred Sturtevant turned that insight into a technique, using recombination frequency as a proxy for physical distance between genes, and built the first genetic linkage map from it, a method whose descendants underpinned the Human Genome Project generations later.

Sex determination: more than one system

How an organism's sex gets fixed turns out to vary a good deal across the animal kingdom, and UPSC has tested more than one of these systems. In humans, as in Drosophila, every individual carries 22 matched pairs of autosomes plus one pair of sex chromosomes; a female carries two X chromosomes, while a male carries one X and a distinctly smaller Y. Because every egg carries a single X, and sperm split evenly between X-bearing and Y-bearing, it is entirely the fertilising sperm, not anything about the mother, that fixes whether the resulting child is male or female, each outcome equally likely at conception. This is called male heterogamety, since it is the male that produces two different kinds of gametes with respect to sex.

Two contrasting systems are worth knowing for pairing-style questions. In several insects such as the grasshopper, males carry only a single sex chromosome alongside their autosomes (denoted XO) while females carry two, so male and female cells differ in total chromosome count, unlike the human case. In birds, moths and some reptiles, the pattern flips around: it is the female that produces two different kinds of eggs (carrying either a Z or a W chromosome) while the male is uniform (ZZ), making this female heterogamety and reversing which parent's gamete decides the offspring's sex. The honeybee adds a further oddity: a fertilised egg develops into a diploid female (queen or worker), while an unfertilised egg develops, with no paternal contribution at all, into a haploid male drone, so a drone has no father yet can have grandsons.

Sex-linked inheritance and reading a pedigree

A gene sitting on the X chromosome behaves differently across sexes purely because of chromosome dosage: a female, with two X copies, can be heterozygous and mask a recessive allele behind a normal one, while a male, with only a single X, expresses whatever allele that X carries, with no second copy to cover for it. This is why several well-known recessive disorders, red-green colour blindness and haemophilia among them, show up far more often in men than in women even though the underlying mutation rate is the same in both sexes: an affected father cannot pass the condition to his sons at all (his son gets his Y chromosome, not his X), but a mother who is an unaffected carrier has a fifty-fifty chance of passing the affected X to each son, who will then show the condition outright, having no second X to compensate. The British royal family supplies the case study every textbook reaches for: Queen Victoria was a carrier of haemophilia, and the mutation surfaced in several of her sons and, through her carrier daughters, spread into the royal houses of Russia, Germany and Spain across subsequent generations, a documented multi-generation trail that is a favourite prompt for pedigree-reading questions.

Because it is neither practical nor ethical to run controlled breeding crosses on human beings, geneticists reconstruct inheritance patterns by charting a family's history across generations instead, using a standard set of symbols, squares for males, circles for females, filled shapes for affected individuals, and connecting lines for matings and offspring. Reading such a pedigree lets a geneticist work out whether a trait behaves as dominant or recessive, and whether it tracks with an autosome or a sex chromosome, purely from the pattern in which it appears and skips across the family tree, without ever needing to run an experimental cross. UPSC's statement-based questions on this chapter frequently hinge on exactly this kind of transmission logic: whether a trait can pass father to son, whether a carrier mother's daughters are themselves at risk, and so on.

Mitochondrial inheritance: heredity outside the nucleus

Everything covered so far assumes a gene sits on a chromosome inside the nucleus and obeys Mendel's segregation rules. Mitochondria break that assumption entirely. Each mitochondrion carries its own small, circular strand of DNA (mitochondrial DNA structure and how it differs from nuclear DNA is covered in the linked cell-biology note above), and a single human cell typically holds hundreds to thousands of mitochondria, each contributing one or more copies of that circular genome. A mature egg cell arrives loaded with roughly a hundred thousand copies of mitochondrial DNA sitting in its cytoplasm, while a sperm cell carries only a tiny handful by comparison, and whatever paternal mitochondria do make it into the fertilised egg are actively tagged and broken down within the first few embryonic cell divisions. The practical result is that essentially every mitochondrion in a person's body traces its lineage to the mother's line alone, not the father's, and this holds regardless of the child's own sex; a son inherits his mother's mitochondrial DNA just as completely as his sister does, he simply cannot pass it on to his own children the way she can. Because inheritance here depends on which parent happens to be the one supplying the cytoplasm rather than on any dominant-recessive interaction between two alleles, geneticists classify it as a cytoplasmic or extranuclear pattern, sitting entirely outside the nuclear, biparental rules Mendel worked out with his pea plants.

