Principles of Inheritance and Variation — Class 12 Biology Notes & Practice

Take a breath. Genetics has a reputation for being the chapter where students either fall in love with Biology or quietly give up — and almost always, the ones who give up did so because nobody slowed down at the beginning. So we are going to slow down. Every capital letter, every little grid, every strange word like heterozygous will be built up from nothing, in order, with worked-out crosses you can copy in your own notebook.

Here is the promise of this page. Principles of Inheritance and Variation sits inside Unit VII, Genetics and Evolution, which carries 20 marks — the single highest-weighted unit in the whole Class 12 Biology paper. Nothing else in your syllabus pays back study time this generously. And unlike chapters that demand memorising long lists, this one rewards a skill. Learn to work one cross properly and you can work all of them.

That is why this page is organised around a single habit rather than a pile of tricks. You will meet it in a moment: a four-step decoding drill that we apply out loud in every single solved example, so by the time you reach the practice worksheet your hand already knows what to do. You will find principles of inheritance and variation class 12 important questions with solved examples, a full set of genetics numericals class 12 with solutions, and a dedicated run of pedigree analysis class 12 questions — all worked out step by step, none of them left as “try it yourself”.

If the words dominant and recessive already feel wobbly, that is completely normal and completely fixable. Spend ten minutes refreshing with the Class 10 Heredity notes first — that chapter is the foundation this one is built on, and coming back afterwards will feel like switching on a light.

What You’ll Learn

Your Game Plan

1. Read the Mendel section slowly and write out one monohybrid cross by hand before you read any further. Ten minutes now saves ten hours later.
2. Learn the four-step decode below until it is automatic. Do not skip steps even when the answer looks obvious — the marks live in the steps.
3. Work through the deviations from Mendelism together in one sitting, because the exam loves to compare them against each other.
4. Do the sex determination and sex-linked sections back to back; the second one only makes sense on top of the first.
5. Practise pedigrees with a pencil, marking genotypes directly on the chart. This is a drawing skill as much as a thinking skill.
6. Finish with the worksheet at the bottom. Attempt each question fully on paper before opening the answer.

Study Notes

Mendel’s Experiments And The Laws Of Inheritance

Gregor Mendel was a monk with a garden and a very stubborn patience. Between 1856 and 1863 he grew thousands of pea plants and, crucially, counted the offspring instead of just describing them. That decision — treating heredity as arithmetic — is why he is called the father of genetics.

He chose the garden pea, Pisum sativum, for reasons that are themselves examinable. It has clearly contrasting characters with no in-between forms, it normally self-pollinates so pure lines are easy to maintain, it can be cross-pollinated by hand when you want to, it has a short life cycle, and it produces plenty of seeds so his numbers were large enough to trust.

Mendel worked with seven characters, each existing in two sharply different forms: stem height (tall / dwarf), flower colour (violet / white), flower position (axial / terminal), pod shape (inflated / constricted), pod colour (green / yellow), seed shape (round / wrinkled) and seed colour (yellow / green).

Before the laws, get the vocabulary straight, because most lost marks are vocabulary accidents. A gene is the unit of inheritance; an allele is one of its alternative forms. An organism with two identical alleles (TT or tt) is homozygous; with two different alleles (Tt) it is heterozygous. The genotype is the allele pair it carries; the phenotype is what you actually see. A dominant allele shows itself even when only one copy is present; a recessive allele only shows when both copies are recessive.

Notice the deep reason segregation must be true: gametes are haploid. A parent has two alleles but a gamete carries one. The physical event behind the law is the separation of homologous chromosomes in anaphase I of meiosis — the same meiosis you met while studying gamete formation in Sexual Reproduction in Flowering Plants. Mendel deduced the rule decades before anyone had seen a chromosome do it.

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Monohybrid And Dihybrid Crosses (With Punnett Squares)

A monohybrid cross follows one character at a time; a dihybrid cross follows two characters at once. The Punnett square, invented by Reginald Punnett, is simply a bookkeeping table: gametes from one parent along the top, gametes from the other down the side, and every box is one possible fertilisation. Nothing mystical — it is a multiplication table for gametes.

