StudyOSStudyOS CBSE Study Material, Class 5–12

Heredity

NCERT Class 10 · Science Based on NCERT Class 10 Science textbook · Free CBSE study kit

Everything you need to ace this chapter, in one place.
Notes · Mind Map · Quiz · Assertion-Reason · Case Studies · Question Bank · Past Papers

Get Full Study Kit →
Studying with friends? Send them this chapter.

Chapter Notes

Overview
This chapter explains the mechanism by which variations are created and inherited from one generation to the next. It builds on the idea that reproductive processes give rise to new individuals that are similar but subtly different, and explores how these differences are passed on. The one big idea a student must walk away with is that traits are controlled by genes, which come in pairs, and that the inheritance of traits follows predictable patterns as discovered by Mendel, including the concepts of dominant and recessive traits and independent assortment.
Key Concepts (12)
Heredity
The process by which traits and characteristics are reliably inherited from one generation to the next.
e.g. A child inheriting free earlobes from a parent who has free earlobes.
Variation
Subtle differences in body design or traits among individuals of the same species, arising during reproduction.
e.g. Some students in a class have free earlobes while others have attached earlobes.
Gene
A section of DNA that provides information for one protein, and thus controls a characteristic or trait.
e.g. The gene for plant height provides information for an enzyme that makes a growth hormone.
Dominant trait
A trait that is expressed in the F1 generation even when only one copy of the gene for that trait is present.
e.g. Tallness in pea plants (T) is dominant over shortness (t).
Recessive trait
A trait that is expressed only when both copies of the gene for that trait are identical and recessive.
e.g. Shortness in pea plants (tt) is recessive; it appears only when both alleles are 't'.
F1 generation
The first filial generation obtained by crossing two parent plants with contrasting traits.
e.g. Crossing a tall pea plant (TT) with a short pea plant (tt) gives all tall F1 plants (Tt).
F2 generation
The second filial generation obtained by self-pollinating the F1 plants.
e.g. Self-pollinating F1 tall plants (Tt) gives F2 progeny with a 3:1 ratio of tall to short plants.
Chromosome
Separate independent pieces of DNA that carry genes; each cell has two copies of each chromosome, one from each parent.
e.g. Human cells have 23 pairs of chromosomes, including one pair of sex chromosomes.
Sex chromosomes
The pair of chromosomes that determine the sex of an individual; in humans, women have XX and men have XY.
e.g. A child inherits an X chromosome from the mother and either an X (girl) or Y (boy) from the father.
Independent inheritance
The principle that different traits are inherited independently of each other, as shown by new combinations of traits in F2 offspring.
e.g. In pea plants, seed shape (round/wrinkled) and seed colour (yellow/green) are inherited independently, giving 9:3:3:1 ratio.
DNA copying
The process by which DNA is replicated during reproduction, with small inaccuracies leading to variations.
e.g. When a bacterium divides, small errors in DNA copying generate minor differences between the two daughter cells.
Selection of variants
The process by which environmental factors favour certain variations, forming the basis for evolution.
e.g. Bacteria that can withstand heat will survive better during a heat wave.
Formulas & Equations (3)
Monohybrid cross ratio: 3:1 (dominant : recessive) in F2 generation
Reads as: In a monohybrid cross, the F2 generation shows a three to one ratio of dominant to recessive traits.
Variables: Dominant trait (e.g., tall) and recessive trait (e.g., short) in pea plants.
Use when: When solving problems involving inheritance of a single trait, like tall vs short plants.
⚠ Students often write the ratio as 1:2:1 for phenotypes instead of genotypes; the phenotypic ratio is 3:1.
Dihybrid cross ratio: 9:3:3:1 in F2 generation
Reads as: In a dihybrid cross, the F2 generation shows a nine to three to three to one ratio of trait combinations.
Variables: 9 (both dominant), 3 (first dominant, second recessive), 3 (first recessive, second dominant), 1 (both recessive).
Use when: When solving problems involving inheritance of two independent traits, like seed shape and colour.
⚠ Students forget that the 9:3:3:1 ratio applies only when traits are independently inherited and both parents are heterozygous for both traits.
