Heredity
Heredity explains how genes carried on chromosomes pass traits reliably from parents to offspring through Mendel's rules, and how chromosomes determine sex in humans.
Science & Technology is a high-frequency Prelims area, and genetics fundamentals — Mendel's laws, dominant/recessive traits, chromosomes and sex determination — are clean, factual and repeatedly tested. For Mains GS-III this is the conceptual base for biotechnology, gene editing, GM crops and genetic disorders. The father-determines-sex fact also links to GS-I/GS-II social issues like India's skewed sex ratio and the PCPNDT Act, 1994.
Understand the chapter
Variation and its Accumulation During Reproduction
Reproduction produces individuals that share a common body design but carry subtle differences, and these differences accumulate across generations. Asexual reproduction yields only minor variation from small inaccuracies in DNA copying, whereas sexual reproduction maximises diversity through recombination. Not all variants survive equally — the environment selects the favourable ones, which is the very basis of evolution.
- Asexual reproduction → little variation (e.g., a sugarcane field); only minor DNA-copying errors
- Sexual reproduction → much greater diversity
- Heat-tolerant bacteria survive a heat wave: environment selects variants
- Selection of variants by environmental factors = basis of evolutionary processes
Heredity and Inherited Traits
Heredity is the reliable transmission of traits from parents to offspring. A child carries all basic human features yet does not look exactly like its parents, and populations show wide variation. Because father and mother contribute practically equal genetic material, each trait exists in two versions in the child — one paternal, one maternal.
- Free vs attached earlobes = a simple pair of inherited human variants
- Each trait influenced by both paternal and maternal DNA → two versions per trait
- Equal genetic contribution from both parents is the starting assumption
Mendel's Monohybrid Cross — Dominant and Recessive Traits
Mendel crossed pure tall and pure short pea plants; the entire F1 generation was tall, with no medium-height (no blending), so only one parental trait was expressed. On self-pollination, the F2 generation gave both tall and short plants in a 3:1 ratio, proving both traits were inherited but only one expressed. This led him to propose two copies of a 'factor' (now gene) per trait.
- F1 all tall → Law of Dominance (T masks t)
- F2 phenotypic ratio 3:1 (tall:short); genotypic ratio 1:2:1 (TT:Tt:tt)
- TT and Tt are both tall; only tt is short
- Dominant (T) expresses with a single copy; recessive (t) needs both copies
Dihybrid Cross — Independent Inheritance
Mendel bred plants differing in two traits together — round-green seeds crossed with wrinkled-yellow seeds. The F1 were all round and yellow (both dominant), but the F2 threw up new combinations such as round-green and wrinkled-yellow. This showed that seed shape and seed colour are inherited independently of each other, giving the classic 9:3:3:1 ratio.
- F2 phenotypic ratio = 9:3:3:1
- Mendel's counts: 315 round-yellow, 108 round-green, 101 wrinkled-yellow, 32 wrinkled-green (556 seeds)
- Appearance of new trait combinations proves independent assortment
- Each trait assorts separately when gametes (zygotes) re-combine
From Genes to Traits — DNA and Chromosomes
Cellular DNA is the information source for making proteins; a section of DNA coding for one protein is a gene. Genes control traits through enzyme efficiency — an efficient enzyme makes more growth hormone (tall plant), while an altered, less-efficient enzyme makes less (short plant). Genes lie on chromosomes as separate pieces, so body cells carry two copies of each chromosome while germ cells carry only one set, which restores the full number at fertilisation.
- Gene = a section of DNA that provides information for one protein
- Genes act via enzyme → hormone amount → trait (e.g., plant height)
- Genes sit on independent chromosomes — explains why traits assort independently
- Germ cells take one chromosome of each pair; fusion restores species chromosome number
Sex Determination in Humans
Species differ in how sex is fixed: in some reptiles the incubation temperature of eggs decides it, and snails can even change sex, but in humans sex is genetically determined. Of the 23 chromosome pairs, 22 are matched autosomes and the 23rd is the sex-chromosome pair. Females are XX (a perfect pair) and males are XY (the Y being shorter), so all children get an X from the mother and the father's contribution decides the child's sex.
- 22 autosome pairs + 1 sex-chromosome pair = 23 pairs (46 chromosomes)
- Female = XX; Male = XY (Y is the short, mismatched chromosome)
- X from father → girl; Y from father → boy (father determines sex)
- Other systems: temperature in some reptiles; sex-changing snails
Key terms
- Heredity
- Transmission of traits and characters from parents to offspring across generations.
- Variation
- Differences in characters among individuals of a species; the raw material for evolution.
