Principles of Inheritance and Variation

This is the fourth chapter of NCERT's Class 12 Biology textbook (pages 53–78 of the 2026-27 reprint) and the first of Unit VII, Genetics and Evolution; the PDF also carries the unit opener and a biography of Watson and Crick (pages 51–52). The chapter covers Mendel's laws (4.1), inheritance of one gene (4.2), inheritance of two genes (4.3), polygenic inheritance (4.4), pleiotropy (4.5), sex determination (4.6), mutation (4.7) and genetic disorders (4.8), with Figures 4.1–4.17, Tables 4.1–4.3, a summary and 16 exercise questions. It is the most numerical biology chapter so far: every ratio on this page is computed, not copied.

• Neighbouring chapters: Human Reproduction (meiosis halves 46 to 23; the sperm decides the sex) and Reproductive Health (amniocentesis to detect genetic disorders) come before; the next chapter, Molecular Basis of Inheritance, explains what a gene is in DNA terms, and Evolution uses variation and mutation. Class 11 Cell Cycle and Cell Division supplies meiosis.
• Prelims — science: UPSC has asked about colour blindness (1981, 1985, 2009), haemophilia (1987, 1992, 1993, 1998, 1999), blood-group inheritance (1994, 2011), X-linked recessive disorders (1994), phenylketonuria (1994), pleiotropy (2002), gene mapping (2011) and who decides a child's sex (1984, 2000).
• GS2 and GS3: genetic disorders and their screening (sickle-cell disease and thalassaemia in India), disability rights, genome mapping and crop breeding all start from this chapter's ideas.

• Mendel (1856–1863), garden pea, 7 pairs of contrasting traits from 14 true-breeding lines. Monohybrid: F1 all dominant; F2 3:1 phenotype, 1:2:1 genotype. Laws: dominance, segregation (one allele per gamete).
• Test cross: dominant phenotype × homozygous recessive → all dominant (it was homozygous) or 1:1 (heterozygous). Dihybrid: F2 9:3:3:1, 16 squares, 9 genotypes, 4 phenotypes → law of independent assortment.
• Incomplete dominance: snapdragon red × white → pink; F2 1:2:1 in both. Co-dominance + multiple alleles: ABO — I^A, I^B, i → 6 genotypes, 4 groups. Polygenic: height, skin colour (additive). Pleiotropy: one gene, many effects (PKU).
• Chromosomal theory: Sutton and Boveri (genes behave like chromosomes in meiosis); rediscovery 1900 (de Vries, Correns, von Tschermak). Morgan, Drosophila: linkage and recombination (y–w 1.3%, w–m 37.2%); Sturtevant's maps.
• Sex determination: XO (grasshopper), XY (humans 44+XX / 44+XY; the sperm decides), ZW (birds; the egg decides), haplodiploidy (honey bee ♀ 32, ♂ 16). Mutation: chromosomal aberrations, point mutation (sickle-cell: Glu→Val at position 6), frame-shift; mutagens such as UV.
• Pedigree analysis traces traits in families. Mendelian disorders: X-linked recessive — haemophilia, colour blindness (about 8% of men, 0.4% of women); autosomal recessive — sickle-cell, PKU, thalassaemia. Chromosomal: Down (trisomy 21, 47), Klinefelter (47, XXY), Turner (45, X).

In the mid-nineteenth century Gregor Mendel carried out hybridisation experiments on garden peas for seven years (1856–1863) and proposed the laws of inheritance. For the first time, NCERT notes, statistical analysis and mathematical logic were applied to a biological problem; his large samples and confirmation over successive generations made his results general rules rather than guesses. He chose characters that showed two opposing traits and used true-breeding lines — lines that, after continuous self-pollination, keep showing the same trait. He selected 14 true-breeding varieties, as pairs that were alike except for one character with contrasting traits.

Mendel crossed tall and dwarf pea plants. All plants of the first hybrid generation — the F1 (Filial1) — were tall; none were dwarf, and none in between. When he self-pollinated the F1, the F2 had dwarf plants again: 1/4 were dwarf and 3/4 tall, with no blending. The other six characters behaved the same way — only one parental trait in F1, both in F2 in the proportion 3:1.

Mendel proposed that something passed unchanged through the gametes — his ‘factors’, now called genes. Genes coding for a pair of contrasting traits are alleles, slightly different forms of the same gene. A capital letter is used for the trait seen in F1 and the small letter for the other: T (tall) and t (dwarf); NCERT warns against T and d, which hide that they are alleles. True-breeding tall and dwarf plants are homozygous, TT and tt; these allele pairs are the genotype, while ‘tall’ and ‘dwarf’ are the phenotype. The F1 heterozygote Tt looks exactly like TT, so T is dominant and t recessive. Tt is a monohybrid, and TT × tt a monohybrid cross.

