Principles of Inheritance and Variation

The chapter builds in four movements. First, Mendel's own experiments: why the garden pea was an ideal system, the monohybrid cross that yields the Law of Dominance and the Law of Segregation, and the exceptions to simple dominance — incomplete dominance, co-dominance, multiple alleles (ABO blood groups), pleiotropy and polygenic inheritance. Second, the dihybrid cross and the Law of Independent Assortment, followed by the physical proof of Mendel's abstract factors: the Chromosomal Theory of Inheritance and Morgan's discovery of linkage and recombination in Drosophila. Third, the chromosomal mechanics of sex — XX-XY, XO, ZW and the honeybee's haplodiploid system — and mutation as the ultimate source of new variation. Finally, the human cost and diagnosis of genetic error: pedigree analysis and the two broad classes of genetic disorder, Mendelian (single-gene) and chromosomal (whole-chromosome), each illustrated with the disorders named in the NCERT syllabus.

Why Mendel chose the garden pea

Gregor Mendel conducted hybridisation experiments on garden pea (Pisum sativum) for seven years (1856 to 1863) and proposed the laws of inheritance in living organisms. His work was the first to apply statistical analysis and mathematical logic to a biological problem: large sample sizes gave his data credibility, and results confirmed over successive generations showed he had found general rules, not isolated coincidences. The pea plant suited this because Mendel could find and maintain true-breeding lines — lines that, after continuous self-pollination, stably show the same trait generation after generation — and because pea plants offer several easily distinguishable, always-either-or contrasting traits with no intermediate forms. Mendel selected fourteen true-breeding varieties as seven contrasting pairs: stem height (tall or dwarf), flower colour (violet or white), flower position (axial or terminal), pod shape (inflated or constricted), pod colour (green or yellow), seed shape (round or wrinkled), and seed colour (yellow or green).

The monohybrid cross: dominance and segregation

Mendel crossed a true-breeding tall pea plant with a true-breeding dwarf one and grew the resulting seeds into the first hybrid or filial-1 (F1) generation. Every F1 plant was tall — the dwarf trait had vanished. When Mendel self-pollinated these tall F1 plants, the dwarf trait reappeared in the filial-2 (F2) generation, in a ratio of 3 tall to 1 dwarf, with no blending: every plant was fully tall or fully dwarf, never intermediate. He obtained the identical pattern — one trait masking the other at F1, both traits reappearing 3:1 at F2 — for all seven trait pairs he tested. From this Mendel inferred that a stable, particulate factor (what we now call a gene) is passed unchanged from parent to offspring through the gametes. Genes controlling a pair of contrasting traits are alleles — alternative forms of the same gene. By convention, the capital letter denotes the trait expressed at F1 (dominant) and the lowercase letter the trait masked at F1 (recessive): T for tall, t for dwarf. A true-breeding tall plant is homozygous TT and a true-breeding dwarf plant is homozygous tt; TT and tt are the plant's genotype, while "tall" and "dwarf" are its phenotype. The F1 hybrid, genotype Tt, is heterozygous, and because the Tt phenotype is indistinguishable from TT, T is dominant over t. A cross between TT and tt — heterozygous for one gene — is a monohybrid cross. To determine whether a tall plant of unknown genotype is TT or Tt, Mendel devised the test cross: cross the dominant-phenotype individual with the homozygous recessive parent (tt) rather than self-pollinating it. A TT times tt test cross gives all tall (Tt) progeny; a Tt times tt test cross gives tall and dwarf progeny in a 1:1 ratio — the appearance of any dwarf offspring proves the tested parent was heterozygous. Mendel's two rules from the monohybrid cross are formalised as:

Law of Dominance — characters are controlled by discrete units (factors/genes); factors occur in pairs; in a dissimilar pair, one factor dominates the other. This law explains why only one parental character appears at F1 and both reappear at F2 in a 3:1 ratio.