The number of different gametes a parent can make is 2ⁿ, where n is the number of heterozygous gene pairs. Tt is heterozygous at one pair, so 2¹ = 2 gametes (T, t). RrYy is heterozygous at two pairs, so 2² = 4 gametes (RY, Ry, rY, ry). TT is heterozygous at zero pairs, so 2⁰ = 1 gamete type. Get this right and the size of your grid is never in doubt.

When you fill the 16 boxes for RrYy × RrYy you get nine different genotypes but only four phenotypes, in the ratio 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green. The genotypic ratio underneath is 1 RRYY : 2 RRYy : 2 RrYY : 4 RrYy : 1 RRyy : 2 Rryy : 1 rrYY : 2 rrYy : 1 rryy. Two of those sixteen boxes — RRyy and rrYY — are brand new combinations that neither parent had. That is recombination, and it is the whole reason sexual reproduction generates variation.

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Test Cross And Back Cross

Here is a real problem a breeder faces. You have a tall pea plant in front of you. Is it TT or Tt? You cannot tell by looking, because both are tall. The dominant allele is hiding whatever sits beside it. You need an experiment that forces the hidden allele to reveal itself.

That experiment is the test cross: cross the individual of unknown genotype with a homozygous recessive individual. The recessive partner is chosen deliberately, because it can only ever contribute recessive alleles to the gametes. It contributes nothing that could mask anything. So whatever the offspring show, they are showing the unknown parent’s contribution directly — the recessive parent acts like a clean window.

A back cross is the broader term: crossing an offspring back with either of its parents. If that parent happens to be the homozygous recessive one, the back cross is a test cross. So every test cross of this kind is a back cross, but not every back cross is a test cross — a favourite one-mark distinction.

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Incomplete Dominance And Co-dominance

Mendel was lucky as well as brilliant: the seven characters he picked all happened to show clean dominance. Nature is not always so tidy. When later biologists studied other traits, they found heterozygotes that did not simply look like the dominant parent. These are the deviations from Mendelism — and here is the honest framing to carry into the exam: they do not break Mendel’s laws of segregation and assortment at all. Alleles still separate and still assort. What changes is only how the heterozygote is expressed.

Incomplete dominance is when neither allele fully dominates and the heterozygote shows an intermediate phenotype — a genuine blend. The classic case is flower colour in the four o’clock plant (Mirabilis jalapa) or the snapdragon: a red-flowered plant (RR) crossed with a white-flowered plant (rr) gives F₁ that are entirely pink (Rr). The pink is not a mixture of red and white cells; there is simply not enough pigment made from a single R allele to produce full red.

Co-dominance is when both alleles express themselves fully and separately in the heterozygote — not a blend but both, side by side. Human ABO blood groups are the standard example: a person of genotype I^AI^B makes both the A antigen and the B antigen on their red cells, and is blood group AB. Roan cattle are another: a cross of red-coated and white-coated cattle gives offspring with a coat carrying both red hairs and white hairs, distinctly, not pink.

The stunning consequence, and the thing examiners test, is that in both cases the F₂ phenotypic ratio becomes 1 : 2 : 1 and therefore matches the genotypic ratio exactly. When every genotype has its own visible phenotype, the two ratios cannot differ.

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Multiple Alleles And ABO Blood Group Inheritance

So far every gene we have met came in exactly two flavours. But a gene is a stretch of DNA, and DNA can be altered in more than one way — so a gene may exist in three or more allelic forms in a population. That is multiple allelism. The crucial detail students forget: however many alleles exist in the population, any one diploid individual still carries only two of them, because they have only two copies of that chromosome.

The textbook example is the human ABO blood group, controlled by the gene I with three alleles: I^A, I^B and i. The gene codes for an enzyme that adds a sugar to a base molecule sitting on the red blood cell surface. I^A adds sugar A, I^B adds sugar B, and i is a non-functional version that adds nothing at all.