Because the recessive trait reappears unblended in F2, the alleles must separate (segregate) during meiosis so that each gamete gets only one — with a 50 per cent chance of either. The Punnett square, devised by the British geneticist Reginald C. Punnett, lists the possible gametes on two sides and their combinations inside. Random fertilisation of F1 gametes gives 1/4 TT, 1/2 Tt and 1/4 tt: a genotypic ratio of 1:2:1 but, since TT and Tt both look tall, a phenotypic ratio of 3:1. NCERT writes it as a binomial: (½T + ½t)² = ¼TT + ½Tt + ¼tt. Dwarf F2 plants bred true in F3 and F4, so they were tt.

The test cross. From its appearance alone one cannot tell whether a tall F2 plant is TT or Tt. Mendel crossed it with the recessive (dwarf) parent instead of selfing it — a test cross: an organism showing the dominant phenotype, whose genotype is to be found, is crossed with the homozygous recessive. If all offspring show the dominant trait, the plant was homozygous; if half show the recessive trait, it was heterozygous. NCERT's example uses flower colour, violet (V) dominant over white (v).

Incomplete dominance. In the dog flower (snapdragon, Antirrhinum sp.), a cross between true-breeding red (RR) and white (rr) plants gives pink (Rr) F1 plants, resembling neither parent. Selfing the F1 gives 1 red : 2 pink : 1 white. The genotypic ratio is the usual 1:2:1, but the phenotypic ratio is no longer 3:1, because R is not completely dominant and Rr can be told apart.

What dominance really is. Suppose a gene codes for an enzyme that transforms a substrate S. A modified allele may make (i) a normal or less efficient enzyme, (ii) a non-functional enzyme, or (iii) no enzyme. In case (i) the two alleles are equivalent. In cases (ii) and (iii) the phenotype depends on the one working allele: the unmodified, functioning allele is dominant, and the modified one is usually recessive — the recessive trait appears only when no working enzyme is made.

Co-dominance. Here the F1 resembles both parents. The ABO blood groups are controlled by the gene I, which decides which sugar polymer protrudes from the red blood cell membrane. It has three alleles: I^A and I^B make slightly different sugars, and i makes none. I^A and I^B are each completely dominant over i, but together they both express — co-dominance — so the cells carry both A and B sugars. Three alleles give six genotypes and four blood groups. ABO is also an example of multiple alleles: more than two alleles of one gene exist in the population, though any individual carries only two — so multiple alleles are seen only in population studies.

Dominance depends on the phenotype chosen. Starch synthesis in pea seeds is controlled by one gene with alleles B and b. BB seeds make starch efficiently, have large starch grains and are round; bb seeds make small grains and are wrinkled; Bb seeds are round, so B looks dominant — but their starch grains are of intermediate size, so for grain size the alleles show incomplete dominance. Dominance, NCERT concludes, is not an autonomous feature of a gene; it depends on the gene product and on which phenotype we examine.

Mendel crossed plants with yellow, round seeds (RRYY) and green, wrinkled seeds (rryy). The F1 seeds (RrYy) were all yellow and round — so yellow is dominant over green and round over wrinkled, as in the separate monohybrid crosses. Selfing the F1 gave 3/4 yellow : 1/4 green and 3/4 round : 1/4 wrinkled — each pair still segregating 3:1. Together the four phenotypes — round yellow, round green, wrinkled yellow, wrinkled green — appeared in the ratio 9:3:3:1, which is (3 round : 1 wrinkled) × (3 yellow : 1 green).

From such crosses came Mendel's Law of Independent Assortment: when two pairs of traits are combined in a hybrid, segregation of one pair is independent of the other. In the F1 RrYy plant, 50 per cent of gametes carry R and 50 per cent r; independently, half of each carry Y and half y. So there are four gamete types — RY, Ry, rY, ry — each 1/4, on both the pollen and egg side, and a 16-square Punnett square gives the F2.

Mendel published his work in 1865, but it was ignored until 1900. NCERT gives four reasons: communication was poor; his idea of stable, discrete, non-blending factors did not seem to explain the apparently continuous variation in nature; using mathematics in biology was new and unacceptable to many; and he could offer no physical proof of what factors were. In 1900 three scientists — de Vries, Correns and von Tschermak — independently rediscovered his results. By then microscopy showed chromosomes (‘coloured bodies’, seen by staining) that double and divide before each cell division, and by 1902 their movement in meiosis had been worked out.

