NCERT Solutions Class 10 Science Chapter 8: Heredity

Navigating through the CBSE Class 10 Science curriculum requires an in-depth understanding of Mendelian genetics, monohybrid and dihybrid crosses, phenotypic and genotypic ratios, the law of segregation, the law of independent assortment, sex determination in humans, and the accumulation of genetic variations. Chapter 8 of Class 10 Biology, “Heredity”, explores how biological characteristics and traits are faithfully transmitted from parents to offspring. It investigates how Gregor Johann Mendel uncovered the statistical rules of inheritance using garden pea plants (Pisum sativum); explains why dominant alleles mask recessive alleles in heterozygous conditions; and details the chromosomal basis of sex determination in human beings (XX female vs. XY male). To help students master every aspect of this high-weightage chapter, this comprehensive solutions guide offers textbook-accurate, highly structured, and pedagogically sound responses strictly aligned with the latest CBSE evaluation standards.

Every question presented in the official NCERT textbook—ranging from all in-text question sets (Pages 129 and 134) to the complete chapter-end exercises (Questions 1 to 4 on Pages 136–137)—has been solved with exhaustive detail. Short-answer conceptual responses follow the official 30–40 word limit, while 3-mark and 5-mark genetic problems use structured Punnett squares and cross diagrams. Key scoring terms, official CBSE board year tags, and comparative charts have been highlighted to ensure students secure maximum marks in their CBSE Board Examinations.

Master Formula & Concept Summary Tables

1. Seven Contrasting Pairs of Traits in Pea Plants (Pisum sativum)

Plant Character / Trait	Dominant Trait (Expressed in F₁)	Recessive Trait (Suppressed in F₁)
1. Stem Height	Tall (`T`)	Dwarf / Short (`t`)
2. Flower Colour	Violet / Purple (`W`)	White (`w`)
3. Flower Position	Axial (`A`)	Terminal (`a`)
4. Pod Shape	Inflated / Full (`I`)	Constricted (`i`)
5. Pod Colour	Green (`G`)	Yellow (`g`)
6. Seed Shape	Round (`R`)	Wrinkled (`r`)
7. Seed Colour	Yellow (`Y`)	Green (`y`)

2. Summary of Mendelian Genetic Crosses & Ratios

Type of Cross	Parental Cross (P)	F₁ Generation	F₂ Phenotypic Ratio	F₂ Genotypic Ratio	Mendel’s Law
Monohybrid Cross (Height)	Pure Tall (`TT`) × Pure Dwarf (`tt`)	100% Tall (`Tt`)	3 : 1 (3 Tall : 1 Dwarf)	1 : 2 : 1 (1 TT : 2 Tt : 1 tt)	Law of Segregation
Dihybrid Cross (Shape & Colour)	Round Yellow (`RRYY`) × Wrinkled Green (`rryy`)	100% Round Yellow (`RrYy`)	9 : 3 : 3 : 1	Complex (16 combinations)	Law of Independent Assortment

NCERT In-Text Questions: Set 1 (Page No. 129)

Question 1 If a trait A exists in 10% of a population of an asexually reproducing species and a trait B exists in 60% of the same population, which trait is likely to have arisen earlier? [CBSE 2024, 2022]

Answer: Trait B is likely to have arisen earlier in the population:

In asexual reproduction, existing traits are replicated and passed from parent to offspring across successive generations with minimal variation.
Since Trait B is present in 60% of the population, it has been replicated and accumulated over a much longer period.
Trait A (present in only 10%) is a newly arisen genetic variation that has appeared more recently.

Question 2 How does the creation of variations in a species promote survival? [CBSE 2024, 2020]

Answer: Environmental conditions (temperature, moisture, food supply, pathogens) can change drastically:

If all organisms in a population were genetically identical clones, an extreme environmental change could kill the entire population.
The presence of genetic variations ensures that some pre-adapted individuals survive. For example, during a sudden increase in water temperature, heat-resistant bacterial variants survive and multiply, preventing the extinction of the species.

NCERT In-Text Questions: Set 2 (Page No. 134)

Question 1 How do Mendel’s experiments show that traits may be dominant or recessive? [CBSE 2024, 2023, 2020]

Answer: Mendel demonstrated dominance and recessiveness through his Monohybrid Cross:

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MENDELIAN MONOHYBRID CROSS (INHERITANCE OF HEIGHT):
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Parents (P):           Pure Tall (TT)   ×   Pure Dwarf (tt)
Gametes:                     (T)                  (t)
F₁ Generation:                     All Tall (Tt)

Self-Pollination (F₁ × F₁):        Tt   ×   Tt
Gametes:                         T , t  ×  T , t

F₂ Punnett Square:
             ┌───────────┬───────────┐
             │     T     │     t     │
       ┌─────┼───────────┼───────────┤
       │  T  │  TT (Tall)│  Tt (Tall)│
       ├─────┼───────────┼───────────┤
       │  t  │  Tt (Tall)│ tt (Dwarf)│
       └─────┴───────────┴───────────┘

F₂ Phenotypic Ratio :  3 Tall : 1 Dwarf (3:1)
F₂ Genotypic Ratio  :  1 TT : 2 Tt : 1 tt (1:2:1)
================================================================================

Deduction:
1. In the F₁ generation, all plants were tall (Tt). The dwarf trait disappeared, proving Tallness (T) is Dominant.
2. In the F₂ generation, dwarfness reappeared in 25% of plants (tt). This proved that the recessive dwarf trait was inherited in F₁ but remained hidden in the presence of the dominant allele.

