Heredity
Why This Matters
You have your mother’s eyes, maybe your father’s nose, perhaps your grandfather’s height. A child clearly belongs to the same species as its parents — two arms, two eyes, the same basic design — and yet it never looks exactly like either of them. How does this work? How do features pass from parents to children, and why are the children still a little different?
That’s the whole question of heredity: the passing of features (called traits) from one generation to the next. And it isn’t vague — it follows surprisingly clean rules. A monk named Gregor Mendel worked them out over a century ago by patiently counting pea plants, long before anyone knew what DNA was.
These rules explain a lot: why a trait can “skip” a generation and reappear in a grandchild, why two brown-eyed parents can have a blue-eyed child, and — this is the big one — why the father, not the mother, decides whether a baby is a boy or a girl. By the end of this chapter you’ll be able to predict the outcome of a cross with a simple grid called a Punnett square.
The Big Idea
Each trait in a sexually reproducing organism is controlled by two copies of a gene — one from each parent. The copies may be the same or different. When they differ, the version that shows up is the dominant trait and the hidden one is the recessive trait. The recessive trait isn’t lost — it can reappear in a later generation.
Here’s the chain of logic. Both parents contribute roughly equal amounts of genetic material to a child. So for every trait, a child carries two versions — one maternal, one paternal. If the two versions disagree (say “tall” and “short”), only one of them gets expressed in the body. But both are still present, and both get passed on. That’s why a hidden trait can resurface generations later.
This single idea — two copies per trait, one expressed, both inherited — is the key that unlocks the whole chapter.
Let’s Break It Down
Variation accumulates over generations
Every time an organism reproduces, the offspring inherit the basic body plan plus tiny new differences (from small errors in DNA copying). Now those offspring reproduce too — and their offspring carry the old inherited differences and fresh new ones. So variation builds up generation after generation.
In asexual reproduction (one bacterium splitting into two, then four) the differences are tiny — just copying slip-ups. In sexual reproduction, mixing DNA from two parents creates far more variation. That’s why a field of sugarcane (often grown asexually) looks uniform, while a class of 40 humans looks wildly varied.
Not all variations survive equally — a heat-tolerant bacterium does better in a heat wave. This selection of useful variations by the environment is the seed of evolution, which the next chapter explores.
In an asexually reproducing species, trait A is found in 10% of the population and trait B in 60%. Which trait probably arose earlier?
Trait B. In asexual reproduction, a new variation spreads only by being passed down through many generations of dividing cells. A trait found in 60% of the population has had time to spread widely, so it most likely appeared earlier. Trait A, at only 10%, is probably a more recent variation that hasn’t spread as far yet.
Mendel and the pea plants
Mendel chose garden peas because they had clear either/or traits: seeds round or wrinkled, plants tall or short, flowers violet or white — no in-betweens. He crossed plants with contrasting traits and, crucially, counted the offspring in each generation. That counting is what let him spot the pattern.
He started by crossing a pure tall plant (TT) with a pure short one (tt). Call the first-generation offspring the F1 generation.
- F1 result: every plant was tall. Not medium height — all tall. So the short trait seemed to vanish.
Then he let the F1 tall plants self-pollinate to make the F2 generation:
- F2 result: about three-quarters were tall, one-quarter short. The short trait came back!
This told Mendel two things. First, the short trait was never lost — it was just hidden in F1 and reappeared in F2. Second, each plant must carry two factors (we now call them genes) for each trait. The tall version “T” is dominant — a single T makes a plant tall. The short version “t” is recessive — a plant needs two t’s (tt) to be short.
So in F2: TT and Tt are both tall, only tt is short. The combinations appear in a 1 : 2 : 1 ratio (TT : Tt : tt), which looks like 3 tall : 1 short.
The short trait was lost in the F1 generation and a new short trait randomly appeared in F2.
The short trait vanishes completely in the F1 plants — every one looks tall — so it really does look as if 'short' was lost and then reappeared out of nowhere in F2.
Nothing was lost: the 'short' factor (t) was hidden in every F1 plant (they are all Tt — carrying both factors but showing the dominant T). When two Tt plants cross, some F2 offspring inherit tt and the recessive short trait shows up again.
- List the gametes (sex cells). Each Tt parent makes two kinds: T and t.