Law of Segregation — the two alleles of a pair do not blend and both are recovered unchanged at F2; during gamete formation the alleles of a pair segregate from each other so that a gamete receives only one of the two. A homozygous parent produces one kind of gamete; a heterozygous parent produces two kinds, in equal (50:50) proportion.

Incomplete dominance and co-dominance

Not every trait shows the clean dominant-versus-recessive pattern of Mendel's peas. In snapdragon (Antirrhinum), crossing true-breeding red (RR) with true-breeding white (rr) gives an F1 that is pink — neither parental colour. Self-pollinating this F1 gives an F2 of 1 red (RR) : 2 pink (Rr) : 1 white (rr): the underlying genotypic ratio is still the classic Mendelian 1:2:1, but because Rr is now visibly distinct from both RR and rr, the phenotypic ratio matches the genotypic ratio instead of collapsing to 3:1. This pattern is called incomplete dominance — R is not completely dominant over r. Why does dominance happen at all? A gene typically carries information to make a functional product, often an enzyme. A mutated allele that still makes a normal or nearly normal enzyme behaves identically to the unmutated allele; but if it makes a non-functional enzyme or none at all, the phenotype depends entirely on the one working allele — that allele is dominant, the non-functional one recessive. Dominance is therefore not a fixed property of a gene alone; it depends on the product and on which phenotype is examined. Starch synthesis in pea seeds shows this: BB makes large starch grains (round seeds) and bb small grains (wrinkled seeds), with Bb producing round seeds — B looks completely dominant for seed shape — yet Bb starch grains are of intermediate size, so the same gene pair shows incomplete dominance if grain size is the phenotype examined.

Co-dominance is different again: here the F1 expresses both parental phenotypes simultaneously rather than blending them, and the clearest textbook example is the human ABO blood group system, covered next.

Multiple alleles: the ABO blood group system

Mendel worked with genes that had only two alleles, but a gene can have more than two allelic forms across a population — even though any one diploid individual can carry only two of them. The ABO blood group gene, symbol I, has three alleles: IA and IB, which each direct synthesis of a slightly different sugar on the red blood cell surface, and i, which produces no sugar at all. IA and IB are each completely dominant over i, but when both IA and IB are present together in the same individual, both sugars are expressed on the red blood cell — this simultaneous, independent expression of two alleles is co-dominance. Because there are three alleles, six genotypes are possible: IAIA and IAi both give blood group A; IBIB and IBi both give blood group B; IAIB gives blood group AB (co-dominance); and ii gives blood group O. Six genotypes therefore collapse into four phenotypes. Multiple alleles of a gene can only be observed by studying a whole population, since any individual carries just two of the alleles. A related but distinct phenomenon is pleiotropy, where a single gene affects several apparently unconnected phenotypic traits at once, usually because the gene's product feeds into more than one metabolic pathway. Phenylketonuria in humans is the textbook example: a mutation in the gene coding for the enzyme phenylalanine hydroxylase causes both mental retardation and reduced pigmentation of hair and skin — two very different symptoms traced to one faulty gene.

Polygenic inheritance

Mendel's traits were discrete alternatives — purple or white flowers, tall or dwarf plants — but many real traits vary continuously across a whole range rather than falling into two neat classes: human height and human skin colour are the classic examples. Such traits are controlled by three or more genes acting together and are called polygenic traits; environmental influence also contributes to the final phenotype. In a polygenic trait each allele's contribution is additive. Consider three hypothetical genes A, B and C for human skin colour, where the dominant forms A, B, C each add to dark pigmentation and the recessive forms a, b, c each contribute to lighter pigmentation: the genotype AABBCC gives the darkest skin, aabbcc the lightest, and genotypes with a mix of dominant and recessive alleles give intermediate shades — the greater the number of dominant alleles, the darker the skin. This additive, multi-gene model is why such traits form a smooth bell-shaped distribution in a population rather than Mendel's sharp either-or classes.

