Principles of Inheritance and Variation
This chapter explains how Gregor Mendel's pea-plant experiments revealed the laws that govern how traits (and variation) are passed from parents to offspring through discrete 'factors' now called genes.
Genetics is core General Science for Prelims, where Mendelian ratios (3:1, 1:2:1), dominant/recessive concepts, incomplete dominance and the Punnett square are repeatedly tested through definition- and match-the-pair questions. For Mains it feeds GS-III Science & Technology — the conceptual base for biotechnology, gene editing, crop and livestock improvement. The Watson–Crick DNA story bridges classical genetics to molecular biology.
Understand the chapter
Genetics: Inheritance and Variation
Genetics is the branch of biology dealing with the inheritance and variation of characters from parents to offspring. Inheritance is the process by which characters pass from parent to progeny and is the basis of heredity, while variation is the degree to which progeny differ from their parents. Humans exploited naturally occurring variation through artificial selection and domestication as early as 8000–1000 BC, long before its scientific basis was understood.
- Inheritance = transmission of characters parent to progeny (basis of heredity).
- Variation = degree of difference of progeny from parents; a cause lies hidden in sexual reproduction.
- Artificial selection produced Indian breeds, e.g., Sahiwal cows of Punjab, from wild ancestors.
Mendel's Experimental Design
In the mid-19th century Gregor Mendel conducted hybridisation experiments on the garden pea (Pisum sativum) for seven years (1856–1863) and proposed the laws of inheritance. He was the first to apply statistical analysis and mathematical logic to biology, using large sample sizes and confirming inferences over successive generations to give his data credibility. He selected 14 true-breeding pea varieties as seven pairs that were alike except for one character with two contrasting traits.
- True-breeding line = stable trait expression over generations of continuous self-pollination.
- Seven contrasting characters: stem height, flower colour, flower position, pod shape, pod colour, seed shape, seed colour.
- Strength of method: large sampling + statistical rigour + multi-generation confirmation.
Inheritance of One Gene (Monohybrid Cross)
Crossing true-breeding tall (TT) and dwarf (tt) peas produced an F1 generation that was entirely tall — only one parental trait appeared, with no blending. Self-pollinating the F1 gave an F2 in which both traits reappeared unchanged in a 3:1 (tall:dwarf) phenotypic ratio. The underlying genotypic ratio is 1 TT : 2 Tt : 1 tt, i.e., 1:2:1.
- F1 = Filial-1 (all dominant trait); F2 = Filial-2 (both traits).
- F2 phenotypic ratio 3:1; genotypic ratio 1:2:1.
- No blending — contrasting traits stay discrete and the hidden trait re-emerges intact.
Genes, Alleles, Genotype and Phenotype
Mendel's stably inherited 'factors' are today called genes — the units of inheritance carrying the information to express a trait. Alleles are slightly different forms of the same gene that code for contrasting traits (T for tall, t for dwarf). Genotype is the genetic constitution (TT, Tt, tt) while phenotype is the observable trait (tall/dwarf); in a dissimilar pair the dominant allele masks the recessive.
- Homozygous = identical alleles (TT, tt); heterozygous = dissimilar alleles (Tt).
- Dominant trait (capital letter) expressed in F1; recessive (small letter) is masked.
- TT and Tt are phenotypically identical (both tall) — phenotype cannot reveal genotype.
Segregation, Punnett Square and Test Cross
During meiosis the paired alleles separate (segregate) so that each gamete carries only one allele — a random process giving a 50:50 chance, verified by cross results. The Punnett square, devised by British geneticist Reginald C. Punnett, graphically predicts all possible offspring genotypes and their probabilities. To determine whether a dominant-phenotype plant is TT or Tt, Mendel used a test cross — crossing it with the recessive (tt) parent.
- Segregation: a gamete receives only one of the two alleles (50% chance each).
- Punnett square output: 1/4 TT : 1/2 Tt : 1/4 tt.
- Test cross = dominant phenotype × recessive (tt) parent to reveal the unknown genotype.
Mendel's Laws of Inheritance
From his monohybrid crosses Mendel framed two general rules. The First Law (Law of Dominance) holds that characters are controlled by discrete factors occurring in pairs, and in a dissimilar pair one factor dominates the other — explaining the 3:1 F2 ratio. The Second Law (Law of Segregation) holds that the paired alleles separate during gamete formation so each gamete receives only one, with no blending, so both traits are recovered in F2.
- Law of Dominance (First): factors in pairs; one dominant, one recessive.
- Law of Segregation (Second): alleles segregate purely into gametes (law of purity of gametes).
- Segregation is universal; Dominance has exceptions such as incomplete dominance.
Incomplete Dominance
Sometimes the F1 resembles neither parent but is intermediate, as in the snapdragon (Antirrhinum sp.). Crossing true-breeding red (RR) and white (rr) flowers gives a pink (Rr) F1, and selfing yields an F2 of 1 red : 2 pink : 1 white. Here the genotypic ratio stays 1:2:1, but the phenotypic ratio also becomes 1:2:1 because R is not completely dominant over r, making the heterozygote distinguishable.