Sex determination in humans: XX (female) and XY (male)
Reads as: Women have two X chromosomes, men have one X and one Y chromosome.
Variables: X and Y are sex chromosomes; mother always contributes X, father contributes either X or Y.
Use when: When explaining or solving problems about how the sex of a child is determined.
⚠ Students incorrectly think the mother determines the sex of the child; actually, the father's sperm determines it.
How It Works
Accumulation of variation during reproduction
  1. Step 1: The previous generation provides a common basic body design and subtle changes to the next generation.
  2. Step 2: The new generation inherits differences from the first generation and also creates new differences during its own reproduction.
  3. Step 3: In asexual reproduction, variations are minor due to small inaccuracies in DNA copying.
  4. Step 4: In sexual reproduction, greater diversity is generated due to the combination of genes from two parents.
💡 Variations are not all equal; some give survival advantages depending on the environment.
Mendel's monohybrid cross (tall vs short pea plants)
  1. Step 1: Mendel crossed a tall pea plant (TT) with a short pea plant (tt).
  2. Step 2: All F1 progeny were tall (Tt), showing that tallness is dominant.
  3. Step 3: He self-pollinated the F1 tall plants to produce the F2 generation.
  4. Step 4: In the F2 generation, one quarter of the plants were short (tt) and three quarters were tall (TT or Tt), giving a 3:1 phenotypic ratio.
💡 The F1 plants are all tall because the dominant trait (T) masks the recessive trait (t); the recessive trait reappears in F2.
Mendel's dihybrid cross (seed shape and colour)
  1. Step 1: Mendel crossed a tall plant with round seeds (RR yy) with a short plant with wrinkled seeds (rr YY).
  2. Step 2: All F1 progeny were tall with round seeds (Rr Yy), showing tallness and round seeds are dominant.
  3. Step 3: He self-pollinated the F1 plants to produce the F2 generation.
  4. Step 4: The F2 generation showed new combinations: tall with wrinkled seeds and short with round seeds, in addition to the parental combinations.
  5. Step 5: The ratio of phenotypes was 9:3:3:1, proving independent inheritance of the two traits.
💡 New combinations arise because factors for seed shape and seed colour recombine independently during gamete formation.
Expression of traits (how genes control characteristics)
  1. Step 1: Cellular DNA is the information source for making proteins in the cell.
  2. Step 2: A section of DNA that provides information for one protein is called a gene.
  3. Step 3: Proteins control characteristics; for example, an enzyme involved in making a plant hormone determines plant height.
  4. Step 4: If the gene for that enzyme works efficiently, a lot of hormone is made and the plant is tall; if the gene is altered, less hormone is made and the plant is short.
💡 Genes do not directly produce traits; they produce proteins that influence traits.
Sex determination in human beings
  1. Step 1: Humans have 22 pairs of autosomes and one pair of sex chromosomes.
  2. Step 2: Women have two X chromosomes (XX); men have one X and one Y chromosome (XY).
  3. Step 3: All children inherit an X chromosome from their mother.
  4. Step 4: A child who inherits an X chromosome from the father will be a girl (XX); a child who inherits a Y chromosome from the father will be a boy (XY).
💡 The sex of the child is determined by the father, not the mother.
Formation of germ cells with single gene set
  1. Step 1: Each body cell has two copies of each chromosome, one from each parent.
  2. Step 2: During germ cell formation, each germ cell takes one chromosome from each pair.
  3. Step 3: This ensures that each germ cell has a single set of genes.
  4. Step 4: When two germ cells combine during fertilisation, the normal number of chromosomes is restored in the progeny.
💡 This mechanism ensures stability of DNA of the species and explains independent inheritance of traits.
Exam Traps — Don't Make These Mistakes
⚠ Confusing F1 and F2 generation ratios
✗ In Mendel's monohybrid cross, the F1 generation shows a 3:1 ratio of tall to short plants.
→ The F1 generation from a cross between TT and tt is all tall (100% tall), not a 3:1 ratio. The 3:1 ratio appears only in the F2 generation.
✓ In Mendel's monohybrid cross, the F1 generation had all tall plants, and the F2 generation showed a 3:1 ratio of tall to short plants.
⚠ Thinking the mother determines the sex of the child
✗ The sex of the child is determined by the mother because she provides the egg.
