Heredity
Walk down any street and you will not find two people who look exactly alike, even siblings from the same parents. Walk through a field of sugarcane, by contrast, and the plants look almost like copies of one another. Both scenes show the same rule from opposite ends: reproduction always creates some variation, but how much depends on how the reproduction happens. Sugarcane is usually propagated asexually, so its offspring are close copies with only tiny differences from small errors in DNA copying. Human beings reproduce sexually, which shuffles genetic material from two parents into new combinations every time — why a family resembles itself without any two members being identical. This chapter asks a sharper question than "why do we vary": what are the actual rules by which traits pass from parents to children, generation after generation? A monk named Gregor Mendel answered this over a century ago by growing thousands of pea plants, crossing them deliberately, and counting every offspring by trait — something nobody had done so carefully before. From those counts came this chapter's two central discoveries. First, each trait is controlled by two copies of a hereditary unit, a gene, one from each parent; when the two copies differ, only one, the dominant trait, is expressed, while the other, the recessive trait, stays hidden but not lost. Second, different traits are inherited independently of each other, because the genes for them sit on separate, independently shuffled chromosomes. By the end you will be able to predict, using nothing more than a small grid called a Punnett square, exactly what ratio of tall to short, round to wrinkled, or even boy to girl you should expect from a given cross — and know precisely why that ratio comes out the way it does.
- Variation arises in every reproduction; asexual reproduction (e.g. a bacterium dividing) gives only small copying-error variation, while sexual reproduction shuffles genetic material from two parents into far greater diversity.
- Variations are not all equally useful — environmental selection of the fittest variants is the seed of evolution, taken up in later chapters.
- Mendel's monohybrid cross (tall crossed with short pea plants) gave an all-tall F1, and a self-pollinated F2 with a 3 tall to 1 short ratio — both traits inherited, only one expressed.
- A dominant trait (tallness, T) is expressed whenever at least one copy is present (TT or Tt); a recessive trait (shortness, t) is expressed only when both copies are recessive (tt).
- Mendel's dihybrid cross (round-green seeds crossed with wrinkled-yellow, giving an all round-yellow F1) gave F2 in the ratio 9 round-yellow : 3 round-green : 3 wrinkled-yellow : 1 wrinkled-green, proving seed shape and seed colour are inherited independently.
- Genes are sections of DNA carrying instructions for one protein; a protein such as an enzyme can control how much growth hormone is made, and that amount can decide a visible trait such as plant height.
- Every body cell carries two copies of each chromosome, one from each parent; a germ cell (gamete) carries only one from each pair, so fertilisation restores the double set.
- In humans, 22 chromosome pairs are matched autosomes, but the 23rd pair is the sex chromosomes: women XX, men XY; since the mother always gives X, the father's sperm — X or Y with equal chance — determines a child's sex.
- Key idea: every trait is carried as two gene copies, one from each parent, on separate chromosomes that assort independently during germ-cell formation, so a dominant copy masks a recessive one without erasing it, producing Mendel's 3:1 and 9:3:3:1 ratios and the 1:1 chance of a boy or a girl.
Every inherited trait is controlled by two gene copies, one contributed by each parent through germ cells that carry only one chromosome from each pair, so that a dominant copy is expressed and a recessive copy stays hidden but is still passed on, producing Mendel's predictable ratios of 3:1 for a single trait and 9:3:3:1 for two independently assorting traits, and in humans deciding sex through whether the father's sperm carries an X or a Y chromosome.
What this chapter covers
The NCERT chapter builds the rules of heredity in two connected stages.
- Accumulation of variation during reproduction (8.1): why asexual reproduction creates small copying-error variations, why sexual reproduction creates far more, and why environmental selection of variants underlies evolution.
- Heredity (8.2): inherited traits (8.2.1); Mendel's pea-plant experiments — the monohybrid cross, dominant and recessive traits, and the dihybrid cross showing independent inheritance (8.2.2); how genes control traits through proteins, and why each germ cell carries only one chromosome set (8.2.3); and how sex is determined across species, with a close look at the human XX/XY system (8.2.4).