The dihybrid cross and the Law of Independent Assortment

Mendel also crossed pea plants differing in two characters simultaneously — for instance, a plant with yellow, round seeds (RRYY) crossed with one having green, wrinkled seeds (rryy). Every F1 seed was yellow and round (RrYy), confirming yellow dominant over green and round dominant over wrinkled, exactly as the separate monohybrid crosses had shown. Self-pollinating the F1 gave an F2 with four phenotypic classes — round yellow, wrinkled yellow, round green, wrinkled green — in the ratio 9:3:3:1. Crucially, within this F2, yellow to green segregated 3:1 and round to wrinkled segregated 3:1, exactly as in an isolated monohybrid cross of either trait alone; the two traits behaved as if entirely unaware of each other. This led Mendel to his second generalisation, the Law of Independent Assortment: when two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair. In the F1 plant RrYy, the R/r pair segregates 50:50 quite independently of whether the gamete carries Y or y, so four gamete types — RY, Ry, rY, ry — are produced in equal frequency (25 percent each), for both pollen and eggs. A 4 by 4 Punnett square of these sixteen combinations reproduces the 9:3:3:1 ratio, which can equally be derived by multiplying the two separate 3:1 ratios: (3 round : 1 wrinkled) times (3 yellow : 1 green) = 9 round-yellow : 3 wrinkled-yellow : 3 round-green : 1 wrinkled-green.

The chromosomal theory of inheritance

Mendel published his work in 1865, but it stayed unrecognised until 1900, when de Vries, Correns and von Tschermak independently rediscovered his results. By then microscopy had advanced enough for scientists to watch structures in the nucleus double and divide just before cell division — these were named chromosomes. Walter Sutton and Theodor Boveri, around 1902, noticed a striking parallel: chromosomes, like Mendel's genes, occur in pairs; both segregate at gamete formation so only one member of each pair reaches a gamete; and independent chromosome pairs assort independently of each other during meiosis, exactly like independent gene pairs. Sutton united chromosomal behaviour with Mendelian principles into the Chromosomal Theory of Inheritance: the two alleles of a gene pair sit at homologous positions (loci) on a pair of homologous chromosomes, and the physical separation of homologous chromosomes during meiosis is the physical basis of the Law of Segregation, while the independent orientation of different chromosome pairs at the metaphase plate is the physical basis of the Law of Independent Assortment. Experimental proof came from Thomas Hunt Morgan and colleagues, who chose Drosophila melanogaster (the fruit fly) for its rapid two-week life cycle, large broods, easily grown synthetic medium, clear sexual dimorphism, and abundant visible hereditary variations under a low-power microscope.

Linkage and recombination

Morgan carried out dihybrid crosses in Drosophila — for instance, crossing yellow-bodied white-eyed females with brown-bodied red-eyed males and then intercrossing the F1. The two genes did not assort independently: the F2 ratio deviated sharply from the expected 9:3:3:1, with parental-type combinations turning up far more often than new combinations. Morgan traced this to both genes sitting on the same chromosome (the X chromosome, relevant also to sex determination below), coining linkage for this physical association of genes on one chromosome, and recombination for the generation of non-parental gene combinations. Not all linked genes behave alike: Morgan found white and yellow genes very tightly linked, recombining only 1.3 percent of the time, while white and miniature-wing showed 37.2 percent recombination — much more loosely linked. His student Alfred Sturtevant used recombination frequency between gene pairs as a measure of the physical distance between them on a chromosome, producing the first genetic maps — a technique still used as a starting point for genome sequencing projects today.