From that single fact, the entire inheritance pattern follows logically — you do not need to memorise it. I^A and I^B each make a product, so each is dominant over i, which makes none. But when I^A and I^B are together, both enzymes work, both sugars appear, and the person is group AB — co-dominance. Only ii has neither antigen: blood group O.

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Pleiotropy And Polygenic Inheritance

These two sit next to each other in the syllabus for a reason: they are mirror images, and holding them as a pair is the safest way to remember both. Pleiotropy is one gene producing many effects. Polygenic inheritance is many genes producing one effect. Say that sentence out loud twice and you will never swap them again.

Pleiotropy happens whenever a single gene influences several apparently unrelated characters. It is not exotic — it is what you would expect, once you realise a gene usually codes for a protein, and one protein may be needed in several different pathways or tissues. Disturb the protein, and every pathway that depended on it wobbles.

A clean example is phenylketonuria in humans. A single recessive gene defect leaves the enzyme phenylalanine hydroxylase non-functional. The immediate consequence is that phenylalanine accumulates. The downstream consequences are several and very different from each other: intellectual disability, reduced hair and skin pigmentation, and abnormal levels of phenylalanine and its derivatives in blood and urine. One gene, many phenotypic effects. In pea, the gene for starch synthesis is another example — it affects both starch grain size and seed shape.

Polygenic inheritance is the opposite arrangement: a character is controlled by three or more genes, each contributing a small, additive amount. Because the contributions add up, the phenotype does not fall into a few sharp classes — it varies continuously, which is why traits like human height, skin colour and grain colour in wheat give a smooth bell-shaped spread rather than tidy 3:1 buckets. Environment usually influences these traits as well, smoothing the curve further.

The standard model is human skin colour with three gene pairs (A, B and C), where every dominant allele adds one unit of melanin and every recessive allele adds none. The darkest possible genotype is AABBCC with six dominant alleles; the lightest is aabbcc with zero. Everything in between depends only on how many dominant alleles you carry, not which ones — and that is exactly why intermediate shades are so common.

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Chromosome Theory Of Inheritance

Mendel published in 1866 and was essentially ignored for thirty-four years. Part of the reason is that his “factors” were abstract — he had no physical object to point to. Then, around 1900, microscopes and staining techniques had improved enough that biologists could watch chromosomes move during cell division, and something clicked.

Working independently in 1902, Walter Sutton and Theodor Boveri noticed that chromosomes behave during meiosis exactly the way Mendel’s factors were supposed to behave. Sutton united the two ideas and gave us the chromosome theory of inheritance: Mendel’s factors (genes) are located on chromosomes, and the behaviour of chromosomes during meiosis explains the laws of inheritance.

The parallel is worth writing out because it is a standard three-mark answer. Chromosomes occur in pairs, and so do alleles. Both members of a chromosome pair separate at anaphase I, and so do the two alleles of a gene — that is the law of segregation, made physical. Different chromosome pairs line up independently of one another at metaphase I, and so do different gene pairs — that is the law of independent assortment, made physical. A gamete receives one chromosome from each pair, and one allele from each gene pair. Fertilisation restores the diploid number, and the paired condition.

The theory was confirmed experimentally by Thomas Hunt Morgan, who moved from peas to the fruit fly Drosophila melanogaster. Fruit flies were an inspired choice: they breed on simple synthetic medium, complete a generation in about two weeks, produce huge numbers of offspring, and have clearly distinguishable males and females with many easily visible hereditary variants. Morgan’s work tied specific genes to specific chromosomes — and, as we are about to see, promptly turned up an exception to independent assortment.

Chromosomes and genes, in one clean paragraph: a chromosome is a long DNA molecule with associated proteins. A gene is a particular segment of that DNA carrying the information for one product. The fixed position a gene occupies on its chromosome is its locus. The two chromosomes of a homologous pair carry the same genes at the same loci, but may carry different alleles of them — which is precisely why a heterozygote is possible at all.

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Linkage And Crossing Over

Here is where Mendel’s third law finally meets a genuine limit — and understanding why is more valuable than memorising that it does.