Walter Sutton and Theodore Boveri noted that chromosome behaviour parallels gene behaviour and used it to explain Mendel's laws: chromosomes, like genes, occur in pairs; the two alleles of a gene sit at homologous sites on homologous chromosomes; the pairing and separation of a chromosome pair would segregate the factors it carries; and two pairs line up at metaphase I independently of each other. Sutton united chromosomal segregation with Mendel's principles and called it the chromosomal theory of inheritance.

Morgan and the fruit fly. Thomas Hunt Morgan and colleagues verified the theory experimentally with Drosophila melanogaster, which grows on a simple synthetic medium, completes its life cycle in about two weeks, gives many progeny from one mating, has easily distinguished sexes, and shows many hereditary variations visible under a low-power microscope.

Morgan made dihybrid crosses in Drosophila with sex-linked genes — for example, yellow-bodied, white-eyed females with brown-bodied, red-eyed males, intercrossing the F1. The two genes did not segregate independently, and the F2 deviated strongly from 9:3:3:1. Knowing both genes were on the X chromosome, Morgan's group saw that when two genes sit on the same chromosome, parental combinations are far more frequent than non-parental ones. Morgan coined linkage for this physical association of genes on a chromosome and recombination for the generation of non-parental combinations.

Some genes were tightly linked (little recombination) and others loosely linked (more recombination): white and yellow showed only 1.3 per cent recombination, while white and miniature wing showed 37.2 per cent. Morgan's student Alfred Sturtevant used recombination frequency between gene pairs as a measure of the distance between them and ‘mapped’ their positions — the first genetic maps, which today guide whole-genome sequencing such as the Human Genome Project.

Mendel studied traits with distinct alternatives, but many traits are spread across a gradient — human height is not just tall or short. Such traits are controlled by several genes (NCERT: generally three or more) and are called polygenic; the environment also influences them. Human skin colour is the classic example: the phenotype reflects the additive contribution of each allele. In NCERT's model three genes A, B, C control skin colour; AABBCC is darkest, aabbcc lightest, and a genotype with three dominant and three recessive alleles is intermediate — the number of each kind of allele decides the shade.

Pleiotropy. A single gene can show multiple phenotypic effects; such a gene is pleiotropic. Usually the gene affects a metabolic pathway that contributes to several phenotypes. NCERT's example is phenylketonuria (PKU): a mutation in the gene for the enzyme phenylalanine hydroxylase (a single-gene mutation) causes both intellectual disability and reduced hair and skin pigmentation. The pea starch gene in Section 3 (seed shape and starch-grain size) is another case.

The first clue came from insects. Henking (1891) traced a nuclear structure through spermatogenesis and saw that 50 per cent of sperms received it and 50 per cent did not; he called it the ‘X body’ without explaining it. It was later found to be a chromosome — the X chromosome. In many insects sex determination is of the XO type: all eggs carry an X besides the autosomes; some sperms carry an X and some do not; eggs fertilised by X-bearing sperms become females and the others males. In the grasshopper males have only one X besides the autosomes and females a pair — so the sexes differ by one chromosome. The X was named the sex chromosome and the rest autosomes.

In other insects and in mammals including humans, sex determination is of the XY type: males have an X and a distinctly smaller Y, females a pair of X; both sexes have the same number of chromosomes. In both XO and XY systems the male makes two kinds of gametes — male heterogamety. In many birds the female makes two kinds — female heterogamety: females are ZW and males ZZ.

In humans, of 23 pairs of chromosomes, 22 pairs (autosomes) are the same in both sexes; females have XX and males XY. Half the sperms carry X and half Y; every ovum carries X. An X-bearing sperm gives a girl (XX), a Y-bearing sperm a boy (XY). So the sperm's genetic make-up decides the sex, and every pregnancy has a 50 per cent chance of either. NCERT adds that it is unfortunate women are blamed, ostracised and ill-treated for having daughters on this false notion.

In the honey bee sex depends on the number of chromosome sets. A fertilised egg develops into a female (queen or worker) — diploid, 32 chromosomes; an unfertilised egg develops into a male (drone) by parthenogenesis — haploid, 16. This haplodiploid system has odd consequences: males make sperms by mitosis; they have no father and so cannot have sons, but they have a grandfather and can have grandsons.

