Every teaching point in sections 4.1 to 4.2.2.2, in book order, with the plain meaning and the exam angle. Keep the chapter open alongside; the page markers match. Nothing in these pages is left out.
The chapter opens with three ideas that are themselves examinable. Students skip this page because it has no section number. It contains two definitions that get asked directly.
Like produces like — an elephant gives birth to an elephant, a mango seed grows into a mango plant.
This is the observation that needs explaining. Something physical must pass from parent to offspring carrying the instructions for what the offspring will be.
Yet offspring are not identical to their parents, and siblings differ from each other.
So whatever passes down is not simply copied whole. Two facts sit side by side: similarity and difference. Genetics is the study of both together.
Genetics is the branch of biology dealing with inheritance and variation of characters from parents to offspring.
Inheritance = the process by which characters are passed on from parent to progeny; it is the basis of heredity. Variation = the degree by which progeny differ from their parents.
Asked as — definition-matching questions. Learn both definitions word-perfect. Note "degree" in variation, not "way" or "amount".
Humans knew from as early as 8000–1000 B.C. that one cause of variation lay hidden in sexual reproduction.
They did not know why, but they used it. They picked out useful variations already present in wild populations of plants and animals, then bred selectively for them.
Asked as — the date range 8000–1000 B.C. is a direct one-mark fact.
Example of artificial selection and domestication: Sahiwal cows of Punjab, developed from ancestral wild cows.
The only named example on this page. Two things to remember: the breed name and the state.
Asked as — "Sahiwal cattle are an example of ___" → artificial selection / domestication.
Our ancestors knew about inheritance and variation, but had very little idea of the scientific basis of these phenomena.
This is the bridge sentence into Mendel. Practice existed for thousands of years; explanation arrived only in the nineteenth century.
Gregor Mendel worked on garden pea for seven years, from 1856 to 1863, and proposed the laws of inheritance in living organisms.
Three numbers in one sentence: the plant, the duration, the years. He worked in the mid-nineteenth century.
Asked as — "Mendel experimented for how many years?" → 7. The window 1856–1863 is also asked directly, and separately from 1865 (publication year, which appears later in the chapter).
Mendel's work was the first time statistical analysis and mathematical logic were applied to problems in biology.
This is his real methodological breakthrough, and it is why the chapter says his approach was unacceptable to biologists of his time (that reason reappears on page 65).
Asked as — "Mendel's greatest contribution to the method of biology was ___" → the use of statistics and mathematics.
Three reasons his conclusions were trustworthy: a large sample size, confirmation across successive generations, and the choice of characters with two clearly opposing forms.
A large sample gives credibility to the data. Following the plants into later generations proved he had found general rules, not a lucky pattern. Clear-cut traits meant no judgement was needed about what he was seeing — a plant was tall or dwarf, never something in between.
Asked as — "advantages of selecting pea plant" (NCERT exercise 1). Combine these with the practical points: pea is naturally self-pollinating, easy to cross-pollinate by hand, has a short life cycle, produces many seeds, and had readily available true-breeding varieties with sharply contrasting traits.
A true-breeding line is one that has undergone continuous self-pollination and shows stable trait inheritance and expression for several generations.
In modern language: homozygous for the trait, so it never throws up a surprise. If a tall true-breeding plant is selfed, every offspring is tall, generation after generation. This is why Mendel could be sure of what he started with.
Asked as — definition question. The three parts that must be in the answer: continuous self-pollination, stable inheritance, several generations.
Mendel selected 14 true-breeding pea varieties, taken as 7 pairs, each pair alike except for one character with contrasting traits.
14 varieties, arranged as 7 pairs. Do not confuse the two numbers — 14 is the count of varieties, 7 the count of character pairs.
Asked as — a very common trap: options offer 7, 12, 14, 22. The answer for varieties is 14; for contrasting trait pairs, 7.
Table 4.1 and Figure 4.1 — the seven pairs of contrasting traits.
Memorise the table as a table, with dominant on the left and recessive on the right. Every one of the seven is fair game, and questions often ask which of a given set is not a trait Mendel studied (flower size, seed weight and plant girth are common distractors).
| Character | Dominant trait | Recessive trait |
|---|---|---|
| Stem height | Tall | Dwarf |
| Flower colour | Violet | White |
| Flower position | Axial | Terminal |
| Pod shape | Inflated (full) | Constricted |
| Pod colour | Green | Yellow |
| Seed shape | Round (smooth) | Wrinkled |
| Seed colour | Yellow | Green |
Watch the reversal — in pods, green is dominant and yellow recessive. In seeds, yellow is dominant and green recessive. This single inversion is one of the most reliably exploited traps in the whole chapter.