Independent assortment works because different gene pairs sit on different chromosome pairs, and different chromosome pairs line up independently at metaphase I. But a chromosome carries thousands of genes. What if two genes sit on the same chromosome? Then they are not free to assort independently. They tend to travel together into the same gamete, simply because they are physically attached to the same piece of DNA. This tendency is called linkage.

Morgan found exactly this in Drosophila. When he test-crossed dihybrid flies whose two genes lay on the same chromosome, he did not get the expected 1:1:1:1. Instead, the two parental combinations appeared far more often than the two recombinant ones. The genes were being inherited as a package.

But the package is not sealed. During prophase I of meiosis, homologous chromosomes pair up and non-sister chromatids physically exchange corresponding segments at points called chiasmata. This exchange is crossing over, and it produces new allele combinations — recombinants. Linkage tries to keep genes together; crossing over prises some of them apart. The balance between the two is the whole story.

Morgan then noticed something with far-reaching consequences. Some linked gene pairs recombined often; others almost never did. He reasoned that the closer two genes lie on a chromosome, the less likely a crossover is to fall between them — there is simply less room. So recombination frequency measures distance. His student Alfred Sturtevant used this to build the first genetic maps, with distance measured in map units (centimorgans), where 1% recombination = 1 map unit.

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Sex Determination In Humans, Birds And Honey Bees

“What decides whether a baby is a boy or a girl?” is one of the oldest questions humans have asked, and for most of history the answer was folklore. Genetics answered it properly, and the answer turns out to be different in different groups of animals — which is exactly why your syllabus asks for three of them.

Humans — the XX-XY system. A human has 23 pairs of chromosomes: 22 pairs of autosomes plus one pair of sex chromosomes. A female is 44 + XX; a male is 44 + XY. Since a female has two identical sex chromosomes, every egg she makes carries 22 + X — she is homogametic. A male has two different ones, so half his sperm carry 22 + X and half carry 22 + Y — he is heterogametic. Fertilisation by an X-bearing sperm gives a girl; by a Y-bearing sperm, a boy. This is called male heterogamety, and it means the sex of the child is decided entirely by which sperm arrives first — a point worth stating plainly, because the old habit of blaming mothers for the sex of a child has no biological basis whatsoever. (Grasshoppers use a related XX-XO variation, where the male simply has one X and no partner.)

Birds — the ZZ-ZW system. Birds run the same idea in reverse. The male is ZZ (homogametic) and the female is ZW (heterogametic). Different letters are used deliberately, to signal that these are not the same chromosomes as X and Y. Because the hen is the one with two different sex chromosomes, it is the egg that decides the chick’s sex here. This is female heterogamety.

Honey bees — haplodiploidy. Bees do something else entirely: they have no sex chromosomes at all. Sex depends on whether the egg was fertilised. A fertilised egg is diploid (32 chromosomes) and develops into a female — a queen or a worker, the difference between those two being diet, not genetics. An unfertilised egg develops by parthenogenesis into a haploid drone (16 chromosomes), the male. This has two consequences students love once they see them: a drone has no father but does have a grandfather, and since a drone is already haploid, he cannot perform meiosis — he produces sperm by mitosis.

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Sex-linked Inheritance: Haemophilia And Colour Blindness

Now combine two things you already know: genes sit on chromosomes, and the X and Y are chromosomes too. Genes carried on the sex chromosomes are said to show sex-linked inheritance, and because the X chromosome is large and gene-rich while the Y is small and carries very little, almost everything examinable here is X-linked.

One asymmetry explains every pattern in this section. A female has two X chromosomes, so a harmful recessive allele on one X can be masked by a normal allele on the other — she is a carrier: unaffected herself, but able to pass it on. A male has only one X, and his Y carries no matching allele to mask it. So whatever single allele sits on a male’s X is expressed, dominant or recessive. He is described as hemizygous. This is why X-linked recessive conditions appear far more often in males.

Haemophilia is an X-linked recessive condition in which a protein needed for blood clotting is missing or faulty, so a minor cut or internal injury bleeds for a very long time. Colour blindness (specifically red-green colour blindness) is also X-linked recessive, caused by a defect in the retinal cone pigments, so red and green shades are confused. Their inheritance behaves identically — learn one properly and you have both.