Mutation is an alteration of DNA sequence that changes the genotype and phenotype; with recombination it is a source of variation. Because one continuous DNA helix runs through each chromatid in a highly supercoiled form, loss (deletion) or gain (insertion or duplication) of a DNA segment alters the chromosome — chromosomal aberrations, commonly seen in cancer cells. A change in a single base pair is a point mutation; the classical example is sickle-cell anaemia. Deletions and insertions of base pairs cause frame-shift mutations (Chapter 5). Chemical and physical agents that induce mutations are mutagens; UV radiation is one.

Controlled crosses are impossible in humans, so the study of a trait over several generations of a family — pedigree analysis — is the alternative. A trait is drawn on a family tree with standard symbols, and pedigree study is a strong tool to trace a specific trait, abnormality or disease. NCERT notes that although DNA is normally passed on unchanged, occasional mutations occur, and many human disorders are associated with altered genes or chromosomes. Genetic disorders fall into two groups: Mendelian (mainly a change in a single gene, inherited by Mendel's rules) and chromosomal (Section 11). Common Mendelian disorders include haemophilia, cystic fibrosis, sickle-cell anaemia, colour blindness, phenylketonuria and thalassaemia; they may be dominant or recessive, and some are sex-linked.

Colour blindness is a sex-linked recessive disorder caused by a defect in the red or green cones of the eye, so red and green cannot be told apart; the genes are on the X chromosome. It occurs in about 8 per cent of males but only about 0.4 per cent of females, because males have one X and females two. The son of a carrier woman has a 50 per cent chance of being colour blind; the mother herself is not, because her normal allele masks the recessive one. A daughter is normally not colour blind unless her mother is a carrier and her father colour blind.

Haemophilia is a sex-linked recessive disease in which one protein of the blood-clotting cascade is affected, so a simple cut can bleed without stopping. It passes from an unaffected carrier female to some of her sons. A female with haemophilia is extremely rare, as her mother must be at least a carrier and her father affected. The family pedigree of Queen Victoria, a carrier, shows several affected descendants.

Sickle-cell anaemia is an autosomal recessive trait transmitted when both parents are carriers (heterozygous). It is controlled by a pair of alleles, Hb^A and Hb^S; only Hb^SHb^S individuals have the disease, while heterozygotes (Hb^AHb^S) appear unaffected but are carriers with a 50 per cent chance of passing the allele on — the sickle-cell trait. The cause is the Glu → Val substitution described in Section 9. Phenylketonuria is an autosomal recessive inborn error of metabolism: the person lacks the enzyme converting phenylalanine to tyrosine, so phenylalanine accumulates and is converted to phenylpyruvic acid and other derivatives; their build-up in the brain causes intellectual disability, and they are excreted in urine because the kidney reabsorbs them poorly.

Thalassaemia is also an autosomal recessive blood disorder passed on when both parents are unaffected carriers. A mutation or deletion reduces the rate of synthesis of one globin chain, giving abnormal haemoglobin and anaemia. In α-thalassaemia, α-globin is affected; it is controlled by two closely linked genes, HBA1 and HBA2, on chromosome 16 of each parent, and arises from mutation or deletion of one or more of the four genes — the more genes affected, the less α-globin. In β-thalassaemia, β-globin is affected; it is controlled by one gene, HBB, on chromosome 11 of each parent, and arises from mutation of one or both copies. Thalassaemia is a quantitative problem (too little globin); sickle-cell anaemia is a qualitative one (a wrongly functioning globin).

Chromosomal disorders are caused by the absence, excess or abnormal arrangement of one or more chromosomes. If chromatids fail to segregate during cell division, a cell gains or loses a chromosome — aneuploidy; for example, Down syndrome results from an extra chromosome 21, and Turner syndrome from loss of an X in females. If cytokinesis fails after telophase, a whole set of chromosomes is added — polyploidy, often seen in plants. A normal human cell has 46 chromosomes (23 pairs): 22 pairs of autosomes and one pair of sex chromosomes. An extra copy of a chromosome is a trisomy; a missing one a monosomy.

The chapter asks many questions in the running text. Short outlines follow, in page order.

Try each question first, then open the outline. For crosses, write gametes first, then the Punnett square, then the ratio.