Sex determination

The chromosomal mechanism of sex determination was first traced through insects. Henking (1891) followed a distinctive nuclear structure through spermatogenesis in some insects and found only 50 percent of sperm received it; later work identified this "X body" as a chromosome, hence X chromosome. In many insects sex determination is the XO type: every egg carries an X chromosome, but only half the sperm do; eggs fertilised by X-bearing sperm become female (with two X chromosomes), the rest become male (with only one X plus autosomes) — the grasshopper is the classic example. In humans, Drosophila and many other insects and mammals, sex determination is the XY type: both sexes have the same total chromosome number, but the male's X has a distinctly smaller, dissimilar partner called the Y chromosome. Females are XX (homogametic — one type of egg), males are XY (heterogametic — two types of sperm, X-bearing and Y-bearing) — an instance of male heterogamety. In humans specifically, 22 of the 23 chromosome pairs are autosomes, identical in both sexes; the twenty-third pair is XX in females and XY in males. Because sperm alone determines whether the offspring is X-bearing (female, XX) or Y-bearing (male, XY), and both outcomes are equally likely, each pregnancy carries an equal (50 percent) chance of either sex — the genetic makeup of the father's sperm, never the mother, decides the child's sex. Birds show the reverse pattern: ZW type, or female heterogamety. Total chromosome number is the same in both sexes, but females are heterogametic (ZW, producing two egg types) while males are homogametic (ZZ). Honeybees use a wholly different mechanism: haplodiploidy. A fertilised egg (union of sperm and egg) develops into a diploid female (queen or worker, 32 chromosomes); an unfertilised egg develops by parthenogenesis into a haploid male drone (16 chromosomes). Because males arise without a father, they cannot have sons, but — since their mother had a father — they do have a grandfather and can have grandsons; and because males are haploid, they produce sperm by mitosis, not meiosis.

Mutation

Mutation is any change in the DNA sequence that alters an organism's genotype and, potentially, phenotype; alongside recombination, it is the other principal source of variation in DNA. Because each chromatid carries one continuous DNA helix in highly supercoiled form, loss (deletion) or gain (insertion or duplication) of a DNA segment alters chromosome structure — such chromosomal aberrations are commonly seen in cancer cells. A change confined to a single DNA base pair is a point mutation; sickle-cell anaemia is the classic example. Insertions or deletions of base pairs also cause frame-shift mutations. Physical and chemical agents that induce mutation are called mutagens — ultraviolet radiation is one example.

Pedigree analysis and human genetic disorders

Controlled breeding crosses are impossible in humans, so geneticists instead study pedigree analysis — tracing the inheritance of a trait, abnormality or disease through several generations of a family tree using standard symbols. Human genetic disorders fall into two broad categories, Mendelian (single-gene) and chromosomal (whole-chromosome).

Mendelian disorders arise from a mutation in a single gene, transmitted exactly as Mendel's principles predict, and traceable — dominant or recessive, autosomal or sex-linked — by pedigree analysis. Haemophilia, X-linked recessive, disables a clotting-cascade protein, so a cut bleeds without stopping; it passes from an unaffected carrier female to roughly half her sons, and a daughter is haemophilic only if her mother is at least a carrier and her father is haemophilic. Colour blindness, also X-linked recessive, is a red or green cone defect affecting about 8 percent of males but only about 0.4 percent of females, since a male's single X fully expresses the recessive allele while a female needs two copies. Sickle-cell anaemia, autosomal recessive, needs both parents heterozygous (HbAHbS); only homozygous HbSHbS shows disease, caused by glutamic acid substituted by valine at the sixth position of the haemoglobin beta chain (base change GAG to GUG at codon 6), which makes the mutant haemoglobin polymerise under low oxygen and distort RBCs from biconcave discs into sickles. Phenylketonuria, autosomal recessive, leaves the affected individual unable to convert phenylalanine to tyrosine, so it accumulates and causes mental retardation. Thalassemia, autosomal recessive, reduces synthesis of one globin chain (alpha or beta) — a quantitative defect (too little normal chain), unlike sickle-cell anaemia's qualitative defect (a normal amount of malformed chain); alpha thalassemia involves genes HBA1 and HBA2 on chromosome 16, beta thalassemia the single gene HBB on chromosome 11.