Notation matters here and is worth doing carefully: write the allele as a superscript on the X, so X^H is the normal allele and X^h the haemophilia allele. A carrier woman is X^HX^h; an affected man is X^hY. Never write “Hh” on its own for an X-linked gene — the whole point is which chromosome carries it.

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Pedigree Analysis

You cannot run breeding experiments on people. So human geneticists do the next best thing: they study a trait as it appears across the generations of a real family and reason backwards. That family diagram is a pedigree, and reading one is a skill you can genuinely master in an afternoon.

The symbols are standardised. A square is male and a circle is female. A shaded or filled symbol means the individual shows the trait; an unshaded one means they do not. A horizontal line joining a square and a circle is a marriage/mating line; a vertical line dropping from it leads to a horizontal sibship line from which the children hang, conventionally oldest on the left. Generations are numbered with Roman numerals (I, II, III) and individuals within a generation with Arabic numerals, so “II-3” names one person exactly.

Pedigree analysis lets you work out the mode of inheritance — autosomal or X-linked, dominant or recessive — and then calculate the risk for a future child. It is used in genetic counselling for exactly this purpose.

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Mendelian And Chromosomal Disorders In Humans

Genetic disorders in humans fall into two clearly separated groups, and getting the grouping right is often worth a mark on its own.

Mendelian disorders are caused by an alteration or mutation in a single gene. They are inherited in the ordinary Mendelian way and can be traced with pedigree analysis. The chromosome number and structure are entirely normal — you would see nothing wrong in a karyotype. They may be dominant or recessive, autosomal or X-linked.

Chromosomal disorders are caused by an abnormality in the number or structure of whole chromosomes. Here the karyotype is visibly abnormal. They usually arise from non-disjunction — the failure of chromosomes or chromatids to separate properly during cell division, so one gamete ends up with an extra chromosome and another with one too few. Gaining an extra copy of one chromosome is trisomy (2n + 1); losing one is monosomy (2n − 1). Because these usually happen afresh during gamete formation, they are typically not inherited from a parent in the ordinary sense.

Thalassemia is the Mendelian disorder your syllabus names. It is an autosomal recessive blood disorder in which the synthesis of one of the globin chains of haemoglobin is reduced or absent. In β-thalassemia the beta-globin chains are affected (the gene sits on chromosome 11); in α-thalassemia the alpha-globin chains are affected (genes on chromosome 16). Either way, haemoglobin is formed in inadequate amounts, red cells are destroyed too quickly, and the result is severe anaemia often requiring repeated transfusions. Because it is autosomal recessive, two unaffected carriers can have an affected child — which is exactly why carrier screening before marriage matters so much in communities where the allele is common.

Crucially, thalassemia is a quantitative problem: the globin chains that are made are normal, there are simply too few of them. That distinguishes it from disorders where an abnormal protein is produced instead.

The three chromosomal disorders named for you are Down’s syndrome, Turner’s syndrome and Klinefelter’s syndrome. Down’s involves an autosome; the other two involve sex chromosomes.

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Exam Strategy For This Chapter

You have the content. Now let us talk about converting it into marks, because in a 20-mark unit that conversion is worth more than another hour of reading.

Always show the cross. In any numerical, a bare answer earns a fraction of the marks available. Write the parental genotypes, write the gametes, draw the Punnett square, then state the ratio in words. That is the four-step decode again, and it is also, almost exactly, the official marking scheme. Even if your final arithmetic slips, the steps carry most of the credit.

Label your ratios. Never write a naked “3:1”. Write “phenotypic ratio = 3 tall : 1 dwarf”. It takes four extra words and removes every ambiguity about what you meant.

Watch the denominator. A startling share of lost marks in this chapter come from answering “out of all offspring” when the question said “out of the tall offspring”, or vice versa. Underline the group the question is asking about before you calculate anything.

Draw the diagrams. Punnett squares and pedigrees carry their own marks. A neat 4×4 grid or a properly symbolised pedigree is fast to draw and impossible to misread.