- Make a 2×2 Punnett square — one parent’s gametes across the top, the other’s down the side, and fill each box by combining them.
- Read the four boxes: TT, Tt, Tt, tt — that’s a 1 : 2 : 1 ratio.
- Group by appearance. TT, Tt, Tt are all tall (they have at least one T); only tt is short.
- So 3 out of 4 are tall, 1 out of 4 is short — exactly the 3:1 ratio Mendel counted. The Punnett square turns a confusing result into simple arithmetic.
Both F1 parents are Tt. Each parent passes one factor to each offspring, either T or t with equal chance. Let’s find every possible combination.
| T | t | |
|---|---|---|
| T | TT | Tt |
| t | Tt | tt |
Two traits at once — independent inheritance
What if you track two traits together? Mendel crossed a plant with round, yellow seeds (RRYY) and one with wrinkled, green seeds (rryy). The F1 were all round and yellow — so round (R) and yellow (Y) are dominant.
But the F2 generation had a surprise: along with round-yellow and wrinkled-green, he got new combinations — round-green and wrinkled-yellow — that neither original parent had. The seed-shape trait and the seed-colour trait were inherited independently of each other, in a ratio close to 9 : 3 : 3 : 1.
The lesson: traits don’t travel in fixed bundles. Genes get reshuffled during sexual reproduction, which is exactly why offspring show fresh combinations their parents never had — another source of variation.
A tall plant with round seeds (TtRr) is crossed. Roughly how many different-looking types of offspring can appear, and why?
Four types: tall-round, tall-wrinkled, short-round, short-wrinkled. Because height and seed shape are inherited independently, the combinations mix freely — you can get tall plants with wrinkled seeds and short plants with round seeds, not just the two parental combinations.
Genes, proteins and chromosomes
How does a gene actually control a trait? A gene is a section of DNA that carries the instructions to make one protein — often an enzyme. Take plant height: a hormone triggers growth, and an enzyme makes that hormone. If the gene builds an efficient enzyme, lots of hormone is made and the plant is tall. If the gene has a change that makes a weaker enzyme, less hormone is made and the plant is short. So genes work by controlling proteins, and proteins build traits.
Now, if each parent contributes equally, each cell must carry two copies of every gene — one from each parent. These copies sit on separate threads of DNA called chromosomes. Every body cell has chromosomes in pairs (one of each pair from each parent). But each germ cell (egg or sperm) carries only one chromosome from each pair. When an egg and sperm join, the pairs are restored — and because each pair can come from either parent, genes get independently reshuffled. That’s the machinery behind Mendel’s results.
How sex is determined in humans
Here’s the famous part. Humans have 23 pairs of chromosomes. Twenty-two pairs are perfectly matched. But the last pair — the sex chromosomes — is special:
- Females are XX — two matching X chromosomes.
- Males are XY — one X and a shorter Y.
Now follow the logic. A mother (XX) can only put an X into every egg. A father (XY) puts an X into half his sperm and a Y into the other half. So:
- Egg (X) + sperm carrying X → XX → girl
- Egg (X) + sperm carrying Y → XY → boy
So every child gets an X from the mother no matter what. It is the father’s chromosome — X or Y — that decides the baby’s sex, and it’s a 50:50 chance each time.
The mother is responsible for whether a baby is a boy or a girl.
Because the mother carries the baby and gives birth to it, it feels natural to assume she must also decide its sex — and the blame is often unfairly placed on her.
The mother is XX, so she can only ever pass on an X. It is the father (XY) who passes either an X (→ girl) or a Y (→ boy), so the father's sperm determines the sex of the child, with a 50:50 probability.
Common Mistakes
'Dominant' means the trait is stronger, healthier, or more common in the population.
In everyday language 'dominant' means stronger or more powerful, so it's natural to assume the genetic term carries the same meaning.
Dominant only describes which version is expressed when both are present (recessive is hidden unless paired with itself). It says nothing about strength or how common a trait is — a dominant trait can be rare and a recessive one very common; frequency depends on the population, not on dominance.
A tall plant must be TT.
We expect a trait's appearance to map to a single genotype, so 'tall' feels like it must mean the two matching letters, TT.