→ The mother always contributes an X chromosome, so she cannot determine sex. The father contributes either an X or a Y chromosome, which determines whether the child is a girl (XX) or a boy (XY).
✓ The sex of the child is determined by the father, because the sperm carries either an X or a Y chromosome.
⚠ Mixing up dominant and recessive traits in dihybrid crosses
✗ In a dihybrid cross, all F1 progeny show a mixture of both parental traits, like medium height and wrinkled-round seeds.
→ In Mendel's experiments, F1 progeny showed only dominant traits (tall and round seeds), not a mixture. Recessive traits reappear only in the F2 generation.
✓ In a dihybrid cross, all F1 progeny show only the dominant traits (e.g., tall and round seeds). Recessive traits appear in the F2 generation in new combinations.
⚠ Assuming all variations are equally likely to survive
✗ All variations in a species have equal chances of surviving in the environment.
→ Depending on the nature of variations, different individuals have different advantages. Environmental factors select variants that are better suited, so not all variations have equal survival chances.
✓ Variations that give a survival advantage in a given environment are more likely to persist, while others may not.
⚠ Confusing genes and chromosomes
✗ Genes and chromosomes are the same thing.
→ Genes are sections of DNA that code for proteins, while chromosomes are long threads of DNA that carry many genes. Each chromosome contains many genes.
✓ Genes are segments of DNA on chromosomes that provide information for making proteins. Chromosomes are structures that carry genes.
Compare & Contrast
Expression in F1 generation
Dominant trait
Recessive trait
A dominant trait is expressed in the F1 generation even when only one copy of the gene is present, while a recessive trait is expressed only when both copies are recessive (e.g., tallness is dominant over shortness).
Amount of variation produced
Asexual reproduction
Sexual reproduction
Asexual reproduction produces very minor variations due to small inaccuracies in DNA copying, while sexual reproduction generates much greater diversity because genes from two parents combine.
Number of traits studied
Monohybrid cross
Dihybrid cross
A monohybrid cross studies inheritance of a single trait (e.g., plant height) and gives a 3:1 ratio in F2, while a dihybrid cross studies two traits simultaneously (e.g., seed shape and colour) and gives a 9:3:3:1 ratio in F2.
Exam Focus
Marks pattern: 2 MCQs (1 mark each), 1 Short Answer (3 marks) on Mendel's experiments or sex determination, 1 Long Answer (5 marks) on dihybrid cross or explanation of inheritance.
Most asked: Mendel's monohybrid and dihybrid crosses with ratios · Dominant and recessive traits · Sex determination in humans · Definition of gene, chromosome, and variation
Don't skip: Explain Mendel's monohybrid cross with a diagram and the 3:1 ratio.; Explain Mendel's dihybrid cross and the 9:3:3:1 ratio.; How is the sex of a child determined in human beings? Explain with a diagram.
Safe to skip: The detailed historical background of Mendel's life (monastery, teaching certificate failure) is low-weightage and can be de-prioritised under time pressure.
One-Line Revision (read aloud before the exam)
  1. Heredity is the process by which traits are reliably inherited from one generation to the next.
  2. Variations arise during reproduction due to small inaccuracies in DNA copying.
  3. Sexual reproduction produces greater diversity than asexual reproduction.
  4. Mendel used pea plants with contrasting traits to study inheritance.
  5. In a monohybrid cross, the F1 generation shows only the dominant trait.
  6. The F2 generation of a monohybrid cross shows a 3:1 phenotypic ratio of dominant to recessive traits.
  7. A gene is a section of DNA that provides information for one protein.
  8. Dominant traits are expressed even with one copy of the gene; recessive traits require two copies.
  9. In a dihybrid cross, traits are inherited independently, giving a 9:3:3:1 ratio in F2.
  10. Sex in humans is determined by the father's sperm carrying either an X or a Y chromosome.
  11. Women have XX sex chromosomes; men have XY sex chromosomes.
  12. All children inherit an X chromosome from their mother.
  13. Germ cells have a single set of genes; fertilisation restores the normal chromosome number.
  14. Variations that give a survival advantage are selected by the environment.
  15. Chromosomes are separate independent pieces of DNA that carry genes.

