Heredity — Class 10 Science Notes & Practice

Let us start with something you already know, even if nobody has ever put a name to it. Somebody has told you that you have your mother’s eyes, or your father’s laugh, or your grandmother’s stubbornness. You did not choose any of those things. They arrived with you. That quiet, invisible handing-down of features from parents to children is called heredity, and this chapter is simply the science of how it happens.

Now, a lot of students open this chapter, see letters like TT, Tt and tt scattered across a page, see a grid full of symbols, and immediately decide that this is going to be hard. It is not. I promise you that. Every single thing in this chapter comes down to one idea: you carry two copies of each instruction, one from each parent, and when you have children you pass on only one of those copies, chosen at random. That is it. Everything else — the ratios, the squares, the vocabulary — is just careful bookkeeping built on top of that one sentence.

So we are going to go slowly. I will teach you the words before I use them. I will draw every grid out cell by cell. And when we reach the famous 3:1 and 9:3:3:1 ratios, you will not be memorising them — you will have counted them yourself and seen exactly where each number comes from. Take your time. There is no prize for rushing.

Your Game Plan

If you follow these five steps in order, this chapter will take you one focused afternoon and it will stay learnt. Skipping ahead is the only way to make it confusing.

1. Learn the five words first — gene, allele, chromosome, genotype, phenotype. Do not touch a single cross until these feel comfortable in your mouth.
2. Master one gene before two. Do every monohybrid cross in this page by hand on paper. Only then move to the dihybrid.
3. Practise the Punnett square as a mechanical routine — gametes on the edges, combine row and column, fill every cell. Speed comes from repetition, not cleverness.
4. Say the ratios out loud with their meaning attached: “three tall to one dwarf in the F2” is worth remembering; a bare “3:1” is not.
5. Finish with the worksheet at the bottom with the answers hidden. Write your answer first, then reveal. That gap between your answer and the model answer is exactly where the marks live.

Why Children Resemble Their Parents — Heredity in Everyday Life

Think about a family recipe. Your grandmother makes a particular dish a particular way. She teaches your mother, who teaches you. The recipe is not the food — the recipe is the instruction for making the food. The dish appears fresh on the table every time, but the instruction behind it has been passed down unchanged for three generations.

Heredity works exactly like that. Parents do not hand their children a nose or a hair colour. They hand over instructions. Those instructions are chemical, they sit inside the cells of the reproductive organs, and they travel into the next generation inside the gametes — the sperm from the father and the egg from the mother. When sperm and egg fuse, the two sets of instructions come together in one cell, and that single cell divides again and again to build a whole new individual who is running a mixture of both recipe books.

So the formal definition, the one you write in an exam, is this: heredity is the transmission of characters (traits) from parents to their offspring. The characters themselves are called inherited traits — height in a pea plant, flower colour, the shape of your earlobe, whether you can roll your tongue.

Notice what heredity is not. If your father learned to play the harmonium, that skill is not written into his gametes; you will not be born knowing it. If a person loses a finger in an accident, their children are born with ten. Changes that happen to a body during its own lifetime do not get copied back into the instructions. Only what is written in the instructions travels forward. That distinction — between what is in the recipe and what merely happened to one dish — is worth holding on to, because examiners like to test it with a trick example.

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Variation — Why No Two People Are Identical

Here is the interesting half of the story. If heredity were perfect copying, every child would be an exact duplicate of one parent. Clearly that does not happen. Two children of the same parents, raised in the same house, can look and behave quite differently. The differences between individuals of the same species are called variations.

Where do variations come from? Two main places, and both are easy to picture.

• Reshuffling during sexual reproduction. Each parent has two copies of every instruction and passes on only one, picked at random. Multiply that random choice across thousands of instructions and the number of possible combinations becomes astronomically large. Two children of the same couple simply draw different hands from the same deck.
• Small errors when the genetic material is copied. DNA copying is extremely accurate but not perfect. Occasional small changes creep in, and those changes become new variations that can be passed on.

Now the question that carries marks: why does variation matter? Think of a species as a group of people all writing the same exam. If every single one of them had prepared exactly one chapter, and the paper came from a different chapter, all of them would fail together. If instead they had prepared a spread of chapters, some would cope with whatever paper arrived. Variation is that spread.

Concretely: suppose a population of bacteria is living happily at a certain temperature and the water suddenly gets much hotter. If every bacterium were identical, the whole population would be wiped out. But if a few of them happen to vary in a way that lets them tolerate heat, those few survive and the species continues. Variation does not guarantee survival — it gives the species a chance. That is the honest way to state it, and it is what an examiner wants to read.