Figure 4.2 — the steps in making a cross in pea.
Parts of the flower to label: petal, stigma, anther, stamen, carpel. The procedure has two steps in strict order. First, emasculation — the anthers are removed from the flower chosen as the female parent, so it cannot pollinate itself. Second, pollination — pollen from the chosen male parent is transferred to the stigma. The resulting pod carries the seeds, which are grown into the F1 plants.
Asked as — "removal of anthers is called ___" → emasculation. Also asked: why is emasculation necessary in pea? Because pea is normally self-pollinating, so without it the cross would fail. Bagging (covering the emasculated flower) is the follow-up step in the artificial hybridisation sequence.
Mendel crossed a tall pea plant with a dwarf pea plant, collected the seeds, and grew them.
The plants grown from those seeds are the first hybrid generation — the Filial1 progeny or F1. The two original plants are the parental or P generation.
Asked as — the full form of F1 is "first filial generation". Filial comes from the word for offspring.
All the F1 plants were tall. Not one was dwarf.
The dwarf character simply disappeared from view. It did not soften the height of the F1 plants — they were as tall as the tall parent, not medium.
The same happened with all the other trait pairs: the F1 always resembled one parent, and the other parent's trait was not seen at all.
This is what makes it a general rule rather than a peculiarity of height.
Mendel then self-pollinated the tall F1 plants. In the F2 (Filial2) generation, dwarf plants reappeared — one quarter dwarf, three quarters tall.
This is the pivotal result of the chapter. The character that vanished in F1 was not destroyed; it was hidden. It must have been carried, unexpressed, inside the F1 plants.
Asked as — 3:1 is the F2 phenotypic ratio of a monohybrid cross. Note precisely that 3/4 were tall and 1/4 dwarf.
The F2 tall and dwarf plants were identical to the parental types. There was no blending — no plant of in-between height, at either F1 or F2.
This kills the blending theory of inheritance that was popular before Mendel. Whatever is inherited stays intact as a discrete unit; it does not mix like paint and get diluted.
Asked as — "Mendel's monohybrid cross disproved which theory?" → the theory of blending inheritance. The reasoning phrase to use: the characters are recovered unchanged in F2.
All seven characters behaved the same way: one trait only in F1, both traits in F2 in a 3:1 proportion.
Seven independent repetitions of the same result. That is what let him state a law rather than describe an observation.
Mendel proposed that something is passed down stably and unchanged from parent to offspring through the gametes, over successive generations. He called these things "factors".
Three words in that statement carry weight: stably, unchanged, and through the gametes. Gametes are the vehicle; the factor is the cargo.
Asked as — "Mendel's factors are today known as ___" → genes. He never used the word gene; that came later.
Genes are the units of inheritance. They contain the information required to express a particular trait in an organism.
Learn this as the definition of a gene for this chapter: a unit of inheritance carrying the information for a trait.
Genes that code for a pair of contrasting traits are called alleles. Alleles are slightly different forms of the same gene.
One gene, more than one version. T and t are not two genes — they are two alleles of the single gene for stem height. Getting this distinction right is essential before the linkage section on Day 2.
Asked as — allele definition, and "T and t are ___ of each other" → alleles.
The symbol convention: a capital letter for the trait that shows up in F1, the same letter in lower case for the other trait.
So T for tall, t for dwarf. The possible pairs for height are therefore TT, Tt and tt — three combinations, no more.
Why the same letter in both cases: writing T for tall and d for dwarf would hide the fact that the two are alleles of the same gene.
The book states this explicitly as a warning. It is a reasoning point, occasionally asked as an assertion-reason item.
In a true-breeding tall or dwarf variety, the allelic pair is identical — the plant is homozygous, TT or tt.
Homozygous = two identical alleles. Heterozygous = two different alleles, as in Tt. Alleles can be similar (in homozygotes) or dissimilar (in the heterozygote).
Asked as — NCERT exercise 2(b), differentiate homozygous and heterozygous. Add that a homozygote produces only one kind of gamete, a heterozygote two kinds in equal proportion.
Genotype is the genetic constitution — TT, Tt, tt. Phenotype is the descriptive appearance — tall, dwarf.
The book's in-text question: what is the phenotype of a plant with genotype Tt? Tall — because T dominates. Two different genotypes, TT and Tt, give the same phenotype. That fact is the reason the test cross has to exist.
Because the F1 heterozygote Tt looked exactly like the TT parent, Mendel concluded that in a pair of dissimilar factors, one dominates the other. That one is the dominant factor, the other recessive.