MCQs — 10 Questions with Answers

Q1. A student observes that in a field of sugarcane plants (asexual reproduction), almost all plants look identical, but in a nearby goat herd (sexual reproduction), every goat looks distinctly different. Which of the following best explains this observation based on the chapter content?

  • A. Sexual reproduction shuffles genetic material from two parents, creating greater diversity than asexual reproduction which copies DNA with only minor errors ✓
  • B. Asexual reproduction produces identical copies while sexual reproduction causes intentional mutations in DNA to create variation
  • C. Sugarcane plants are genetically programmed not to vary, while goats are programmed to vary maximally
  • D. The environmental conditions in the sugarcane field prevent variation, but the goat herd environment promotes it

Answer: A — The chapter explicitly states that sexual reproduction generates greater diversity than asexual reproduction due to the combination of genetic material from two parents, while asexual reproduction shows only minor variations from DNA copying errors; option B wrongly claims mutations are intentional, and C-D misattribute the cause to genes or environment rather than reproductive mechanism.

Q2. In Activity 8.1, a student finds that both parents have attached earlobes, yet their child has free earlobes. Based on Mendel's principles discussed in the chapter, what does this observation suggest?

  • A. Both parents must be carrying a hidden version (allele) for free earlobes, which appeared in the child ✓
  • B. The child acquired free earlobes from environmental exposure during development
  • C. Free earlobes are a new mutation that arose spontaneously in the child
  • D. One parent must actually have free earlobes, but it was not visible due to surgery

Answer: A — Mendel's principle states that each trait has two versions in sexually reproducing organisms; if both parents show attached earlobes but carry the hidden recessive allele for free earlobes, this trait can appear in the child—option B incorrectly attributes it to environment, C to spontaneous mutation, and D to surgery rather than inheritance.

Q3. A farmer notices that a trait appears in 5% of a wild plant population while another trait appears in 85% of the same population. Which trait most likely arose earlier in the species' history?

  • A. The trait appearing in 85% of the population, as it has had more time to spread through the population ✓
  • B. The trait appearing in 5% of the population, as rare traits are always older
  • C. Both traits arose at the same time since they coexist in the population
  • D. The question cannot be answered without knowing the reproduction method

Answer: A — The chapter's Question 1 and concept of accumulation of variation indicate that older traits have had more generations to become established and common in the population, while newer traits remain rarer; option B incorrectly assumes rarity indicates age, and C ignores the principle that trait frequency reflects historical prevalence.

Q4. During a biology lesson, a student crosses two tall pea plants from Mendel's F1 generation (which were produced by crossing a tall and short parent). The student expects all offspring to be tall, but instead finds a 3:1 ratio of tall to short plants. What does this result demonstrate?

  • A. Both F1 tall plants carried a hidden version of the shortness trait that was inherited from their short parent ✓
  • B. The tall F1 plants mutated during self-pollination to produce short offspring
  • C. The soil conditions caused some tall plants to become short in the F2 generation
  • D. One of the F1 tall plants was actually heterozygous and the other was homozygous

Answer: A — The chapter explains that F1 plants inherit both tallness and shortness traits from their parents but express only tallness, and when self-pollinated produce the 3:1 F2 ratio—this proves both alleles were present but hidden; option D partially describes this but uses technical terminology beyond the chapter scope, while B and C wrongly attribute it to mutation and environment.