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Genes, Chromosomes and Alleles — The Vocabulary You Need First

Almost everyone who finds this chapter difficult is struggling with vocabulary, not with biology. So let us build the words carefully, from the outside in, using one image: a library.

Inside the nucleus of a cell there are thread-like structures called chromosomes. Think of each chromosome as a book. A chromosome is made of a very long molecule of DNA along with protein. DNA is the chemical in which the instructions are actually written.

A gene is a particular stretch of that DNA that carries the instruction for one character — one chapter in the book. There is a gene for the height of a pea plant, a gene for the colour of its flowers, and so on. A gene is a segment of DNA located at a definite position on a chromosome.

Now the word that trips people up. A gene can exist in more than one version. The height gene in a pea plant has a “tall” version and a “dwarf” version. These alternative forms of the same gene are called alleles. So allele is not a different thing from a gene — an allele is a variant of a gene. Same chapter, different editions.

Here is the part that makes the whole chapter work. In the body cells of an organism that reproduces sexually, chromosomes occur in pairs. One member of each pair came from the mother, the other from the father. Because the chromosomes are paired, the genes on them are paired too — which means you carry two alleles for every character. Picture two copies of the same house key: they may be identical, or one may be slightly different, but you are definitely carrying two.

In human beings there are 23 pairs of chromosomes in each body cell — that is 46 chromosomes in total. Of these 23 pairs, 22 pairs are autosomes (they carry instructions for ordinary body characters) and 1 pair is the sex chromosome pair, which decides whether the individual develops as male or female. Keep that split of 22 + 1 firmly in mind; we will use it directly when we reach sex determination.

And gametes? A gamete carries only one chromosome from each pair — so a human egg or sperm carries 23 chromosomes, not 46. This is essential, and it is common sense once you see it: if both the egg and the sperm carried 46, the child would have 92, the grandchild 184, and the number would double forever. Halving in the gametes and doubling at fertilisation keeps the number constant across generations.

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Mendel and His Pea Plants — Why He Got It Right When Others Didn’t

Gregor Johann Mendel was a monk who worked in a monastery garden in the nineteenth century. He was not the first person to wonder how traits are inherited — people had been breeding animals and crops for thousands of years. He was the first to get a clear answer, and the reason is worth understanding, because it is really a lesson in how to do an experiment well.

Before Mendel, most people believed in blending inheritance — the idea that the characters of the two parents mix in the offspring the way two tins of paint mix, giving something in between, and that once mixed the original colours are gone forever. It sounds reasonable. It is also wrong, and Mendel’s experiments demolished it. In his crosses a character could disappear completely in one generation and then reappear unchanged in the next. Paint that has been mixed never un-mixes. So inheritance is not blending — the instructions stay separate and intact, even when one of them is hidden.

Mendel worked with the garden pea, Pisum sativum. This was not a lucky accident; it was a shrewd choice, and questions about “why the pea plant” are extremely common in exams. Here is the reasoning:

• It has clearly contrasting characters. A pea is tall or dwarf, its seeds are round or wrinkled — sharply either/or, with no confusing in-between. That makes counting possible.
• It normally self-pollinates. Left alone, a pea plant fertilises itself, which meant Mendel could easily obtain pure-breeding lines that gave the same character generation after generation.
• It can also be cross-pollinated by hand. He could remove the anthers from one flower and dust it with pollen he chose, so he controlled exactly which plant crossed with which.
• It has a short life cycle and gives many seeds. Several generations in a few years, and large numbers of offspring — and large numbers are what make a ratio trustworthy.
• It is easy to grow and takes little space. Practical, but genuinely part of the answer.

The other half of his genius was method. Mendel studied one character at a time instead of trying to track the whole plant at once, and he counted every offspring and applied mathematics to the numbers. Studying one variable at a time and counting carefully — that is why he succeeded where careful, intelligent people before him had failed.

He selected seven pairs of contrasting characters. You should be able to name at least three or four; the full set is here for reference.

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Reproduction passes these traits on — revise our Life Processes chapter notes.

The Monohybrid Cross — F1, F2, 3:1 Phenotypic and 1:2:1 Genotypic Ratio

A monohybrid cross is a cross in which we follow just one character. “Mono” for one. This is the experiment that built modern genetics, and we are going to walk through it twice — once for the first generation, once for the second — filling in every box.