The core ideas
Variation: the raw material every generation works with
Reproduction never produces a perfect photocopy. When one bacterium divides into two, and those two divide again into four, small inaccuracies creep into the DNA copying, so the descendants are not perfectly identical — just very close. This is asexual reproduction, and since only one parent is involved, the variation it generates is modest. Sexual reproduction, combining genetic material from two parents, generates far more diversity each generation, because every offspring gets a fresh combination of traits rather than a near-copy of one ancestor. Not every variation is equally useful for survival. A bacterial population that happens to include heat-tolerant individuals does better than average in a heat wave; the environment effectively "selects" which variants thrive and reproduce further — the seed of the evolutionary process taken up in later study. This also answers a subtler question: if a trait is found in 60 percent of an asexually reproducing population and another in only 10 percent, the more common trait has most likely had more generations to spread, so it probably arose earlier. Frequency in a population is a clue about how long a variation has existed — a completely different question from whether it is dominant or recessive, which the next section makes precise.
Inherited traits and Mendel's monohybrid cross
A simple classroom activity makes inheritance concrete: look at the earlobes of everyone around you. Some have a free earlobe hanging loose from the head, others an attached earlobe fused to the side of the head. Comparing each student's earlobe type with their parents' suggests a rule for how the trait passes down — exactly the kind of observation that, counted carefully, becomes genetics. Gregor Mendel did this systematising with garden pea plants in the 1860s, choosing contrasting, clearly visible traits — tall versus short, round versus wrinkled seeds, violet versus white flowers — and counting, generation after generation, exactly how many offspring showed each trait. He crossed a pure-breeding tall plant with a pure-breeding short plant. The F1 generation was not a "medium height" blend: every plant was tall, one parental trait completely expressed, the other completely hidden. Letting these tall F1 plants self-pollinate to produce the F2 generation, Mendel found shortness had not vanished: about one quarter of F2 plants were short — a ratio of 3 tall to 1 short. This result only makes sense if each plant carries two copies of the hereditary factor for height, now called a gene, one inherited from each parent. Writing the tall copy as T and the short copy as t, the pure-breeding parents are TT and tt. Every TT gamete carries T and every tt gamete carries t, so every F1 offspring is Tt — one copy of each, yet visibly tall. A single T copy is enough to make the plant tall: T is the dominant trait, expressed whenever at least one copy is present, while t is the recessive trait, expressed only when both copies are recessive. When two Tt plants are crossed (or a Tt plant self-pollinates), each parent's gametes are equally likely to carry T or t.
where: each bracket is one parent's gametes, each with probability one half; multiplying the brackets gives every offspring combination, and the genotype ratio . Since both TT and Tt look tall, and only tt looks short, the phenotype ratio collapses to:
where: genotype is the actual pair of gene copies an individual carries (TT, Tt or tt), while phenotype is what is actually seen (tall or short); the familiar 3 tall to 1 short ratio is simply the genotype ratio re-grouped by what each genotype looks like. The first :::svg figure below shows this monohybrid cross laid out as a Punnett square, the standard grid Mendel's logic is drawn in.
The dihybrid cross: two traits, inherited independently
Mendel did not stop at one trait. He crossed a tall, round-seeded plant against a short, wrinkled-seeded one, and F1 was uniformly tall with round seeds — both dominant. The real test came at F2, from self-pollinating F1. If the two traits always travelled together, F2 should show only the two original combinations: tall-round and short-wrinkled. Instead Mendel found F2 plants with new combinations too — some tall-wrinkled, some short-round — proving plant height and seed shape are inherited independently. Using R for round seeds (dominant) and r for wrinkled (recessive), Y for yellow seed colour (dominant) and y for green (recessive), Mendel's actual parents were RRyy (round, green) and rrYY (wrinkled, yellow), giving an F1 of RrYy — round and yellow, since R and Y are both dominant. Because the two gene pairs assort independently, an RrYy plant produces four kinds of gametes in equal proportion: RY, Ry, rY and ry.