Q5. A breeder wants to understand why crossing two healthy, normal-looking rabbits occasionally produces offspring with a rare genetic condition. Using Mendel's principles, which explanation best fits this observation?

  • A. Both parents are carriers of recessive alleles for the condition; when each parent contributes their recessive allele, the condition appears in the offspring ✓
  • B. The condition is caused by environmental stress during pregnancy, not inheritance
  • C. The rare condition is a new mutation that occurs randomly during reproduction
  • D. One parent must have had the condition but it was not visible due to camouflage or medical treatment

Answer: A — Mendel's work shows that organisms inherit two versions of each trait, and a recessive condition appears only when both alleles are recessive—both normal-looking parents can carry hidden recessive alleles; B and D attribute it to environment and hidden phenotypes, while C ignores the hereditary pattern demonstrated by repeated occurrence.

Q6. Assertion (A): In sexually reproducing organisms, each trait is influenced by genetic material from both the mother and the father. Reason (R): Sexual reproduction involves the combination of genetic material from two different parents, which is why greater variation is produced compared to asexual reproduction. Choose the correct option:

  • A. Both A and R are true and R is the correct explanation of A ✓
  • B. Both A and R are true but R is not the correct explanation of A
  • C. A is true but R is false
  • D. A is false but R is true

Answer: A — The chapter explicitly states that both parents contribute equal amounts of genetic material and that sexual reproduction creates greater diversity due to combining genetic material from two parents; R correctly explains why A is true, making option A correct.

Q7. Assertion (A): All variations that appear in a population have equal chances of surviving in the environment. Reason (R): Environmental conditions select for certain variations, forming the basis for evolutionary processes. Choose the correct option:

  • A. Both A and R are true and R is the correct explanation of A
  • B. Both A and R are true but R is not the correct explanation of A
  • C. A is true but R is false
  • D. A is false but R is true ✓

Answer: D — The chapter states that variations do NOT have equal chances of survival and that environmental selection occurs—A is explicitly contradicted by the text which asks 'Do all these variations have equal chances? Obviously not,' while R is the correct principle explaining differential survival.

Q8. Assertion (A): In Mendel's F2 generation from self-pollination of F1 tall pea plants, approximately 75% of plants are tall and 25% are short. Reason (R): The F1 tall plants are homozygous for the tallness allele and produce only tall offspring when self-pollinated. Choose the correct option:

  • A. Both A and R are true and R is the correct explanation of A
  • B. Both A and R are true but R is not the correct explanation of A
  • C. A is true but R is false ✓
  • D. A is false but R is true

Answer: C — The chapter clearly demonstrates that F1 tall plants produce the 3:1 ratio in F2 generation (A is correct), but this occurs because F1 plants are heterozygous (not homozygous as stated in R)—they carry both alleles but express only tallness; R's explanation is factually incorrect.

Q9. A scientist examines a population of beetles and finds that 92% have black shells while 8% have red shells. Based on the principle of trait accumulation during reproduction, which inference is most reasonable?

  • A. The black shell trait likely arose earlier in the population's history and has become more prevalent through successive generations ✓
  • B. The red shell trait is older because it is rarer and has been diluted over time
  • C. Both traits arose in the current generation and will stabilize at different frequencies
  • D. The frequency of traits has no relationship to when they first appeared in the population

Answer: A — The chapter's principle of accumulation of variation indicates that traits appearing in higher percentages have had more time to spread through successive generations; option B reverses this logic incorrectly, C assumes both are recent, and D contradicts the mechanism of inheritance over time.

Q10. A gardener observes that in a hybrid tomato plant variety (from sexual reproduction), individual plants show significant variation in fruit size, colour, and shape, while in an experimental line created through tissue culture (asexual reproduction), all plants are virtually identical. What is the most scientifically accurate explanation for this observation?