This maternal-only transmission has a direct clinical consequence: mutations carried on mitochondrial DNA cause a distinct category of inherited disease, and because affected mitochondria arrive purely through the mother's egg, a father affected by such a condition cannot pass it to any child, while an affected or carrier mother risks passing it to every child she has. Fertility clinics in the United Kingdom now offer techniques designed around exactly this asymmetry. One approach swaps out the nuclear chromosomes from a woman's own egg, which carries unhealthy mitochondria, into a donor egg that has had its own nuclear material removed but keeps its healthy mitochondrial cytoplasm intact, carried out before fertilisation takes place. A second approach works on the same principle after fertilisation instead: once both the intending mother's egg and a donor egg have each been fertilised separately, the chromosome-carrying material from the mother's own fertilised egg is moved into the donor's fertilised egg, whose own chromosome-carrying material has first been removed, so that the resulting embryo carries the intending parents' nuclear genome inside a donor's healthy mitochondrial cytoplasm. The United Kingdom's fertility regulator authorised both routes for licensed clinical use starting in 2015, the first country to regulate the procedure this way, and every individual case still needs separate regulatory approval before it can proceed.

The exam angle

This chapter rewards precision over general familiarity, because UPSC's traps sit in small distinctions a casual reading blurs together. A statement pairing "law of segregation" with the dihybrid 9:3:3:1 ratio, or "law of independent assortment" with the monohybrid 3:1 ratio, is the single most common substitution to watch for; keep the one-trait, two-trait split anchored to the right law and the right ratio. Incomplete dominance and codominance also get confused constantly: incomplete dominance produces a new, blended phenotype in the heterozygote (pink snapdragons), while codominance produces both parental phenotypes simultaneously and undiluted (AB blood group); a question describing AB blood group as "incomplete dominance" is a deliberate trap. On the chromosomal theory, remember the division of labour: Sutton and Boveri proposed the parallel between chromosome and gene behaviour, Morgan supplied the experimental proof using Drosophila, and Sturtevant turned recombination frequency into a mapping tool, three separate contributions UPSC likes to scramble across options. Sex determination questions often test whether a candidate knows male heterogamety (humans, most insects) is not the only system, so a statement asserting that "the male gamete always determines offspring sex" fails for birds entirely. Finally, on mitochondrial inheritance, the recurring setup asks whether a father can transmit a mitochondrial condition (he cannot) and whether every child of an affected mother inherits it (in practice essentially all do, since maternal transmission is near-total, not a coin-flip the way a nuclear recessive trait is); treat mitochondrial and nuclear inheritance as following genuinely different rules, not variations on the same theme.

Quick revision points

• Mendel worked with garden pea plants because they self-pollinate naturally, permit controlled cross-pollination via emasculation, have a short life cycle, and show sharply either-or traits rather than continuous variation.
• Law of dominance: one allele masks the other in a heterozygote. Law of segregation: the two alleles of a pair separate into different gametes (monohybrid cross, 3:1 phenotypic ratio, 1:2:1 genotypic ratio).
• Law of independent assortment: two gene pairs segregate independently of each other (dihybrid cross, 9:3:3:1 ratio), true for genes on different chromosomes.
• Incomplete dominance gives a blended F1 phenotype (snapdragon flower colour); codominance gives both parental phenotypes expressed together (ABO blood group, also the standard multiple-allele example: alleles IA, IB, i, six genotypes, four blood types).
• Chromosomal theory of inheritance: Sutton and Boveri (the parallel), proved experimentally by Morgan using Drosophila, mapped by Sturtevant using recombination frequency.
• Linkage: genes on the same chromosome are inherited together more often than chance; recombination (crossing over) breaks up linked combinations, more often the further apart two genes sit.
• Sex determination varies by system: XX/XY with male heterogamety (humans, Drosophila), XO (grasshopper), ZW with female heterogamety (birds), and haplodiploidy (honeybee, unfertilised eggs become haploid males).
• Sex-linked recessive disorders (haemophilia, colour blindness) show up far more often in men, since a single X carries the whole outcome; Queen Victoria's haemophilia pedigree is the standard worked example.
• Mitochondrial DNA passes down almost exclusively via the mother's egg, never meaningfully via the father, a cytoplasmic pattern that sits outside Mendel's nuclear rules; maternal spindle transfer and pronuclear transfer are the two UK-licensed techniques that exploit this asymmetry to keep a faulty mitochondrial line from reaching a child.

Work through the linked questions below to see how UPSC turns these laws, ratios and named contributions into statement-matching traps.