Follow the logic direction: the observation is that Tt looks like TT; the conclusion is dominance. T (tallness) is dominant, t (dwarfness) recessive. He saw identical behaviour in all seven character pairs.
Asked as — NCERT exercise 2(a), differentiate dominant and recessive. Key line: a recessive allele expresses only in the homozygous condition; a dominant allele expresses in both homozygous and heterozygous conditions.
A Tt plant, heterozygous for the gene controlling one character, is a monohybrid. The cross TT × tt is a monohybrid cross.
Mono = one character. When two characters are followed together it becomes a dihybrid cross (Day 2).
Asked as — NCERT exercise 2(c), monohybrid vs dihybrid. Include the ratios: monohybrid F2 3:1, dihybrid F2 9:3:3:1.
Because the recessive trait reappears in F2 without blending, the two alleles of a pair must separate — segregate — from each other during gamete formation by meiosis, and only one allele reaches any one gamete.
This is an inference, not an observation, and questions test whether the student knows which is which. The observed fact is the clean reappearance of dwarfs; the inferred mechanism is segregation during meiosis.
Segregation is a random process, so there is a 50 per cent chance of a gamete carrying either allele.
Gametes of a TT plant all carry T. Gametes of a tt plant all carry t. Gametes of a Tt plant are half T and half t.
At fertilisation, T from one parent (through the pollen) and t from the other (through the egg) unite to give a zygote with one T and one t — the hybrid Tt.
Since the hybrid carries alleles for contrasting traits, it is heterozygous.
The Punnett square was developed by the British geneticist Reginald C. Punnett. It is a graphical way to calculate the probability of all possible genotypes among the offspring of a cross.
How it is built: the possible gametes are written along two sides — usually the top row and the left column — and every possible combination is filled into the boxes below, producing a square.
Asked as — the name Reginald C. Punnett and his nationality (British) are both asked. Distractors are usually Bateson, Correns and Boveri.
Figure 4.4 reads in this order: parental tall TT (male) × dwarf tt (female) → gametes T and t → F1 all Tt, tall → selfing → F2.
The symbols ♀ and ♂ mark the eggs and the pollen of the F1 generation. When a Tt plant is self-pollinated it makes T and t gametes in equal proportion, and each pollen type has a 50 per cent chance of meeting each egg type.
The F2 outcome: phenotypic ratio tall : dwarf = 3 : 1. Genotypic ratio TT : Tt : tt = 1 : 2 : 1.
Both ratios, together, for the same cross. Students who remember only 3:1 lose marks whenever the question says "genotypic".
Asked as — the commonest question in the chapter. Read the word before "ratio" every single time.
Of the random fertilisations, 1/4 give TT, 1/2 give Tt, and 1/4 give tt.
The F1 is Tt in genotype but tall in phenotype. In F2, three quarters are tall — some of them TT, others Tt.
It is not possible to tell TT and Tt apart by looking at the plant.
Within the pair Tt, only T is expressed. So dominance is precisely what makes the genotype invisible — and that is the problem the test cross solves.
The 1/4 : 1/2 : 1/4 result can be written as the expansion of a binomial, because the gametes carrying T or t occur at equal frequency of ½.
(½T + ½t)² = ¼ TT + ½ Tt + ¼ tt. The general form given in the book is (ax + by)².
Asked as — which expression gives the F2 genotypic ratio of a monohybrid cross. The answer is the squared binomial, not the unsquared one.
Mendel self-pollinated the F2 plants. The dwarf F2 plants went on producing only dwarf plants in F3 and F4, so he concluded the dwarfs were homozygous tt.
A recessive phenotype always reveals its genotype — it can only be homozygous recessive. That is why no test cross is ever needed for a recessive plant.
The book's in-text question: what would he have got had he self-pollinated a tall F2 plant?
The tall F2 plants are 1/3 TT and 2/3 Tt. Selfing a TT gives all tall offspring. Selfing a Tt gives tall and dwarf in 3:1. So one third of the tall F2 plants breed true, and two thirds again produce dwarfs.
Asked as — "among the tall F2 plants, what fraction is homozygous?" → 1/3. Note this is 1/3 of the tall plants, but 1/4 of all F2 plants. Both versions appear in question papers.
Genotypic ratios can be predicted mathematically, but looking at a dominant phenotype cannot tell you the genotype. Whether a tall plant is TT or Tt cannot be predicted.
Hence Mendel crossed a tall F2 plant with a dwarf plant. He called this a test cross.
Definition of a test cross: an organism showing the dominant phenotype, whose genotype is to be determined, is crossed with the recessive parent — not self-crossed.