  • A. Sexual reproduction generates variation through recombination of genetic material from two parents, while asexual reproduction produces clones with only minor DNA copying errors ✓
  • B. Hybrid plants are inherently unstable and decompose more easily, causing visible variation
  • C. Tissue culture deliberately introduces mutations to increase variation in plants
  • D. Sexual reproduction creates variation due to environmental stress, while asexual reproduction is protected from environmental effects

Answer: A — The chapter explains that sexual reproduction shuffles genetic material from two parents creating diversity, while asexual reproduction copies DNA with only minor copying errors—option A directly matches chapter principles; B incorrectly claims instability, C wrongly states mutations are deliberate, and D misattributes variation to environment rather than reproductive mechanism.

Flashcards

What is heredity?

The process by which traits and characteristics are reliably passed from parents to offspring through genes.

Define dominant trait with one example.

A trait that is expressed when present in at least one copy; example: tall plant (T) is dominant over short (t).

What is a recessive trait?

A trait expressed only when both copies of the gene are identical and recessive (like tt for short plants).

What does F1 generation mean?

The first filial generation produced by crossing two parent organisms.

What is the F2 generation?

The second filial generation produced by self-pollination or crossing of F1 individuals.

Why does sexual reproduction create more variation than asexual?

Sexual reproduction combines genetic material from two different parents, while asexual reproduction has only minor DNA copying errors.

What is the 3:1 ratio in Mendel's F2 generation?

Three dominant phenotype plants (TT or Tt) appear for every one recessive phenotype plant (tt).

How many gene copies does each sexually reproducing organism have for each trait?

Two gene copies (one from each parent) control each trait.

What are inherited traits?

Characteristics passed from parents to children through genes, such as eye color, height, or earlobe attachment.

How does environmental selection create evolution?

Organisms with beneficial variations survive better and reproduce more, gradually increasing that trait in the population.

Important Board Questions

Define heredity and explain why offspring produced by sexual reproduction show more variations than those produced by asexual reproduction. [2 marks]

Heredity = reliable passage of traits through genes. Sexual reproduction: two parents contribute different genetic material → maximum diversity. Asexual reproduction: one parent, only DNA copy errors → minimal variation.

In Mendel's pea plant experiments, crossing a tall plant (TT) with a short plant (tt) produced F1 plants that were all tall. When F1 plants were self-pollinated, the F2 generation contained both tall and short plants in a 3:1 ratio. Explain why short plants reappeared in F2 even though F1 plants were all tall. [3 marks]

F1 genotype is Tt (tall phenotype because T is dominant). F1 self-pollination: Tt × Tt → 1 TT : 2 Tt : 1 tt. Short phenotype (tt) reappears because both parents carried hidden recessive t allele; F1 inherited both and expressed short trait only when homozygous recessive.

A garden shows variation in plant height. Some plants are very tall, some are medium height, and some are very short. Explain how this variation could have arisen and accumulated in successive generations. How would environmental factors help in maintaining or changing the proportion of each height variant in future generations? [5 marks]

Variation source: sexual reproduction combines different parent genes; mutations create new traits; each generation inherits differences + creates new ones. Accumulation: favorable variations survive better (natural selection), reproduce more frequently, increase in population. Environmental factors: drought favors shorter plants (less water need), favorable season favors tall plants (more photosynthesis). Over generations, selection changes trait frequency, driving evolution.

Assertion-Reason Questions for Heredity

Q1. Assertion (A):

In the F1 generation of Mendel's crosses, no medium-height plants appeared when crossing tall and short parents.

Reason (R):

One allele shows complete dominance over the other, masking the recessive trait.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both the assertion and reason are correct, and the reason correctly explains why only one parental phenotype appeared in F1. The dominant allele completely masks the recessive one, preventing intermediate phenotypes.

Q2. Assertion (A):

Asexual reproduction produces much greater genetic diversity than sexual reproduction.

Reason (R):

Asexual reproduction creates new combinations through meiosis and fertilization.

