This MCQ module is based on: Mendels Laws
TOPIC 15 OF 64
Mendels Laws
🎓 Class 12
Biology
CBSE
Theory
Ch 4 – Principles of Inheritance and Variation
⏱ ~14 min
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Mendels Laws
5.1 Introduction — The Story of Inheritance
Have you ever wondered why you look so much like your parents — perhaps your father's nose, your mother's eyes, your grandmother's hair colour? The transmission of characters from one generation to the next is called inheritance. The branch of biology that studies inheritance and the variation between individuals is called genetics.
While children resemble parents, they are never identical to them — there is always some variation. Understanding inheritance and variation began with the careful experiments of Gregor Johann Mendel, an Austrian monk often called the "Father of Genetics."
5.2 Mendel's Laws of Inheritance
5.2.1 Why Mendel Chose Garden Pea (Pisum sativum)
Between 1856 and 1863, Mendel conducted hybridisation experiments on the garden pea. Pea plants were ideal because:
- Several pure-breeding varieties were available with sharply contrasting traits.
- Pea plants are self-pollinating, so true-breeding lines could be maintained — but cross-pollination is also easy with manual emasculation.
- Short life cycle — many generations could be observed in a few years.
- Each plant produced large numbers of offspring, allowing statistical analysis.
- Each contrasting trait was distinctly different (e.g., tall/dwarf, round/wrinkled, yellow/green).
Mendel selected seven pairs of contrasting traits for study:
| Character | Dominant trait | Recessive trait |
|---|---|---|
| Stem height | Tall | Dwarf |
| Flower colour | Violet (Purple) | White |
| Flower position | Axial | Terminal |
| Pod shape | Inflated | Constricted |
| Pod colour | Green | Yellow |
| Seed shape | Round | Wrinkled |
| Seed colour | Yellow | Green |
5.3 The Monohybrid Cross — Cross Between One Pair of Traits
A cross involving inheritance of one pair of contrasting traits is called a monohybrid cross. Mendel crossed a true-breeding tall plant (TT) with a true-breeding dwarf plant (tt).
Some essential vocabulary:
- P generation — the original parent plants.
- F1 generation — first filial generation, produced by the P × P cross.
- F2 generation — second filial generation, produced by F1 × F1.
- Allele — alternative forms of a gene (T vs. t).
- Genotype — genetic constitution (TT, Tt, tt).
- Phenotype — the observable trait (tall, dwarf).
- Homozygous — same alleles (TT or tt). Heterozygous — different alleles (Tt).
5.3.1 Mendel's Result for the Monohybrid Cross
P generation: Tall (TT) × Dwarf (tt)
F1 generation: ALL plants were Tall (Tt) — none were dwarf!
When Mendel allowed F1 tall plants to self-pollinate:
F2 generation: Tall : Dwarf = 3 : 1 (phenotype ratio)
5.3.2 Mendel's Conclusions — The First Two Laws
Law of Dominance: When two true-breeding individuals with contrasting alleles are crossed, the offspring display only one trait — the dominant allele. The other allele is recessive — it is hidden in the F1 heterozygote but reappears in F2.
Law of Segregation: The two alleles of a gene separate during gamete formation (meiosis), so each gamete carries only one allele. When fertilisation occurs, alleles pair up again randomly. This law is universally true — it has no known exceptions in sexually reproducing organisms.
5.4 The Test Cross — Detecting the Hidden Genotype
A tall pea plant could be TT (homozygous) or Tt (heterozygous). To find out, Mendel devised the test cross — cross the unknown plant with a homozygous recessive (dwarf, tt).
| Tested plant | Cross | Result | Conclusion |
|---|---|---|---|
| TT (homozygous tall) | TT × tt | All offspring tall (Tt) | Tested plant was TT |
| Tt (heterozygous tall) | Tt × tt | 1 Tall (Tt) : 1 Dwarf (tt) | Tested plant was Tt |
🧬 Interactive: Punnett Square Solver
Choose parental genotypes and predict the offspring ratios:
📐 Activity 5.1 — Mendel's Pea Garden
Setup: A scientist crosses a true-breeding violet-flowered pea (VV) with a true-breeding white-flowered pea (vv).
