Principles of Inheritance and Variation — NEET Notes

Mendel and the pea plant

Gregor Johann Mendel, working in his monastery garden at Brünn, carried out hybridisation experiments on the garden pea (Pisum sativum) for seven years between 1856 and 1863. He was the first biologist to apply statistical analysis to a problem in heredity. The pea was well-chosen: it self-pollinates naturally, the flower allows controlled cross-pollination, generation time is short, and a single cross yields hundreds of offspring — enough to give the ratios statistical weight.

Mendel selected 14 true-breeding pea varieties, paired so that each pair differed in exactly one contrasting trait. A true-breeding line, on continuous self-pollination, shows stable inheritance of the trait over many generations. He settled on seven pairs in which the contrast was clean and binary — no intermediate forms.

Mendel did three things his predecessors had not. He worked with true-breeding lines, tracked one or two traits at a time, and counted — recording thousands of F₂ offspring and reducing them to ratios. The result: three principles that still underpin genetics — the Laws of Dominance, Segregation, and Independent Assortment.

Monohybrid cross and the Law of Dominance

Mendel began with one trait at a time. When he crossed a true-breeding tall plant (TT) with a true-breeding dwarf plant (tt), every F₁ offspring was tall. The dwarf trait had not blended away — it had simply not been expressed. When he selfed the F₁ plants, the dwarf trait reappeared in F₂ in the ratio 3 tall : 1 dwarf. Identical 3 : 1 ratios appeared for all seven traits.

Mendel inferred that inheritance is carried by discrete units — "factors", today called genes. Each plant carries two factors for a trait, one from each parent. Different forms of the same gene are called alleles. In a dissimilar pair, the dominant allele is expressed and the recessive is masked. Capital letters denote dominant alleles (T), lowercase the recessive (t). This is the Law of Dominance.

The Law of Segregation and the test cross

The dwarf F₂ plants were the consequence of a second rule. When the heterozygous F₁ (Tt) produces gametes through meiosis, the two alleles segregate cleanly: half the gametes carry T, half carry t. This is the Law of Segregation — the alleles of a pair separate during gamete formation so that each gamete receives only one factor. Also called the principle of purity of gametes.

Reginald C. Punnett devised the tabular Punnett square — NEET 2021 asked its name directly. Gametes of one parent run along the top, gametes of the other along the left column; each intersection shows a zygote genotype.

"During gamete formation the factors of a pair segregate from each other such that a gamete receives only one of the two factors."

Mendel's Law of Segregation — NCERT XII, Ch. 4

From the Tt × Tt Punnett square, one quarter of F₂ are TT, one half Tt, one quarter tt — the genotypic ratio 1 : 2 : 1. Since TT and Tt both look tall, the phenotypic ratio collapses to 3 tall : 1 dwarf. NEET 2016 Q.100 tested this exact genotype-vs-phenotype distinction.

Can you tell whether a tall plant is TT or Tt? No — the phenotype is identical. Mendel devised the test cross: cross the unknown with a homozygous recessive (tt). If TT, every offspring is tall; if Tt, half are tall and half dwarf — a 1 : 1 ratio. The test cross is the standard tool for resolving genotype from phenotype.

Incomplete dominance — when the F₁ is in between

Mendel's seven pea traits all showed clean dominance. In the four o'clock plant Mirabilis jalapa — and the snapdragon Antirrhinum majus — a cross of true-breeding red (RR) with true-breeding white (rr) produced an F₁ of pink (Rr). Neither red nor white. Selfing the pink F₁ gave an F₂ of 1 red : 2 pink : 1 white.

This is incomplete dominance. The genotypic ratio is still 1 : 2 : 1 — segregation is faithful — but the heterozygote is now visibly distinct, so the phenotypic ratio matches the genotypic. Mechanism: the recessive allele produces a non-functional enzyme, and a single dose of functional enzyme gives pink; two doses give red.

Crucially, the Law of Segregation is not violated — only the Law of Dominance fails. NEET 2019 Q.57 hinged on this: the exception in the snapdragon cross is the Law of Dominance, never Segregation.

Codominance, multiple alleles and ABO blood groups

Codominance goes one step further. The F₁ heterozygote shows both parental phenotypes simultaneously, not an intermediate. The textbook case is the ABO blood group in humans, controlled by a single gene I with three alleles: I^A, I^B and i. Alleles I^A and I^B each produce a different surface sugar on the red cell; allele i produces none.

I^A and I^B are both dominant over i, so I^Ai and I^Bi show only the dominant sugar. But when I^A and I^B sit together, both express fully — the red cell carries A and B sugars, giving group AB. The system also illustrates multiple allelism: three alleles in the population, but only two in any one diploid individual.

Three alleles give six genotypes: I^AI^A, I^Ai (group A); I^BI^B, I^Bi (group B); I^AI^B (AB); ii (O). NEET 2017 Q.99 asked this for parents I^AI^B × I^Ai: offspring show 4 genotypes but only 3 phenotypes because two genotypes both code for group A.

Dihybrid cross and the Law of Independent Assortment

Mendel did not stop at one trait. He crossed plants differing in two traits simultaneously — round yellow seeds (RRYY) with wrinkled green seeds (rryy). The F₁ was all round-yellow (RrYy). The surprise came in F₂: four phenotypic classes appeared in the ratio 9 : 3 : 3 : 1.