Answer: D — A is false but R is true — The assertion is false because sexual reproduction produces greater diversity, not asexual reproduction. The reason is also incorrect because the mechanism described (meiosis and fertilization) applies to sexual, not asexual reproduction.

Q3. Assertion (A):

The F2 generation obtained from selfing F1 plants shows both parental traits, even though F1 showed only one parental trait.

Reason (R):

The F1 plants inherited both alleles from their parents but only one was expressed due to dominance.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both the assertion and reason are true. F1 plants inherited both the tall and short alleles but only expressed tallness. In F2, self-pollination of F1 segregates these hidden recessive alleles, allowing short plants to reappear.

Unlock more Assertion-Reason questions in Premium — unlock →

Case-Based Questions (CBQ) for Heredity

Case Study:

A farmer observed that in his field of chickpeas, some plants produced round seeds while others produced wrinkled seeds. He selected a pure-breeding round seed plant and cross-pollinated it with a pure-breeding wrinkled seed plant. All the seeds produced from this cross were round. When he planted these seeds and allowed them to self-pollinate, he found that 75% of the resulting plants produced round seeds while 25% produced wrinkled seeds. He also noticed that some of the round-seeded plants from the F2 generation, when self-pollinated, produced only round seeds, while others produced both round and wrinkled seeds in the same 3:1 ratio.

Q1. What does the appearance of only round seeds in the F1 generation indicate?

  • A. Both traits blend equally in the offspring
  • B. The round seed trait is dominant over the wrinkled seed trait ✓
  • C. Wrinkled seeds are completely eliminated in F1
  • D. Environmental factors determine seed shape

Answer: B

Q2. If R represents round seeds and r represents wrinkled seeds, what is the genotype of the F1 plants?

  • A. RR
  • B. rr
  • C. Rr ✓
  • D. RRrr

Answer: C

Q3. Why do some F2 round-seeded plants produce only round seeds in the next generation?

  • A. They are heterozygous (Rr)
  • B. They are homozygous dominant (RR) ✓
  • C. They have lost the recessive allele
  • D. Environmental conditions have changed

Answer: B

Q4. What is the expected ratio of pure-breeding to hybrid round-seeded plants among all round-seeded plants in the F2 generation?

  • A. 1:2 ✓
  • B. 1:3
  • C. 2:1
  • D. 3:1

Answer: A

Unlock more case studies in Premium — unlock →

Previous Year Questions (PYQ) for Heredity

Real CBSE board exam questions from past papers — free to attempt.

CBSE 2026 · Set 31/4/1 · 1 mark

Identify the traits that cannot be transmitted from parents to the off-springs: (i) Swimming technique (ii) Blood group (iii) Eye colour (iv) Dancing skills

  • (i) and (ii)
  • (i) and (iv)
  • (iii) and (iv)
  • (ii) and (iii)

CBSE 2026 · Set 31/4/1 · 1 mark

Two pea plants, one with round green seeds (RRyy) and another with wrinkled yellow (rrYY) seeds were crossed with each other which produced F1 progeny that have only round yellow (RrYy) seeds. When F1 plants are self-pollinated, the F2 progeny will have which new combination of characters, as compared to the parents: (i) Round, yellow (ii) Round, green (iii) Wrinkled, yellow (iv) Wrinkled, green

  • (i) and (ii)
  • (i) and (iv)
  • (ii) and (iv)
  • (i) and (iii)

CBSE 2026 · Set 31/4/1 · 3 marks

Study the given table and answer the following questions: Parents: A pea plant with red flowers was crossed with a pea plant with white flowers. F1: 30 seeds were sown and all bore plants with red flowers. F2: 44 seeds were sown, 33 plants bore red flowers and 11 plants bore white flowers. (a) Depict the information given in the table in the form of a cross. (b) Deduce the F2 phenotypic ratio/F2 ratio showing external look as well as genotypic/genetic makeup, ratio.