Predict: (a) What flower colours will F1 show? (b) If F1 are self-pollinated, what will be the F2 ratio? (c) Out of 1200 F2 plants, how many should be white?
(a) All F1 are Vv — violet (violet is dominant; recessive white is masked).
(b) F2 = 3 violet : 1 white (1 VV : 2 Vv : 1 vv).
(c) Expected white = 1/4 × 1200 = 300 white plants; remaining 900 violet.
Worked Examples
Worked Example 1: Identify the Genotype
A round-seeded pea plant (R is dominant) is crossed with a wrinkled-seeded plant (rr). Out of 80 offspring, 40 are round and 40 are wrinkled. What is the genotype of the round-seeded parent?
Step 1: Round seed could be RR or Rr. Wrinkled is rr.
Step 2: If parent is RR → all offspring would be Rr (round, 100%).
Step 3: If parent is Rr → cross Rr × rr → ½ Rr (round) : ½ rr (wrinkled) = 1:1 ratio.
Step 4: Observed 40:40 = 1:1. ✅
Conclusion: The round-seeded parent was Rr (heterozygous). This is a classic test cross result.
Step 2: If parent is RR → all offspring would be Rr (round, 100%).
Step 3: If parent is Rr → cross Rr × rr → ½ Rr (round) : ½ rr (wrinkled) = 1:1 ratio.
Step 4: Observed 40:40 = 1:1. ✅
Conclusion: The round-seeded parent was Rr (heterozygous). This is a classic test cross result.
Worked Example 2: Probability Calculation
In a Tt × Tt cross, what is the probability that the first three offspring are all tall?
Step 1: P(tall offspring) from Tt × Tt = 3/4.
Step 2: Each fertilisation is independent — multiply probabilities.
Step 3: P(all 3 tall) = (3/4) × (3/4) × (3/4) = 27/64 ≈ 42.2%
Note: P(at least one dwarf) = 1 − 27/64 = 37/64 ≈ 57.8%.
Step 2: Each fertilisation is independent — multiply probabilities.
Step 3: P(all 3 tall) = (3/4) × (3/4) × (3/4) = 27/64 ≈ 42.2%
Note: P(at least one dwarf) = 1 − 27/64 = 37/64 ≈ 57.8%.
🎯 Competency-Based Questions
Q1. Mendel's Law of Segregation states that:L1 Remember
Answer: (b). The two alleles of a gene separate during meiosis (gamete formation) so each gamete contains only one allele. This is also called the Law of Purity of Gametes.
Q2. Fill in the blank: In a Tt × tt cross, the phenotypic ratio of offspring is _____. L2 Understand
Answer: 1 : 1 (Tall : Dwarf). This is the standard test cross ratio — half the gametes from Tt are T and half are t; tt parent contributes only t. So Tt and tt offspring appear equally.
Q3. A test cross between a tall plant and a dwarf produces 50% tall and 50% dwarf offspring. The genotype of the tall parent is _____. Explain why. L3 Apply
Answer: Tt (heterozygous). If the tall plant were TT, all offspring would be Tt and tall (100%). Since 50% are dwarf (tt), the tall parent must contribute t alleles in half its gametes — meaning it is Tt.
Q4. Analyse: Why did Mendel's choice of pea plant prove crucial for discovering the laws of inheritance? L4 Analyse
Multi-factor success:
- Discrete contrasting traits — clean ratios visible.
- Self-pollination — allowed pure lines.
- Easy hand cross-pollination — controlled crosses.
- Short life cycle — multiple generations in years.
- Many offspring per cross — robust statistics.
Q5. HOT (Create): Design a single experiment that distinguishes a homozygous dominant pea from a heterozygous pea WITHOUT using a test cross. L6 Create
Sample design — Self-pollination test:
- Allow each unknown tall plant to self-pollinate.
- Grow F1 seeds and observe phenotypes.