From this Mendel proposed his third principle: the Law of Independent Assortment. When two trait pairs combine in a hybrid, segregation of one pair is independent of the other — provided the loci lie on different chromosomes. The mathematics is just the product of two monohybrids: (3 : 1) × (3 : 1) = 9 : 3 : 3 : 1.

Chromosomal theory and linkage

Mendel published in 1865 but was ignored until 1900, when de Vries, Correns and von Tschermak independently rediscovered his work. By then microscopy had revealed chromosomes. Sutton and Boveri (1902) noticed that chromosomes and Mendel's factors behave identically — both occur in pairs, both segregate at gamete formation, both assort independently. They proposed the chromosomal theory of inheritance: factors are physically carried on chromosomes.

Thomas Hunt Morgan, working on Drosophila melanogaster, gave it experimental proof. His dihybrid crosses showed that two genes on the same chromosome fail to assort independently. He coined linkage for this physical association and recombination for the rare crossover events that separated them. His student Alfred Sturtevant realised recombination frequency is proportional to gene distance and drew the first genetic map. One map unit (centimorgan) = 1% recombination frequency. NEET 2019, 2022 and 2023 all tested this story.

Pleiotropy — one gene, many effects

Pleiotropy reverses the usual picture: a single gene exerts multiple phenotypic effects. Such a gene is called pleiotropic — tested by NEET 2023 Q.110 and NEET 2016 Q.55. The mechanism is usually a gene product (often an enzyme) sitting in a metabolic pathway with several downstream phenotypes.

The textbook case is phenylketonuria (PKU): a single mutation in phenylalanine hydroxylase produces mental retardation, reduced hair pigmentation, and reduced skin pigmentation — all tracing back to one metabolic block. Sickle-cell anaemia is similarly pleiotropic.

Polygenic inheritance — many genes, one continuous trait

Mendel's traits were qualitative — tall or dwarf, nothing in between. Most natural traits are quantitative: a continuous gradient rather than discrete categories. Human height and skin colour are ranges, not binaries. Such traits are controlled by three or more genes acting additively — polygenic inheritance.

NCERT illustrates with skin colour. Three genes A, B, C contribute to pigmentation; AABBCC gives the darkest skin (six dominant alleles), aabbcc the lightest. Intermediates fall between in proportion to dominant-allele count. Polygenic phenotypes also include an environmental component — nutrition affects height, sun affects skin tone.

Sex determination — four systems

Sex is often fixed at fertilisation by a special pair of sex chromosomes; the rest are autosomes. The first clue came from Henking (1891), who tracked a peculiar nuclear structure through insect spermatogenesis — half the sperm received it, half did not. He called it the "X body"; later workers recognised it as the X chromosome. Four mechanisms appear across organisms.

In humans, of 23 chromosome pairs, 22 are autosomes and one pair is sex chromosomes. Female = 44 + XX; male = 44 + XY. Males produce two kinds of sperm in equal proportion (50% X, 50% Y); the female produces only one kind of ovum (X). The sex of the child is determined by the sperm. NCERT explicitly notes that the social practice of blaming the mother for the sex of the child has no biological basis.

Mutation — the source of new variation

Mutation is any heritable change in DNA sequence — alongside recombination, the second great source of variation. Three structural forms. A point mutation is a single-base-pair change — sickle-cell anaemia, where GAG → GUG and Glu becomes Val at β-globin position 6. Insertions or deletions produce frame-shift mutations. Chromosomal aberrations are larger gains, losses or rearrangements, common in cancer cells.

Agents that induce mutations are mutagens. Physical mutagens include ionising radiations — gamma rays, X-rays, UV. NEET 2021 asked this directly: answer was gamma rays. Infrared rays only cause heating and are not mutagenic. Chemical mutagens include base analogues and alkylating agents. Induced mutagenesis is used in agriculture to generate improved crop varieties.

Pedigree analysis

Humans cannot be subjected to controlled crosses, so geneticists use pedigree analysis — the systematic representation of a trait across generations. Squares are males, circles females, filled symbols affected, half-filled carriers, a horizontal line links partners, a vertical descends to offspring. A double horizontal line indicates consanguineous mating — NEET 2023 Q.163 tested this symbol.

From a pedigree you can deduce whether a trait is autosomal or X-linked, dominant or recessive. A trait that skips generations, appears equally in both sexes, and surfaces in offspring of two unaffected parents is autosomal recessive. A trait in every generation and both sexes is autosomal dominant. A trait affecting males overwhelmingly more than females, with no male-to-male transmission, is X-linked recessive.

Mendelian disorders — single-gene diseases

Mendelian disorders are caused by alteration of a single gene. They obey the same rules as Mendel's pea traits, and pedigree analysis decodes their transmission. Four major patterns: autosomal dominant, autosomal recessive, X-linked dominant, X-linked recessive. NCERT covers five disorders that NEET tests on rotation.

Chromosomal disorders — whole-chromosome errors

If a Mendelian disorder is a typing mistake in a single word, a chromosomal disorder is a torn or extra page. They arise from non-disjunction — failure of chromatids to segregate during meiosis — giving aneuploidy (gain or loss of one chromosome) or polyploidy (gain of an entire chromosome set, common in plants).