where: is the number of distinct gamete types a plant can produce and is the number of gene pairs it is heterozygous for; with independently assorting pairs (R/r and Y/y), , exactly the four gamete types RY, Ry, rY and ry that combine in every possible way in the F2 Punnett square. Working through all sixteen combinations of these four gamete types (the second derivation below carries out this count in full) gives the classic dihybrid phenotype ratio:
where: the four fractions sum to , reproducing Mendel's dihybrid ratio of 9 : 3 : 3 : 1, shown in the bar chart below and checked against Mendel's own seed counts in the third worked example.
From gene to trait: how DNA actually controls what you see
Mendel worked out the pattern of inheritance without knowing what a gene physically was. The modern answer starts with DNA, the information-carrying molecule inside every cell. A section of DNA carrying instructions for making one particular protein is called a gene for that protein. Proteins can act as enzymes that build or control other molecules. Take plant height: a plant grows taller by making more of a growth-triggering hormone, and how much hormone gets made depends on how efficiently an enzyme carries out the step that produces it. An efficient enzyme means plenty of hormone and a tall plant; a gene alteration that makes the enzyme less efficient means less hormone and a short plant. The third derivation below traces this chain — gene, protein, enzyme, hormone, trait — in full, along with why every germ cell carries only one copy of each gene, not two.
Chromosomes, germ cells, and why traits sort independently
If both parents contribute equally to a child's DNA — and Mendel's ratios only make sense if they do — then every gene must exist as two copies, one from each parent. For that arithmetic to stay balanced across generations, each germ cell (sperm, egg, or pollen and egg cell) must carry only a single copy of every gene, so fertilisation restores the double set rather than doubling it again. The physical reason this works is that a full gene set is not one continuous thread but separate, independently manageable pieces of DNA called chromosomes. Every body cell carries two copies of each chromosome, one maternal and one paternal, and when germ cells form, each draws exactly one chromosome from every pair — maternal or paternal, decided independently for each pair.
where: this is precisely how human chromosomes are organised, as the next section on sex determination sets out; the key structural point is that because different chromosome pairs are inherited independently, genes on different chromosomes — such as the height and seed-shape genes — are shuffled independently too. This chromosome-level independence is the physical mechanism behind Mendel's dihybrid 9 : 3 : 3 : 1 result, not a coincidence of pea genetics.
Sex determination: why a child is a boy or a girl
Different species decide the sex of their offspring in strikingly different ways. In some reptiles, the incubation temperature of the eggs decides whether hatchlings are male or female — an entirely environmental cue. In some animals, such as certain snails, an individual can change sex during its life, ruling out a fixed genetic determination. In human beings, sex is determined almost entirely genetically, through the sex chromosomes. Of the 23 pairs of chromosomes in a human cell, 22 are perfectly matched autosomes. The 23rd pair differs: women carry a matched pair, both a full-sized X, written XX; men carry a mismatched pair, one normal X and one much shorter Y, written XY. Every egg a woman produces carries an X, since that is all she has to give. A man's sperm is of two kinds in equal number — half carry an X, half a Y — because his own X and Y separate into different sperm during germ-cell formation.
where: the probability comes entirely from which of the father's two kinds of sperm fertilises the egg — an X-carrying sperm gives XX, a daughter; a Y-carrying sperm gives XY, a son — since the mother's contribution is always X and therefore cannot tip the outcome either way. The svg figure below sets this out as a cross, exactly parallel to the pea-plant Punnett squares used earlier in the chapter.
The goal is to show, purely by combining probabilities, why crossing two Tt pea plants reliably gives close to three tall offspring for every one short offspring, turning Mendel's observation into an arithmetic certainty rather than a coincidence of one experiment, while making precise the difference between the underlying genotype ratio and the visible phenotype ratio a student actually counts.