A plant only needs one T to look tall, so a tall plant could be TT or Tt — you can't tell which just by looking. Only a short plant is certain to be tt.
Quick Check
In Mendel's cross of pure tall (TT) × pure short (tt), what did the entire F1 generation look like?
Every F1 plant is Tt. Since T (tall) is dominant, all of them are tall — the short trait is hidden, not lost. It reappears only in the F2 generation.
Two parents who both have free earlobes have a child with attached earlobes (the recessive trait). What does this tell you about the parents?
For the child to be recessive (two recessive copies), it must have received one recessive factor from each parent. So both free-earlobed parents are carriers — they show the dominant trait but secretly carry the recessive one.
Who determines the sex of a child in humans, and why?
The mother (XX) always gives an X. The father (XY) gives either an X (→ girl) or a Y (→ boy), so the father’s chromosome decides the sex.
Practice Problems
A Mendelian cross bred tall pea plants with violet flowers and short pea plants with white flowers. All the progeny had violet flowers, but almost half were short. Write the genetic make-up (genotype) of the tall, violet parent.
Look at each trait separately.
Flower colour: all progeny were violet → violet is dominant and the violet parent bred true, so it must be WW (two violet copies).
Height: almost half the progeny were short. Short is recessive (tt), so the offspring must have received a “t” from the tall parent. That means the tall parent carries one T and one t → Tt.
So the tall, violet parent is TtWW. (This is option (c) in the NCERT exercise.)
A man with blood group A marries a woman with blood group O, and their daughter has blood group O. From this single family, can you decide whether blood group A or O is dominant? Explain.
No, this one family is not enough to decide.
The daughter is group O, so she carries two O factors — one from each parent. She got one O from her mother (group O) and one O from her father. So the father, though he shows group A, also secretly carries an O factor.
This tells us the father is a carrier, but it does not tell us which trait is dominant. To know that, we’d need to see what happens across many families — for example, whether a child can be group A when neither the A factor is “covering up” an O, etc. A single cross can’t establish dominance; you need population-level counting, just as Mendel did.
(In fact A is dominant over O, but this family alone doesn’t prove it.)
A study found that children with light-coloured eyes usually have parents with light-coloured eyes. Can we conclude from this whether the light-eye trait is dominant or recessive? Why or why not?
No, we cannot conclude dominance from this observation alone.
The fact that light-eyed children tend to have light-eyed parents only shows the trait is inherited — it runs in families. It says nothing about whether light is dominant or recessive.
Think about why: if light eye colour were recessive, then a light-eyed child (two recessive copies) must get a recessive copy from each parent — and if both parents are also light-eyed, the pattern fits perfectly. But the same family pattern could also appear if light were dominant. To decide, you’d need to track crosses across generations and count the ratios of offspring types — for instance, looking at the children of parents with contrasting eye colours and seeing which version is hidden. Correlation within families shows inheritance, not dominance.
Summary
- Heredity is the passing of traits from parents to offspring, and it follows definite rules — first worked out by Mendel with pea plants by counting offspring.
- Each trait is controlled by two copies of a gene, one from each parent. When the copies differ, the dominant version is expressed and the recessive one is hidden — but not lost.
- Crossing pure tall (TT) × pure short (tt) gives an all-tall F1 (Tt); selfing the F1 gives an F2 in a 3 : 1 ratio (tall : short), from genotypes 1 TT : 2 Tt : 1 tt. A Punnett square predicts this.
- Two traits are inherited independently (≈ 9:3:3:1), producing new combinations in F2 — a major source of variation.
- Genes are sections of DNA that code for proteins; they sit on chromosomes, which come in pairs. Germ cells carry one chromosome per pair, restoring the pair at fertilisation.
- In humans, females are XX and males are XY. The child always gets an X from the mother; the father’s sperm (X or Y) decides the sex — a 50:50 chance.
What’s Next
You’ve now seen how variations are created and inherited — the raw material of life. But what happens to those variations over thousands of generations? In Chapter 9: Light — Reflection and Refraction, the syllabus shifts gears into physics: how mirrors and lenses bend light to form images, why a spoon in a glass of water looks bent, and the formulas that let you predict exactly where an image will appear. The careful, rule-based thinking you used for Punnett squares is the same thinking you’ll use for ray diagrams.