First, the symbols. For height in a pea plant we use T for the tall allele and t for the dwarf allele. A pure-breeding (true-breeding) tall plant carries two tall alleles, so we write it TT. A pure-breeding dwarf plant carries two dwarf alleles: tt. The generations are labelled P (parent), F1 (first filial, the children) and F2 (second filial, the grandchildren).

Now for the step that changed biology. Mendel allowed those F1 plants to self-pollinate, so Tt was effectively crossed with Tt.

One more idea belongs here: the test cross. Look at a tall pea plant in a field. Is it TT or Tt? You cannot tell by looking, because both are tall. So you cross it with a dwarf (tt) plant — the recessive one, which can only contribute t. The dwarf parent contributes nothing that could mask anything, so the offspring reveal what the unknown parent was carrying.

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Dominant and Recessive Traits, Genotype vs Phenotype, Homozygous vs Heterozygous

These three pairs of words are the grammar of genetics. Once they are secure, the crosses become mechanical.

Dominant and recessive. When an organism carries two different alleles, one of them may express itself fully and completely hide the other. The one that shows is the dominant allele; the one that stays hidden is recessive. In our pea plant, T (tall) is dominant and t (dwarf) is recessive, so a Tt plant is tall. The crucial point — and it is the point students most often miss — is that the recessive allele is not weak, damaged or absent. It is present, intact, and fully capable of being passed on. It is simply outvoted in that particular plant.

A recessive character can therefore only be seen when there is no dominant allele to hide it — that is, when both alleles are recessive. So a dwarf plant must be tt. It cannot be anything else. This gives you a handy shortcut: if you see the recessive character, you instantly know the full genotype. If you see the dominant character, you know only that at least one dominant allele is present, and the second allele remains unknown — which is exactly why the test cross exists.

Genotype and phenotype. The genotype is the genetic make-up — the actual pair of alleles, written as TT, Tt or tt. The phenotype is the observable character — tall or dwarf, what you would record standing in the field with a notebook. Think of genotype as the code in the recipe and phenotype as the dish you can taste.

Homozygous and heterozygous. If the two alleles are the same (TT or tt), the organism is homozygous for that character — “homo” meaning same. Such a plant is also called pure-breeding, because selfing it gives offspring identical to itself, generation after generation. If the two alleles are different (Tt), the organism is heterozygous — “hetero” meaning different — also called a hybrid. TT is homozygous dominant; tt is homozygous recessive; Tt is heterozygous.

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The Dihybrid Cross — the 9:3:3:1 Ratio and What It Proves

Mendel then asked a sharper question: if I follow two characters at the same time, do they travel together as a package, or independently? A cross that follows two characters together is a dihybrid cross.

He used seed shape and seed colour. R = round (dominant), r = wrinkled; Y = yellow (dominant), y = green. He crossed a pure round-yellow plant (RRYY) with a pure wrinkled-green plant (rryy).

Now the important step. Each F1 plant is RrYy and must be selfed. What gametes can RrYy make? This is the one genuinely new skill in the chapter, so let us be very deliberate about it. The plant must put one allele of each gene into every gamete — one from the R/r pair and one from the Y/y pair. If the two genes are independent, all four combinations should occur, in equal numbers:

• RY — took R from the shape pair, Y from the colour pair
• Ry — took R and y
• rY — took r and Y
• ry — took r and y

Four kinds of gamete from one parent and four from the other means 4 x 4 = 16 boxes. That is where the 16 comes from — not from anything mysterious. Here is the complete grid.

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Mendel’s Laws — Dominance, Segregation and Independent Assortment

Everything you have just watched happen can be compressed into three statements. Do not memorise them as strings of words — attach each one to the experiment that produced it, and they become impossible to forget.

1. The Law of Dominance. When two organisms differing in a pair of contrasting characters are crossed, only one of the two characters appears in the F1 generation. The character that appears is dominant; the one that remains hidden is recessive.
Where you saw it: TT x tt gave an F1 that was entirely tall. The dwarf character did not appear at all.

2. The Law of Segregation. The two alleles of a pair separate (segregate) from each other during the formation of gametes, so that each gamete receives only one allele of the pair. The alleles do not blend or contaminate each other while they are together — they separate unchanged.
Where you saw it: the Tt plant produced T gametes and t gametes in equal numbers, which is why the dwarf character came back, perfectly intact, in one quarter of the F2. This is the law that is always true, for every cross, and it is sometimes called the law of purity of gametes because a gamete is always “pure” — it never carries both members of a pair.