Step 1. List each parent's possible gametes. A Tt plant makes two kinds of gametes, T and t, in equal number, because the two chromosome copies separate independently when germ cells form. Each parent contributes T with probability one half and t with probability one half.
Step 2. Combine the two parents' gametes into every possible pairing. An offspring's genotype is fixed by which gamete it receives from each parent, and every pairing is equally likely since the two parents' choices are independent: T from one parent can pair with T or t from the other, and likewise for t. This gives four equally likely outcomes: TT, Tt, Tt and tt.
Step 3. Collapse the four outcomes into a genotype ratio. Because Tt occurs in two of the four equally likely ways, the genotype ratio is , matching .
Step 4. Translate genotype into phenotype using dominance. A single T copy is enough to make a plant look tall, so both TT and Tt look tall while only tt looks short. Adding the TT and Tt probabilities gives tall, leaving short — the phenotype ratio of 3 tall to 1 short. Shortness was never destroyed in tall F1; it was masked in every Tt individual and resurfaces when two t copies meet again.
Go deeper · Derivation B · 1 minFrom two independent gene pairs to the 9:3:3:1 ratio
The goal is to extend the same gamete-counting logic from one trait to two traits at once, so the dihybrid ratio of 9 round-yellow to 3 round-green to 3 wrinkled-yellow to 1 wrinkled-green stops looking like a mysterious number Mendel happened to count and becomes the direct, countable consequence of two gene pairs assorting into gametes independently and then combining at random.
Step 1. List the four gamete types an RrYy plant can make. Because the R/r pair and the Y/y pair sit on separate chromosomes and are inherited independently, an RrYy plant produces gametes of four equally likely kinds: RY, Ry, rY and ry, matching .
Step 2. Build the full Punnett square. Crossing two RrYy plants means pairing each of one parent's four gamete types with each of the other's four, giving equally likely combinations — the sixteen boxes of the dihybrid Punnett square.
Step 3. Sort the sixteen genotypes by what they look like. Any genotype with at least one R and one Y looks round-yellow; working through the grid, nine of sixteen boxes satisfy this. Three boxes have at least one R but only yy, giving round-green. Three have only rr but at least one Y, giving wrinkled-yellow. Exactly one box has rryy, giving wrinkled-green.
Step 4. Read off the ratio. Counting the sorted boxes gives round-yellow : round-green : wrinkled-yellow : wrinkled-green out of 16, precisely Mendel's observed dihybrid ratio — and the two new combinations, round-green and wrinkled-yellow, seen in neither original parent, are direct proof that the seed-shape and seed-colour genes assort independently.
Go deeper · Derivation C · 1 minFrom an altered gene to a changed trait, and why germ cells carry only one gene set
The goal is to connect Mendel's abstract "factors" to the physical biology of DNA, protein and chromosomes, tracing one unbroken chain from a gene being slightly different in its DNA sequence to a visibly different trait in the whole organism, then showing why that same chromosome mechanism is why each germ cell carries only half the normal gene set that every other body cell holds.
Step 1. A gene is a set of instructions written in DNA. A particular stretch of a cell's DNA carries the coded information needed to make one specific protein; this stretch is what is meant by a gene for that protein.
Step 2. The protein made from that gene can control a chemical process. If the protein is an enzyme involved in producing a plant growth hormone, how efficiently it works decides how much hormone gets made. A gene whose sequence produces a highly efficient enzyme leads to plenty of hormone.
Step 3. The amount of hormone decides the visible trait. Plenty of growth hormone drives strong stem growth and a tall plant; a gene alteration that makes its enzyme less efficient means less hormone and a short plant — the physical route by which a difference buried in DNA sequence becomes a difference you can see.
Step 4. Every body cell needs two gene copies, so every germ cell must carry only one. Since both parents contribute equally to a child's DNA, every body cell holds two copies of each gene, packaged as two copies of each chromosome. If a germ cell carried both copies of every chromosome, fertilisation would double the chromosome number each generation, which is impossible. Instead, when germ cells form, exactly one chromosome from each matched pair is drawn in, maternal or paternal decided independently for every pair, so fertilisation restores — never doubles — the normal double set.