The progeny are then easy to read. Figure 4.5 uses flower colour, where violet (V) is dominant over white (v):
Asked as — NCERT exercise 5, define and design a test cross. The ratio to quote is 1:1. Also know the neighbouring term: a back cross is a cross of the F1 with either parent; a test cross is the special case where that parent is the recessive one. Every test cross is a back cross; not every back cross is a test cross.
From his monohybrid crosses Mendel proposed two general rules, today called the Principles or Laws of Inheritance: the First Law, the Law of Dominance, and the Second Law, the Law of Segregation.
Both of these come out of the monohybrid cross. The third law, independent assortment, comes from the dihybrid cross and belongs to Day 2. Questions frequently test which law came from which cross.
Law of Dominance, in three clauses: (i) characters are controlled by discrete units called factors; (ii) factors occur in pairs; (iii) in a dissimilar pair of factors, one member dominates the other.
Write all three clauses in a written answer — partial marks are lost for giving only the third. What this law explains: why only one parental character appears in F1, why both appear in F2, and why the F2 proportion is 3:1.
Asked as — NCERT exercise 4, explain the Law of Dominance using a monohybrid cross. Answer structure: state the three clauses, draw TT × tt → F1 Tt tall, then the F2 square with 3:1, and connect each clause to what it explains.
Law of Segregation: the alleles do not blend, and both characters are recovered as such in F2 even though one of them was not visible at F1. Although a parent contains two alleles, the alleles of a pair segregate during gamete formation so that a gamete receives only one of the two.
Consequences to state alongside: a homozygous parent produces gametes that are all alike; a heterozygous parent produces two kinds of gametes, each carrying one allele, in equal proportion.
Asked as — this is the one Mendelian law with no exceptions, which makes it a favourite for "which law is universally applicable" questions. Dominance fails in incomplete dominance and co-dominance; independent assortment fails in linkage; segregation holds throughout.
When the pea experiments were repeated with other traits in other plants, the F1 sometimes resembled neither parent and was in between the two.
The example is flower colour in the dog flower — snapdragon, Antirrhinum sp.
Asked as — the botanical name and the common names (snapdragon, dog flower) are all asked. Mirabilis jalapa (four o'clock plant) is the other standard textbook example and appears in question banks even though this chapter names only Antirrhinum.
Cross: true-breeding red-flowered RR × true-breeding white-flowered rr → F1 Rr is pink. Selfing the F1 gives F2 of 1 red : 2 pink : 1 white.
Read this against the monohybrid result. The genotype ratio is exactly what any Mendelian monohybrid cross gives — 1:2:1. What changed is the phenotype ratio: it is no longer 3:1, it is 1:2:1 as well.
Asked as — "In incomplete dominance, which ratio differs from the Mendelian monohybrid cross?" → the phenotypic ratio only. Here the phenotypic and genotypic ratios coincide, because every genotype now has its own appearance.
The cause: R was not completely dominant over r, which made it possible to tell Rr (pink) apart from RR (red) and rr (white).
Notice what incomplete dominance buys you — the heterozygote becomes visible. Under complete dominance it is hidden and you need a test cross; under incomplete dominance you can read the genotype straight off the flower.
Explanation of the concept of dominance — the chapter now asks why any allele should be dominant at all. To answer that, you have to look at what a gene does.
Every gene carries the information to express a trait. In a diploid organism there are two copies of each gene, as a pair of alleles. The two need not be identical: one may have been altered by some change it has undergone, and that change modifies the information the allele carries.
Take a gene carrying the information for an enzyme. The normal allele makes the normal enzyme, which is needed to transform a substrate S. The modified allele has three theoretical possibilities.
(i) it produces the normal enzyme, or a less efficient version of it;
(ii) it produces a non-functional enzyme; or
(iii) it produces no enzyme at all.
Asked as — the three possibilities are asked as a set. Learn them in this order.
Working through those cases: in the first, the modified allele is equivalent to the unmodified one and gives the same phenotype — such equivalent allele pairs are very common. In the second and third, the phenotype depends only on the functioning of the unmodified allele.
So the unmodified, functioning allele — the one representing the original phenotype — is the dominant allele, and the modified allele is generally the recessive one. A recessive trait therefore appears because of a non-functional enzyme, or because no enzyme is made.
Asked as — "the recessive allele generally represents ___" → the modified allele producing a non-functional enzyme or no enzyme. This is the molecular basis of dominance and comes up in assertion-reason questions.
The three-way comparison the book sets up: in dominance, the F1 resembles one of the two parents; in incomplete dominance, it is in between; in co-dominance, the F1 resembles both parents.