3. The Law of Independent Assortment. When two or more pairs of contrasting characters are considered together, the alleles of one pair separate independently of the alleles of the other pair during gamete formation.
Where you saw it: the RrYy plant made RY, Ry, rY and ry gametes in equal numbers, giving the 9:3:3:1 F2 with two brand-new combinations. Shape did not care what colour was doing.

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Carbon chemistry uses the same habit of tracking structures step by step; see Carbon and its Compounds notes.

How to Draw and Read a Punnett Square (Step by Step)

A Punnett square is nothing more than a tidy way of listing every possible way one parent’s gamete can meet the other parent’s gamete. It is bookkeeping, not magic. Here is the routine — follow it in exactly this order, every single time, and you will not get a cross wrong.

1. Write down both parent genotypes. If the question gives you words (“a heterozygous violet plant”), translate to symbols first (Vv) and write the key: V = violet, v = white.
2. Work out the gametes of each parent separately. Split the pair. A homozygous parent (VV or vv) gives only one kind. A heterozygous parent (Vv) gives two kinds. For two genes, use the fixed order RY, Ry, rY, ry.
3. Draw the grid. Number of columns = number of gamete types from parent 1. Number of rows = number of gamete types from parent 2. So 2 x 2 = 4 boxes for a monohybrid, 4 x 4 = 16 for a dihybrid.
4. Label the edges with the gametes — never with the parent genotypes. This is the single most common error.
5. Fill each box by combining the allele at the top of its column with the allele at the side of its row. Write the dominant (capital) letter first, so Tt not tT.
6. Count the genotypes and write the genotypic ratio.
7. Translate each box into a phenotype and count those to get the phenotypic ratio.
8. Write a one-line conclusion in words answering the actual question asked. Marks are often reserved for this sentence.

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Sex Determination in Human Beings — XX, XY, and Why the Father Determines the Sex of the Child

This is a short topic with a large social importance, so let us get it exactly right.

Recall from earlier: a human body cell has 23 pairs of chromosomes. Twenty-two of those pairs are autosomes, identical in males and females. The twenty-third pair is the sex chromosome pair, and this is where males and females differ.

• A human female has two X chromosomes: XX.
• A human male has one X and one much smaller Y chromosome: XY.

Now think about the gametes, using the rule you already know: a gamete receives only one chromosome from each pair.

• The mother is XX. Whichever of her two sex chromosomes goes into an egg, it is an X. So every egg carries an X — she has no other option to give.
• The father is XY. His pair separates, so half his sperms carry an X and half carry a Y. He produces two kinds of sperm.

Look at what that means. The mother’s contribution is fixed and identical every time. The only thing that varies is which kind of sperm happens to reach the egg first. It is a coin toss, and the coin belongs entirely to the father.

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What Was Removed From This Chapter (Evolution)

If you pick up an older textbook, a second-hand guide or an online video made a few years ago, you will find this chapter called “Heredity and Evolution” and running to roughly twice the length. That is not a mistake on their part — it is simply an older version of the course.

One small clarification, since it causes confusion. Earlier in this chapter we distinguished a trait written in the genes from a change acquired by the body during its lifetime — the weightlifter and his muscles. That distinction is retained here because it belongs to heredity: it explains what can and cannot be inherited. What is no longer required is the further use of that idea as a stepping stone into theories of evolution.

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Test yourself under exam conditions with the CBSE Class 10 Science practice paper with answer key.

Practice Worksheet — 10 Questions with Full Model Answers

Here is the part that actually moves your marks. Keep a sheet of paper next to you. Read the question, close your eyes for a moment, write your full answer, and only then click to open the model answer. Comparing your written attempt with the model is worth ten times more than reading the model first and nodding along. Where a cross is asked for, draw the square properly — do not do it in your head.

Before You Close This Page

Look back at how far you have come. You started with the idea that children resemble their parents, and you now know the mechanism: two alleles per character, separated into gametes, recombined at fertilisation. You can draw a Punnett square, produce 3:1 and 1:2:1 and 9:3:3:1 from scratch rather than from memory, use a test cross to expose a hidden genotype, and explain — carefully and correctly — why a mother can never be responsible for the sex of her child.

If any one of those still feels shaky, go back to that section alone. Do not reread the whole page. Genetics rewards targeted repair far more than it rewards re-reading.

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