Worked examples
Board level - genotype and phenotype ratios from a Tt self-cross
A tall pea plant with genotype Tt is allowed to self-pollinate. Using T for the dominant tall allele and t for the recessive short allele, find the genotype ratio and the phenotype ratio expected in the offspring.
Step 1. List the gametes each Tt parent can make: T with probability one half, t with probability one half.
Step 2. Combine the gametes from both parents to get four equally likely genotype outcomes: TT, Tt, Tt, tt.
Step 3. Group identical genotypes: the genotype ratio is . Since both TT and Tt are tall and only tt is short, the phenotype ratio is tall to short . Answer: genotype ratio TT : Tt : tt = 1 : 2 : 1; phenotype ratio tall : short = 3 : 1.
Board level - deducing a parent's genotype from its offspring (NCERT exercise pattern)
A Mendelian experiment crossed a tall pea plant bearing violet flowers with a short pea plant bearing white flowers. All the progeny bore violet flowers, but almost half of them were short. Using T/t for tall/short and W/w for violet/white (both violet and tall are dominant), find the genetic make-up of the tall parent.
Step 1. Fix the short, white parent's genotype first. Since short and white are both recessive phenotypes, this parent must be homozygous recessive for both: genotype ttww, contributing only a t and a w gamete.
Step 2. Use the flower-colour result to fix the tall parent's W-gene genotype. Every offspring is violet, none white. Since the ttww parent always contributes w, every offspring can only be violet if the tall parent contributes W in every gamete — so the tall parent must be homozygous WW (a Ww parent would give roughly half white offspring, which was not observed).
Step 3. Use the height result to fix the tall parent's T-gene genotype. Almost half the offspring were short, matching the 1:1 ratio a Tt by tt cross gives. If the tall parent were TT instead, every offspring would be tall, contradicting the observation.
Step 4. Combine both genes. The tall parent is heterozygous for height (Tt) but homozygous dominant for flower colour (WW). Answer: the tall, violet-flowered parent has genotype TtWW.
Board HOTS / foundation JEE-NEET - checking Mendel's own dihybrid seed counts against the 9:3:3:1 ratio
Mendel's dihybrid cross of a round-green-seeded pea plant (RRyy) with a wrinkled-yellow-seeded plant (rrYY) gave F1 RrYy, and self-pollinating F1 produced 556 F2 seeds in total, actually counted as 315 round-yellow, 108 round-green, 101 wrinkled-yellow and 32 wrinkled-green. Using the theoretical 9:3:3:1 ratio, calculate the expected count in each class for a total of 556 seeds, and compare with what Mendel actually counted.
Step 1. Find the value of "one part" of the ratio. The ratio 9 : 3 : 3 : 1 has total parts, so for 556 seeds, one part is seeds.
Step 2. Scale each class by its number of parts. Round-yellow (9 parts): . Round-green and wrinkled-yellow (3 parts each): . Wrinkled-green (1 part): .
Step 3. Compare expected against Mendel's actual counts. Expected 313 versus actual 315 round-yellow; 104 versus 108 round-green; 104 versus 101 wrinkled-yellow; 35 versus 32 wrinkled-green — every class lies within about 4 seeds of the theoretical prediction. Answer: the 9:3:3:1 ratio predicts about 313 : 104 : 104 : 35 out of 556 seeds, and Mendel's actual counts of 315 : 108 : 101 : 32 match closely, confirming independent assortment. Timing note: about three minutes. Find "one part" by dividing the grand total by 16, then multiply by 9, 3, 3 and 1 in turn; do not work out each class from scratch. A real experiment is never expected to match a theoretical ratio exactly — small scatter of a few percent, as seen here, is normal, not evidence against it.
Common traps
- Treating genotype and phenotype as the same thing. TT and Tt have different genotypes but the identical tall phenotype — always check which one a question is asking for.