Learn the comparisons as tables. Incomplete dominance versus co-dominance, pleiotropy versus polygenic inheritance, Mendelian versus chromosomal disorders, Down’s versus Turner’s versus Klinefelter’s — all four turn up as “differentiate between” questions, and all four are quicker to write as a two- or three-column table than as prose.

Practise on the real thing. Once you have worked the questions on this page, take a timed run at the CBSE Class 12 Biology sample paper to see how genetics is actually distributed across sections and how long the five-markers really take you. And since Unit VII sits alongside the reproduction unit in your syllabus, the Human Reproduction chapter is the natural companion read — gametogenesis there is the physical process that segregation depends on here.

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Principles Of Inheritance And Variation Class 12 Important Questions — Rapid Revision

Use this as a last-night sweep. Cover the right-hand side of each line and see whether the answer arrives before you read it. If it does not, go back to the section named — do not simply re-read the line.

• Why did Mendel choose the garden pea? Contrasting characters with no intermediates, natural self-pollination giving pure lines, easy artificial cross-pollination, short life cycle, many seeds per plant.
• Which of Mendel’s laws has no exception? The law of segregation — alleles always separate cleanly during gamete formation.
• Which law does linkage violate? The law of independent assortment, because genes on the same chromosome are not free to assort independently.
• Monohybrid F₂ ratios? Phenotypic 3:1; genotypic 1:2:1.
• Dihybrid F₂ ratio? 9:3:3:1 phenotypic — and it equals (3:1) × (3:1).
• Test cross ratio for a heterozygote? 1:1 for a monohybrid; 1:1:1:1 for a dihybrid.
• Why is the test-cross partner homozygous recessive? Because it contributes only recessive alleles, so it masks nothing and the offspring reveal the unknown parent’s gametes directly.
• Incomplete dominance F₂ ratio? 1:2:1, with phenotypic and genotypic ratios identical.
• Example showing multiple alleles and co-dominance? ABO blood groups — three alleles I^A, I^B, i in the population; I^AI^B expresses both antigens.
• How many alleles does one person carry for ABO? Exactly two, however many exist in the population.
• Pleiotropy versus polygenic inheritance? One gene, many effects (phenylketonuria) versus many genes, one continuously varying character (human skin colour).
• Who proposed the chromosome theory, and who proved it? Proposed by Sutton and Boveri (1902); proved experimentally by Morgan using Drosophila.
• Recombination frequency formula? (recombinants ÷ total offspring) × 100; 1% recombination = 1 map unit (centimorgan).
• Who is heterogametic? Human male (XY); bird female (ZW); honey bees have no sex chromosomes at all.
• Chromosome numbers in a honey bee? Queen and worker 32 (diploid); drone 16 (haploid), producing sperm by mitosis.
• Why are more males colour blind? Males are hemizygous — one X, no second allele to mask a recessive one.
• What rules out X-linked recessive inheritance instantly? Father-to-son transmission of the trait.
• The named Mendelian disorder in this chapter? Thalassemia — autosomal recessive, reduced synthesis of α- or β-globin chains.
• Karyotypes of the three chromosomal disorders? Down’s 47, +21; Turner’s 45, X; Klinefelter’s 47, XXY — all from non-disjunction.

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Genetics Numericals Class 12 With Solutions — Every Ratio On One Page

Most numericals in this chapter are one of a small number of shapes. Learn to recognise the shape and the four-step decode does the rest. This table is the recognition key — the ratio in the question tells you what kind of cross produced it.

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Practice Worksheet

Ten questions, mixed marks, all original. Work each one fully on paper — genotypes, gametes, grid, ratio — before you open the answer. Reading a solution feels like learning; producing one actually is.

Kaizen closing thought. You will not master genetics tonight, and you were never supposed to. What you can do tonight is work one cross more carefully than you did yesterday — naming the trait, writing the parents, listing the gametes, gridding it out. Tomorrow, one more. That is the whole method: not a heroic weekend, just a slightly better answer than the last one, repeated until the paper arrives and the questions look familiar. One more correct cross than yesterday. Start now, and be patient with yourself — the pea plants took Mendel eight years.