CBSE 2026 · Set 31/4/2 · 3 marks

Study the given table and answer the following questions: A pea plant with red flowers was crossed with a pea plant with white flowers. 30 seeds were sown and all bore plants with red flowers. 44 seeds were sown, 33 plants bore red flowers and 11 plants bore white flowers. (a) Depict the information given in the table in the form of a cross. (b) Deduce the F2 phenotypic ratio/F2 ratio showing external look as well as genotypic /genetic makeup, ratio.

CBSE 2026 · Set 31/4/3 · 1 mark

Two pea plants, one with round green seeds (RRyy) and another with wrinkled yellow (rrYY) seeds were crossed with each other which produced F1 progeny that have only round yellow (RrYy) seeds. When F1 plants are self-pollinated, the F2 progeny will have which new combination of characters, as compared to the parents: (i) Round, yellow (ii) Round, green (iii) Wrinkled, yellow (iv) Wrinkled, green. Correct options are:

  • (i) and (ii)
  • (i) and (iv)
  • (ii) and (iv)
  • (i) and (iii)

CBSE 2026 · Set 31/4/3 · 1 mark

Assertion (A): Variations are not often seen in off-springs produced by asexual reproduction. Reason (R): DNA molecule generated by replication is not similar to the original DNA.

  • Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
  • Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
  • Assertion (A) is true, but Reason (R) is false.
  • Assertion (A) is false, but Reason (R) is true.

CBSE 2026 · Set 31/4/3 · 3 marks

Use the following information for Pea plants: Dominant allele for tall plants – T, Recessive allele for short plants – t, Dominant allele for purple flowers – P, Recessive allele for white flowers – p. (a) Perform a cross between pure plant dominant for both the traits with another plant which is recessive for both the traits and work out the progeny upto F2 generation. (b) What is the probability of obtaining tall plants with white flowers in F2 progeny?

CBSE 2026 · Set 31/5/1 · 1 mark

When a human egg is fertilized by a sperm having 'Y' chromosome, the zygote has the following combination of chromosomes:

  • 44 + XX
  • 22 + XX
  • 44 + XY
  • 22 + XY

CBSE 2026 · Set 31/5/1 · 1 mark

Assertion (A): Bacteria that can withstand heat have better chances of survival in a heat wave. Reason (R): Accumulation of variations in a species increases the chances of its survival in changing environment.

  • Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
  • Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
  • Assertion (A) is true, but Reason (R) is false.
  • Assertion (A) is false, but Reason (R) is true.

CBSE 2026 · Set 31/5/1 · 3 marks

Based on Mendel's experiment show the cross between round, yellow seeds of pea plant (RRYY) with wrinkled and green seeds of pea plant (rryy) upto F2 generation. Write the number of seeds/plants with following genetic (Genotype) makeup in F2 generation: (i) RRYY (ii) rryy

CBSE 2026 · Set 31/5/2 · 1 mark

When a human egg is fertilized by a sperm having 'Y' chromosome, the zygote has the following combination of chromosomes :

  • 44 + XX
  • 22 + XX
  • 44 + XY
  • 22 + XY

CBSE 2026 · Set 31/5/3 · 3 marks

(a) Show a cross between a pea plant having round & green seed with a pea plant having wrinkled and yellow seeds upto F2 generation. (b) Also write down results of F2 generation.

See all 78 board PYQs for this chapter — open PYQ tab →

Class 10 Science Formula Sheet

Every formula and chemical equation from the 10 chapters on one page, checked against the NCERT textbook and the CBSE 2026-27 syllabus. Free PDF download.

View the formula sheet →

Class 10 Science Previous Year Questions, Chapter-wise

All 875 CBSE board questions from 2022–2026, sorted by chapter and question type. Free PDF download.

See all past board questions →

Next chapterLight — Reflection and Refraction →
Found this useful? Share it with your class group.

You've seen the free preview. Open the full interactive study kit — mind map, instant-graded quiz, progress tracking — free, no sign-in required.

Open Full Study Kit — Free →

Premium unlocks the full Question Bank, Daily Practice Papers, Chapter Quiz and Full Mock Papers for every chapter.