- Classic example: snapdragon / Antirrhinum sp.
- Phenotypic ratio = genotypic ratio = 1:2:1 (not 3:1).
- Still no blending — alleles keep their identity and reappear unchanged in F2.
Key terms
- Inheritance
- Process by which characters are passed from parent to progeny; the basis of heredity.
- Variation
- The degree by which progeny differ from their parents.
- Gene
- Unit of inheritance (Mendel's 'factor') carrying the information to express a trait.
- Allele
- Slightly different form of the same gene coding for a pair of contrasting traits.
- Genotype vs Phenotype
- Genotype = genetic constitution (TT/Tt/tt); phenotype = observable trait (tall/dwarf).
- Homozygous / Heterozygous
- Identical alleles (TT, tt) versus dissimilar alleles (Tt).
- True-breeding line
- A line showing stable trait inheritance over generations of continuous self-pollination.
- Test cross
- Crossing a dominant-phenotype organism with the recessive parent to determine its genotype.
- Incomplete dominance
- Heterozygote shows an intermediate phenotype, giving a 1:2:1 phenotypic ratio.
- Punnett square
- Graphical grid (by R.C. Punnett) to calculate probabilities of offspring genotypes.
Must-know facts exam-ready
- Mendel conducted pea hybridisation for seven years, 1856–1863, on the garden pea (Pisum sativum).
- He used 14 true-breeding varieties forming 7 pairs of contrasting traits.
- Monohybrid F2: phenotypic ratio 3:1, genotypic ratio 1:2:1.
- F1 expresses only the dominant parental trait; traits show no blending.
- Incomplete dominance (snapdragon/Antirrhinum): F2 phenotype = genotype = 1:2:1.
- Punnett square was devised by British geneticist Reginald C. Punnett.
- Test cross = dominant phenotype × recessive (tt) parent to deduce genotype.
- Mendel's First Law = Law of Dominance; Second Law = Law of Segregation.
- Watson and Crick proposed the DNA double-helix in March 1953 and won the Nobel Prize in 1962.
- Artificial selection is known from 8000–1000 BC; e.g., Sahiwal cows of Punjab.
- Mendel's 'factors' are today called genes; their contrasting forms are alleles.
Timeline
- 8000–1000 BCHumans exploit natural variation through artificial selection and domestication.
- 1856–1863Mendel's seven-year pea hybridisation experiments; laws of inheritance proposed.
- March 1953Watson and Crick propose the complementary double-helical structure of DNA.
- 1962Watson and Crick awarded the Nobel Prize.
Memory tricks remember it for good
Traps to avoid
- The genotypic ratio stays 1:2:1 in both normal and incomplete dominance; only the phenotypic ratio changes (3:1 becomes 1:2:1).
- Incomplete dominance is NOT blending inheritance — alleles keep their identity and reappear unchanged in F2.
- Law of Dominance is the First Law (and has exceptions); Law of Segregation is the Second (no exceptions).
- A test cross uses the recessive (tt) parent — not self-pollination and not the dominant parent.
- TT and Tt are phenotypically identical (both tall); genotype cannot be read off phenotype.
- 'Factors' was Mendel's term; the words 'gene' and 'allele' were coined later, not by Mendel.
Exam focus
🧠 Prelims angles
- Distinguishing monohybrid ratios: 3:1 phenotype versus 1:2:1 genotype.
- Match-the-pair on Mendel's laws (Dominance, Segregation) with their statements.
- Incomplete dominance example and ratio (snapdragon/Antirrhinum, 1:2:1).
- Definitions: genotype vs phenotype, homozygous vs heterozygous, gene vs allele.
- Concept and inventor of the Punnett square (Reginald C. Punnett) and the test cross.
- Watson–Crick DNA double helix and the Nobel year (1962).
✍️ Mains angles GS-III
- How Mendel's principles laid the foundation for modern genetics and biotechnology.Trace 'factors' → genes → DNA double helix (Watson–Crick), then link to crop and livestock improvement.
- Artificial selection and the value of India's indigenous genetic resources.Use Sahiwal cattle and ancient domestication to discuss conservation of native breeds and food security.
Last-minute revision tick as you recall
- Mendel: garden pea, 1856–1863, seven years, seven contrasting traits.
- Monohybrid F2 = 3:1 phenotype, 1:2:1 genotype.
- F1 shows only the dominant trait; traits never blend.
- Genes = Mendel's 'factors'; alleles = contrasting forms; capital letter = dominant.
- Homozygous TT/tt; heterozygous Tt.
- Law 1 = Dominance; Law 2 = Segregation (purity of gametes).
- Test cross = dominant phenotype × recessive (tt) parent.
- Incomplete dominance (snapdragon): 1:2:1 phenotype equals genotype.
- Watson–Crick double helix 1953, Nobel 1962; Punnett square by R.C. Punnett.
Distilled from NCERT Class 12 · Biology (Class 12) for UPSC. Always cross-check facts with the original NCERT.