Chromosomal disorders arise from the absence, excess, or abnormal arrangement of whole chromosomes, not a single-gene mutation. Failed chromatid segregation causes aneuploidy — gain or loss of one chromosome; failed cytokinesis after telophase causes polyploidy — a whole extra chromosome set, common in plants. A normal human cell has 46 chromosomes (22 autosome pairs plus one sex-chromosome pair); gaining or losing one chromosome is trisomy or monosomy. Down syndrome (Langdon Down, 1866) is trisomy 21 (47 chromosomes); affected individuals are short-statured with a small round head, furrowed tongue, partially open mouth, a characteristic palm crease, and retarded physical, psychomotor and mental development. Klinefelter syndrome is an extra X chromosome (47, XXY): overall masculine development with some feminine development (gynaecomastia) and sterility. Turner syndrome is a missing X chromosome (45, X0): sterile females with rudimentary ovaries and reduced secondary sexual characteristics.

Board level - Dihybrid cross between tall yellow-seeded and tall green-seeded pea plants

A cross is made between a tall plant with yellow seeds (TtYy) and a tall plant with green seeds (Ttyy). Find the expected proportion of offspring that are (a) tall and green, and (b) dwarf and green.

Step 1. Split the dihybrid cross into its two independent monohybrid crosses, since height and seed colour assort independently. For height: Tt times Tt gives 3/4 tall : 1/4 dwarf. For seed colour: Yy times yy is a test cross, giving 1/2 yellow : 1/2 green.

Step 2. Combine the two independent ratios by multiplication for part (a): tall and green = (3/4 tall) times (1/2 green) = 3/8.

Step 3. Combine the two independent ratios by multiplication for part (b): dwarf and green = (1/4 dwarf) times (1/2 green) = 1/8. Answer: 3/8 of the offspring are expected to be tall and green, and 1/8 are expected to be dwarf and green.

Board level - Determining parental genotypes from an ABO blood-group cross

A child has blood group O. The father has blood group A and the mother has blood group B. Work out the genotypes of the parents and the possible genotypes of any other offspring from this couple.

Step 1. The child is blood group O, genotype ii, so the child received one i allele from each parent. This means both parents, despite showing dominant phenotypes, must carry a hidden i allele — the father must be IAi (not IAIA, since IAIA could not pass an i allele) and the mother must be IBi (not IBIB, for the same reason).

Step 2. Set up the cross IAi times IBi and build the Punnett square: father's gametes are IA and i in equal proportion; mother's gametes are IB and i in equal proportion. The four equally likely zygote combinations are IAIB, IAi, IBi, and ii.

Step 3. Read off phenotypes: IAIB is blood group AB (co-dominance), IAi is blood group A, IBi is blood group B, and ii is blood group O. Answer: the parents' genotypes are IAi (father, group A) and IBi (mother, group B); their children can be group AB, A, B, or O in equal 1:1:1:1 proportion, confirming that two group A and B parents can indeed have a group O child.

Competitive level (NEET) - X-linked recessive inheritance of haemophilia across a cross

A woman who is a carrier for haemophilia (unaffected, genotype XHXh) marries a normal, non-haemophilic man (XHY). Work out the expected genotypes and phenotypes of their children, and state which sex can never receive the disorder from this particular cross.

Step 1. Haemophilia is X-linked recessive, so the allele is carried only on the X chromosome; the Y chromosome carries no corresponding allele. The mother, XHXh, produces two egg types in equal proportion: XH and Xh. The father, XHY, produces two sperm types in equal proportion: XH and Y.

Step 2. Build the Punnett square from these four gametes (XH, Xh from mother crossed with XH, Y from father), giving four equally likely zygote genotypes: XHXH, XHXh, XHY, and XhY.