- Believing traits blend, like paint mixing. Mendel's F1 was proof against blending: crossing tall with short gave all-tall offspring, not medium-height plants; one trait is expressed completely, the other stays hidden.
- Confusing "dominant" with "more common" or "more advantageous." Dominance describes which trait is expressed in a heterozygote; it says nothing about how frequent a trait is in a population or whether it is beneficial.
- Assuming a gene and a chromosome are the same object. A gene is one specific section of DNA coding for one protein; a chromosome is an entire thread of DNA carrying many genes together.
- Assuming every pair of traits must be inherited together. Mendel's dihybrid cross disproved this: seed shape and seed colour, on different chromosomes, recombine freely, producing brand-new combinations in F2.
- Assuming the mother's chromosomes decide a baby's sex. The mother always contributes an X chromosome; it is entirely the father's sperm — X or Y with equal probability — that decides a child's sex.
You should be able to…
- Explain why asexual reproduction produces only small variation while sexual reproduction produces much more.
- Set up a monohybrid Punnett square for a Tt by Tt cross and read off the genotype ratio (1:2:1) and phenotype ratio (3:1).
- Distinguish clearly between genotype, phenotype, dominant trait and recessive trait.
- Explain why a recessive trait can reappear in F2 after being completely absent in F1.
- Set up a dihybrid Punnett square (or use for gamete types) to derive the 9:3:3:1 phenotype ratio.
- Explain, step by step, how a gene, protein, enzyme and hormone can together control a visible trait such as height.
- Explain why every germ cell carries only one copy of each chromosome while every other body cell carries two.
- State the human sex chromosome pattern (XX in women, XY in men) and explain why the father's sperm decides a child's sex.
Ask the tutor: Want to work through a Punnett square from your own textbook, or check a tricky genotype-deduction exercise together?
If shortness disappears completely in the F1 generation, where does it actually go, and how does it come back in F2?
It never leaves — it is only hidden from view. Every F1 plant from a tall-by-short cross is Tt, carrying one dominant T and one recessive t. A single T copy is enough to make the plant look tall, so the t copy has no visible effect, but it is still faithfully copied into roughly half of that plant's gametes. When two F1 plants combine gametes to make F2, there is a one-in-four chance that a t-carrying gamete meets another, giving a tt offspring — and only then does shortness reappear. Nothing was destroyed; the trait was carried silently for one generation.
If trait A is found in only 10 percent of an asexually reproducing population and trait B in 60 percent, does that tell us which trait is dominant?
No — that is a different question entirely, and a common confusion. Frequency in a population tells you roughly how long a trait has had to spread through repeated reproduction: a trait present in 60 percent of individuals has most likely existed and multiplied for longer than one present in only 10 percent. Dominance, by contrast, describes what happens inside a single individual carrying two different gene copies — which one gets expressed. A trait can be common and still recessive, or rare and still dominant; frequency and dominance answer unrelated questions.
A father has blood group A and a mother has blood group O, and their daughter has blood group O. Does this single family tell us whether A or O is dominant?
No, one family is not enough evidence. You would need to know the father's exact genotype, which a single cross does not reveal, and Mendel's own method shows why: he established dominance only by breeding many plants across two full generations and counting large numbers of offspring, never from one cross. A recessive-looking trait appearing once in a child is consistent with more than one parental genotype, so a reliable answer needs a properly designed set of crosses studied across F1 and F2 — the same careful, repeated counting Mendel did with his pea plants.
Since the mother also carries an X chromosome, why is she not considered responsible for the child's sex?
Because the mother's contribution never varies — every egg she produces carries an X, with no exception. The variability sits entirely with the father: his cells carry one X and one Y, and when his germ cells form, half his sperm carry X and half carry Y. Whichever type fertilises the egg fixes the outcome — X gives XX, a daughter; Y gives XY, a son. Since the mother's side is always fixed, the outcome is decided by which of the father's two sperm types succeeds.