Numbers

One-liners

• A gamete carries one allele of each gene — the law of segregation.
• Monohybrid F2 is 3:1 in phenotype and 1:2:1 in genotype.
• A test cross is a cross with the homozygous recessive.
• Dihybrid F2 is 9:3:3:1 because two 3:1 segregations are independent.
• In incomplete dominance the phenotype ratio equals the genotype ratio, 1:2:1.
• IA and IB are co-dominant; both are dominant over i.
• Multiple alleles can be found only in population studies.
• Sutton and Boveri proposed the chromosomal theory; Morgan verified it in Drosophila.
• Linkage reduces recombination; Sturtevant used recombination to map genes.
• Polygenic traits are additive and influenced by the environment.
• Pleiotropy: one gene, several effects (PKU).
• In birds the female is ZW and decides the sex of the chick.
• Honey bee drones develop from unfertilised eggs and make sperm by mitosis.
• Haemophilia and colour blindness are X-linked recessive; sickle-cell, PKU and thalassaemia are autosomal recessive.
• Down syndrome is trisomy 21; Klinefelter is XXY; Turner is XO (45, X).

Traps to avoid

• The dihybrid genotypic ratio is not 9:3:3:1 — there are nine genotypes.
• Green pods are dominant in pea, though green seeds are recessive.
• Incomplete dominance is not blending inheritance: the parental forms reappear in F2.
• ABO has six genotypes but only four phenotypes.
• Linkage is an exception to independent assortment, not to segregation.
• Low recombination means tight linkage and close genes.
• In XO insects the male has one chromosome fewer; in XY both sexes have equal numbers.
• In humans the sperm decides sex; in birds the egg does.
• Sickle-cell anaemia is autosomal, not sex-linked (the UPSC 1995 trap).
• GUG is the mRNA codon in sickle-cell; the DNA change is GAG → GTG.
• Myotonic dystrophy (NCERT) is autosomal dominant; Duchenne muscular dystrophy is X-linked recessive.
• Thalassaemia is a quantitative globin defect; sickle-cell is qualitative.

Prelims — what UPSC asks

• Sex-linked disorders are UPSC's favourite: colour blindness is very rare in women (1981) and transmitted by sex chromosomes (1985); a colour-blind mother's son is colour blind, her daughter not (2009); haemophilia is hereditary (1987), caused by defective genes (1992), carried by females and expressed in males (1993), leads to non-clotting of blood (1998), and ‘royal haemophilia’ is sex-linked (1999); a recessive gene on a sex chromosome causes muscular dystrophy (1994). All follow from Section 10.
• Blood groups, PKU, pleiotropy, mapping: an A father and O mother can have an O son (1994; A is also possible but not offered); an AB and an O parent cannot have an O child, so the O son is the adopted one (2011); PKU is an inherited lack of an enzyme (1994); one gene controlling several characters is pleiotropy (2002); gene maps help livestock pedigrees and disease-resistant breeds but cannot explain ‘all’ human diseases (2011). Sex determination by the father's chromosomes (1984, 2000) is shared with Human Reproduction.
• Key notes: 1995 ‘Which set is correctly matched?’ is a flawed question: the keyed option (c) puts sickle-cell anaemia among ‘sex-linked’ disorders, but sickle-cell anaemia is autosomal recessive; the other options are also wrong (AIDS is viral; plague is bacterial; diphtheria, pneumonia and leprosy are infections). Option (c) is the intended answer because colour blindness and haemophilia are sex-linked — treat it as the least-wrong choice. 1994 muscular dystrophy: the key (c) is correct for Duchenne muscular dystrophy (X-linked recessive); do not confuse with NCERT's myotonic dystrophy (autosomal dominant).
• Likely next: ratios (3:1, 1:2:1, 9:3:3:1, 1:1), ABO genotype puzzles, ZW and haplodiploid systems, point mutation in sickle-cell, karyotypes of Down, Klinefelter and Turner, thalassaemia types, and India's sickle-cell elimination mission as current affairs.

Mains — where it appears and how to answer

• No GS Mains question in the bank so far is built on this chapter. Its use is as the science base for GS2 (screening and care for sickle-cell disease and thalassaemia, disability rights, genetic counselling) and GS3 (genome sequencing and mapping, gene therapy, crop and livestock breeding).
• Answer frame: one line of genetics (inheritance pattern, carrier risk) → the burden in India (where carriers are common) → programme or law → positions on screening, attributed → gaps (access, counselling, stigma) → way forward.
• Use person-first, non-stigmatising language (‘a person with sickle-cell disease’, ‘intellectual disability’); a two-line Punnett square can make a risk point quickly.

Tap an option. NCERT's own exercise questions come first; the rest test the figures and common confusions.

Open any question to attempt it in the Question bank — your attempt is saved to your progress.