- Gene
- A section of DNA that carries information for making one protein; the unit of inheritance.
- Allele (factor)
- An alternative version of a gene controlling the same trait (e.g., T or t).
- Dominant trait
- A trait expressed even when present in a single copy (e.g., T, tallness).
- Recessive trait
- A trait expressed only when both copies are present (e.g., t, shortness).
- Chromosome
- An independent thread of DNA carrying genes; humans have 23 pairs.
- Autosomes
- The 22 paired chromosomes not involved in sex determination.
- Sex chromosomes
- The pair that decides sex — XX in females, XY in males.
- Germ cell (gamete)
- A reproductive cell that carries only a single set of chromosomes.
Must-know facts exam-ready
- Gregor Johann Mendel (1822–1884), the 'Father of Genetics', used the garden pea (Pisum sativum).
- Mendel was the first to keep count of individuals showing a trait in each generation; he studied at the University of Vienna.
- Monohybrid cross F2 phenotypic ratio = 3:1; genotypic ratio = 1:2:1 (TT:Tt:tt).
- Dihybrid cross F2 phenotypic ratio = 9:3:3:1.
- Mendel's dihybrid counts: 315 round-yellow, 108 round-green, 101 wrinkled-yellow, 32 wrinkled-green = 556 seeds.
- In F1 only the dominant trait appears — no blending or intermediate (no medium-height plants).
- Humans have 23 pairs of chromosomes: 22 autosome pairs + 1 sex-chromosome pair (46 total).
- Females are XX; males are XY, with the Y chromosome being shorter.
- The father determines the sex of the child; every child inherits an X from the mother.
- In some reptiles incubation temperature decides sex, and snails can change sex — so sex is not always genetic.
- Mendel classically studied seven pairs of contrasting pea characters; the chapter lists round/wrinkled seeds, tall/short plants and violet/white flowers.
Memory tricks remember it for good
Traps to avoid
- 3:1 is the F2 phenotypic ratio while 1:2:1 is the genotypic ratio — do not interchange them.
- TT and Tt look identical (both tall): you cannot read genotype off phenotype alone.
- The father, not the mother, determines a child's sex; the mother always contributes an X — used to debunk blaming women.
- 'Dominant' does not mean more common, stronger or better — only that it is expressed over the recessive when present.
- Sex is not always genetic: temperature decides it in some reptiles and snails can change sex — do not over-generalise the human XY system.
- A gene is only a section of DNA on a chromosome, not the whole chromosome; Mendel's 'factors' are today's genes.
Exam focus
🧠 Prelims angles
- Direct numerical MCQs on F2 ratios: 3:1, 1:2:1 and 9:3:3:1.
- Genotype-from-phenotype problems (e.g., identify the tall parent's make-up — TTWW/TtWW type, as in the chapter exercise).
- Sex determination: XX vs XY, role of the father, autosome vs sex-chromosome counts.
- Identifying dominant vs recessive from cross data (violet/white flowers, eye colour, coat colour).
- Mendel factual hooks: garden pea, Father of Genetics, first to quantify trait counts.
- Gene–DNA–chromosome relationship and definitions (gene = DNA segment coding one protein).
✍️ Mains angles GS-III
- Mendel's laws as the foundation of modern genetics and biotechnology.Bridge monohybrid/dihybrid principles to gene mapping, GM crops and gene therapy under GS-III Science & Technology.
- Genetic basis of sex determination and its social/ethical dimensions in India.Use 'father determines sex' to counter the social blaming of women, linking to the skewed sex ratio and the PCPNDT Act, 1994.
- How variation and environmental selection drive survival and evolution.Use heat-tolerant bacteria surviving a heat wave to connect variation to natural selection and adaptation.
Last-minute revision tick as you recall
- Heredity = transmission of traits; variation = raw material for evolution.
- Mendel = Father of Genetics; garden pea (Pisum sativum); first to count traits.
- Monohybrid F2: 3:1 phenotype, 1:2:1 genotype (TT:Tt:tt).
- Dihybrid F2: 9:3:3:1 (Mendel's 556-seed data).
- Dominant = expressed with one copy (T); recessive = needs both (tt).
- Gene = DNA segment → protein (enzyme/hormone) → trait.
- Genes on chromosomes; body cells have 2 sets, germ cells 1 set.
- Humans: 23 pairs = 22 autosomes + XX (female)/XY (male).
- Father determines child's sex; mother always gives X.
Distilled from NCERT Class 10 · Science (Class 10) for UPSC. Always cross-check facts with the original NCERT.