"Both", not "in between". In co-dominance both alleles express their own products fully and independently, and both products are present together.
Asked as — the single most examined confusion in this section. Pink snapdragon = incomplete dominance (a new intermediate appearance, neither parent's). AB blood group = co-dominance (both parental products present).
The example is ABO blood grouping in humans, controlled by the gene I.
The plasma membrane of red blood cells carries sugar polymers that protrude from its surface, and the kind of sugar is decided by this gene. The gene has three alleles: IA, IB and i.
IA and IB each make a slightly different form of the sugar. The allele i makes no sugar at all.
That single fact explains the whole dominance pattern of the system, and it links straight back to the enzyme logic in D1-49 and D1-50 — the recessive allele is the one producing nothing.
Asked as — "Why is i recessive?" → because it produces no sugar. A frequent one-mark question.
Because humans are diploid, each person carries any two of the three alleles. IA and IB are both completely dominant over i.
With IA and i together, only IA expresses, because i makes nothing. With IB and i, only IB expresses. But when IA and IB are together, both express their own sugars — this is co-dominance — and the red cells carry both A and B sugars.
Three alleles give six possible combinations, so there are six genotypes — and, the in-text question asks, how many phenotypes? Four: A, B, AB and O.
| Genotype | Blood group | Why |
|---|---|---|
| IAIA | A | A sugar only |
| IAi | A | i makes nothing, so A shows |
| IBIB | B | B sugar only |
| IBi | B | i makes nothing, so B shows |
| IAIB | AB | co-dominance, both sugars |
| ii | O | no sugar at all |
Asked as — the numbers 6 and 4 are asked as a pair, and the order in the options is often reversed to catch a careless read. Two genotypes each for A and B; one each for AB and O.
ABO grouping is also the chapter's example of multiple alleles: more than two — here three — alleles governing the same character.
The crucial qualifier: since only two alleles can be present in one individual, multiple alleles can be recognised only when studies are made at the level of a population, not an individual.
Asked as — "Multiple allelism can be studied only in a population" is a standard true/false and assertion-reason item. The reason is that a diploid individual carries just two alleles.
The starch grain example: starch synthesis in pea seeds is controlled by one gene with two alleles, B and b.
BB homozygotes synthesise starch efficiently and make large starch grains. bb homozygotes are less efficient and make smaller grains. After the seeds mature, BB seeds are round and bb seeds are wrinkled. Heterozygotes produce round seeds, so B looks dominant — but their starch grains are of intermediate size.
The conclusion: for seed shape, B is dominant; for starch grain size, the same alleles show incomplete dominance. So dominance is not an autonomous feature of a gene or of its product.
It depends on the gene product, on how a particular phenotype is produced from that product, and on which phenotype you choose to examine — when one gene influences more than one phenotype.
Asked as — this is a favourite conceptual question, and the answer students get wrong is the sweeping one ("B is incompletely dominant"). The correct answer is that dominance depends on the phenotype being examined. Note also that this same paragraph opens with a single gene product having more than one effect, which is the idea developed as pleiotropy in section 4.5 on Day 2.
1. How many years did Mendel work, and between which years? 2. How many true-breeding varieties, and how many trait pairs? 3. Which is dominant in pods — green or yellow? And in seeds? 4. Define a true-breeding line. 5. What is the technical name for removing anthers? 6. Give the F2 phenotypic and genotypic ratios of a monohybrid cross. 7. Who developed the Punnett square, and of what nationality? 8. State the three clauses of the Law of Dominance. 9. State the Law of Segregation. 10. Which Mendelian law has no exceptions? 11. In Antirrhinum, what are the F2 phenotypic and genotypic ratios, and which one differs from Mendel's? 12. Why is the allele i recessive? 13. How many genotypes and how many phenotypes in the ABO system? 14. Why can multiple alleles be studied only in a population? 15. In Bb pea seeds, which phenotype shows dominance and which shows incomplete dominance?
Answers: 7 years, 1856–1863 · 14 varieties, 7 pairs · pods green, seeds yellow · continuous self-pollination giving stable inheritance and expression over several generations · emasculation · 3:1 and 1:2:1 · Reginald C. Punnett, British · factors are discrete units / factors occur in pairs / one of a dissimilar pair dominates · alleles do not blend and segregate during gamete formation so each gamete gets only one of the two · Segregation · 1:2:1 and 1:2:1, the phenotypic ratio differs · it produces no sugar · 6 and 4 · because a diploid individual carries only two alleles · seed shape shows dominance, starch grain size shows incomplete dominance.