- If TT: all F1 offspring are tall (no dwarfs).
- If Tt: F1 shows the 3:1 tall : dwarf ratio (recessive dwarf reappears).
🧠 Assertion–Reason Questions
Choose: (A) Both true, R explains A. (B) Both true, R doesn't explain A. (C) A true, R false. (D) A false, R true.
A: In a monohybrid cross of Tt × Tt, the F2 generation shows a 3:1 phenotypic ratio.
R: The two alleles of a gene segregate during gamete formation and recombine randomly at fertilisation.
Answer: (A). Both true; R correctly explains A. Random segregation of alleles in gametes (T : t = 1:1) and random fertilisation give the genotypes 1 TT : 2 Tt : 1 tt → phenotype 3 tall : 1 dwarf.
A: A test cross is used to determine the genotype of a recessive plant.
R: The recessive phenotype always corresponds to a homozygous recessive genotype.
Answer: (D). A is FALSE — a test cross is used for the dominant phenotype (unknown if it is TT or Tt), not the recessive. R is TRUE — a recessive phenotype is always homozygous recessive (genotype is already known to be tt).
A: Mendel's experiments produced consistent ratios because he counted thousands of plants.
R: Statistical reliability of inheritance ratios increases with sample size.
Answer: (A). Both true; R explains A. Mendel's careful counting of thousands of offspring (totalling ~28,000 pea plants over 8 years) gave him statistically reliable ratios where chance fluctuations averaged out.
Frequently Asked Questions - Mendels Laws
What is the main concept covered in Mendels Laws?
In NCERT Class 12 Biology Chapter on Principles of Inheritance and Variation, "Mendels Laws" covers the core biological structures, processes, and pathways students need for board exam success. The MyAiSchool lesson explains the topic with definitions, labelled diagrams, comparison tables, and interactive simulations. Scientific terminology and physiological/genetic significance are highlighted throughout to build conceptual depth aligned with CBSE 2025-26 syllabus.
How is Mendels Laws useful in real-life or applied biology?
Real-life applications of "Mendels Laws" from NCERT Class 12 Biology Principles of Inheritance and Variation include medical diagnostics, agriculture, biotechnology, public health, evolutionary insights, and ecological monitoring. The MyAiSchool lesson links every biological concept to a tangible application so students see biology as a problem-solving framework for living systems and real-world challenges.
What are the key terms students should memorize for Mendels Laws?
Key terms in "Mendels Laws" (NCERT Class 12 Biology Principles of Inheritance and Variation) are tabulated in the MyAiSchool key-terms grid. Students should memorize each term with its precise definition, function, and example. Terminology is high-yield in CBSE board exams — 1-mark MCQs and 2-mark short answers test definitions directly. The Summary section provides a printable quick-reference card.
How does this part connect to other parts of the chapter?
NCERT Class 12 Biology Principles of Inheritance and Variation is structured so each part builds biological understanding sequentially. "Mendels Laws" connects to neighbouring parts via shared mechanisms, structural hierarchies, and physiological processes. The MyAiSchool lesson cross-references related concepts with internal links so students can navigate the whole chapter as one connected biological story rather than disconnected fragments.
What types of CBSE board questions come from Mendels Laws?
CBSE board questions from "Mendels Laws" typically include: (1) 1-mark MCQs on definitions and processes, (2) 2-mark short-answer differences/comparisons, (3) 3-mark labelled-diagram questions, (4) 5-mark long-answer essays combining mechanism + diagram + significance. The MyAiSchool lesson tags each Competency-Based Question (CBQ) with Bloom level (L1-L6) so students know how to study for each weight.
How can students use the interactive simulation effectively?
The interactive simulation in the "Mendels Laws" lesson allows students to explore biological processes, classifications, or pathways using selectors and sliders, with live visual feedback. To use it effectively: (1) explore each option/state, (2) compare with textbook diagrams, (3) note the function/outcome changes, (4) try the integrated practice quiz. The simulation reinforces visual-spatial understanding that pure text-based study cannot.
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