Step 3. Read off phenotypes. Daughters (XHXH or XHXh) always receive a paternal XH, so every daughter has at least one normal allele and is phenotypically unaffected — though half of all daughters (XHXh) are carriers like their mother. Sons receive only the maternal X, so half the sons are XHY (unaffected) and half are XhY (haemophilic, since a single recessive allele on the lone X is fully expressed with no second allele to mask it). Answer: 1/4 XHXH (unaffected daughter), 1/4 XHXh (carrier daughter, unaffected), 1/4 XHY (unaffected son), 1/4 XhY (haemophilic son); overall, half of all sons are haemophilic and no daughter from this cross is ever haemophilic, though half of daughters are carriers. Timing note: about 2 minutes. NEET frequently tests X-linked recessive crosses this way — set up the cross using X-superscript notation rather than plain letters, since the Y chromosome carries no allele at all. The recurring trap is forgetting that a carrier mother can produce an affected son with no affected father anywhere in the pedigree, and conversely that a daughter can only be affected if her father is also affected (or a rare XhXh combination arises) — with a normal father, as in this cross, daughters are always safe.

Students often say the F1 generation "blends" the two parental traits — it does not, under complete dominance; only incomplete dominance produces a genuinely intermediate F1, and even then the underlying alleles remain unblended, as proved by their unchanged reappearance at F2 in the correct 1:2:1 ratio. A second common slip is treating genotype and phenotype as interchangeable: TT and Tt are different genotypes but the identical phenotype under complete dominance, which is exactly why a test cross, not simple observation, is needed to tell them apart. Students frequently misstate the Law of Independent Assortment as applying to all genes in an organism — it only holds for genes on different chromosome pairs, or far enough apart on the same chromosome to recombine freely; genes close together on the same chromosome are linked and do not assort independently, as Morgan's Drosophila crosses demonstrated. Many also confuse aneuploidy (gain or loss of one chromosome, as in Down, Klinefelter or Turner syndrome) with polyploidy (gain of one or more entire extra chromosome sets), and confuse a point mutation (single base-pair change, as in sickle-cell anaemia) with a chromosomal aberration (loss, gain or rearrangement of a DNA segment large enough to be visible under a microscope). Finally, sex-linked disorder pedigrees are routinely misread: haemophilia and colour blindness are X-linked recessive, so a carrier mother (unaffected) can have an affected son with no affected father anywhere in the family, and an affected daughter requires an affected father plus at least a carrier mother — a pattern many students wrongly assume needs an affected parent of either sex.

• State Mendel's Law of Dominance and Law of Segregation and explain how a monohybrid cross (Tt times Tt) produces a 3:1 phenotypic but 1:2:1 genotypic F2 ratio.
• State the Law of Independent Assortment and derive the 9:3:3:1 dihybrid F2 ratio from two independent 3:1 ratios.
• Design and interpret a test cross to determine whether a dominant-phenotype individual is homozygous or heterozygous.
• Distinguish incomplete dominance, co-dominance, multiple alleles, pleiotropy and polygenic inheritance, and give the NCERT example for each (Antirrhinum, ABO blood group, ABO again, phenylketonuria, human skin colour).
• Explain the Chromosomal Theory of Inheritance and connect the behaviour of homologous chromosomes in meiosis to both Mendelian laws.
• Explain linkage and recombination as Morgan defined them, and explain why genetically close genes recombine less often than distant ones.
• Compare XX-XY, XO, ZW and haplodiploid sex-determination systems and identify the heterogametic sex in each.
• Distinguish a point mutation from a chromosomal aberration, and aneuploidy from polyploidy.
• Read a simple pedigree to determine whether a Mendelian disorder is autosomal or X-linked, and dominant or recessive.
• List the NCERT Mendelian disorders (haemophilia, colour blindness, sickle-cell anaemia, phenylketonuria, thalassemia) and chromosomal disorders (Down, Klinefelter, Turner syndrome) with their defining genetic cause.

Ask the tutor: Want to work through a few more Punnett-square or pedigree problems together, including test crosses and X-linked disorder probabilities, to build speed for board and NEET questions alike?