Question 2 How do Mendel’s experiments show that traits are inherited independently? [CBSE 2024, 2022]

Answer: Mendel demonstrated independent inheritance using a Dihybrid Cross (Seed Shape and Seed Colour):

When pure breeding Round Yellow (RRYY) plants were crossed with Wrinkled Green (rryy) plants, the F₁ generation produced all Round Yellow (RrYy) seeds.
Upon self-pollinating F₁ plants, the F₂ generation produced four distinct phenotypes in a 9 : 3 : 3 : 1 ratio:
- Round Yellow (9) — Parental type
- Round Green (3) — New Recombinant type
- Wrinkled Yellow (3) — New Recombinant type
- Wrinkled Green (1) — Parental type
The emergence of new combinations (Round Green and Wrinkled Yellow) proves that the gene for seed shape assorts completely independently of the gene for seed colour.

Question 3 A man with blood group A marries a woman with blood group O and their daughter has blood group O. Is this information enough to tell you which of the traits — blood group A or O — is dominant? Why or why not? [CBSE 2024, 2020]

Answer: No, this information is NOT sufficient to determine which blood group is dominant:

If A is dominant, the father can be heterozygous (I^A i) and the mother homozygous recessive (i i). Their child can inherit allele i from both parents to have blood group O (i i).
If O were dominant, the father could be homozygous (i i) and the mother (I^O I^O or I^O i), and the daughter could still have blood group O.
To confirm dominance, we need data from multiple progeny and generations. A single child’s phenotype is statistically insufficient.

Question 4 How is the sex of the child determined in human beings? [CBSE 2024, 2023, 2020]

Answer: In human beings, sex is determined genetically by the 23rd pair of chromosomes (Sex Chromosomes):

Females possess two identical X chromosomes: XX. All maternal eggs carry only an X chromosome.
Males possess one X and one Y chromosome: XY. Paternal sperms are of two types: 50% carry X and 50% carry Y.

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SEX DETERMINATION CROSS IN HUMANS:
--------------------------------------------------------------------------------
Parents:                Mother (XX)       ×       Father (XY)
Gametes:                (X)   (X)                 (X)   (Y)

Cross:
                   ┌───────────────┬───────────────┐
                   │    Sperm X    │    Sperm Y    │
       ┌───────────┼───────────────┼───────────────┤
       │   Egg X   │   XX (Girl)   │   XY (Boy)    │
       ├───────────┼───────────────┼───────────────┤
       │   Egg X   │   XX (Girl)   │   XY (Boy)    │
       └───────────┴───────────────┴───────────────┘

Probability: 50% Female (XX) : 50% Male (XY)
================================================================================

Conclusion: If an egg is fertilized by an X-carrying sperm, the child is a girl (XX). If fertilized by a Y-carrying sperm, the child is a boy (XY). Hen

NCERT Chapter-End Exercises (Page No. 133)

Question 1 A Mendelian experiment consisted of breeding tall pea plants bearing violet flowers with short pea plants bearing white flowers. The progeny all bore violet flowers, but almost half of them were short. This suggests that the genetic make-up of the tall parent can be depicted as: [CBSE 2024, 2022] (a) TTWW (b) TTww (c) TtWW (d) TtWw

Answer: (c) TtWW

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STEP-BY-STEP GENETIC DEDUCTION:
--------------------------------------------------------------------------------
1. Flower Colour Trait (Violet vs. White):
   • Short parent has white flowers, which is a recessive trait (ww).
   • ALL offspring bore violet flowers (100% violet).
   • For all progeny to express the dominant violet phenotype, the tall parent 
     must supply a dominant 'W' allele to every single offspring.
   • Therefore, the tall parent must be HOMOZYGOUS dominant for flower colour: WW.

2. Plant Height Trait (Tall vs. Short):
   • Short parent is homozygous recessive (tt) and provides only 't' alleles.
   • The progeny resulted in ALMOST HALF TALL and ALMOST HALF SHORT (1:1 ratio).
   • For short (tt) offspring to appear, the tall parent must carry and pass on a 
     recessive 't' allele along with a dominant 'T' allele.
   • Therefore, the tall parent must be HETEROZYGOUS for height: Tt.

3. Combined Genotype of the Tall Parent:
   Height (Tt) + Flower Colour (WW) ===> TtWW
================================================================================

Question 2 A study found that children with light-coloured eyes are likely to have parents with light-coloured eyes. On this basis, can we say anything about whether the light eye colour trait is dominant or recessive? Why or why not? [CBSE 2024, 2020]

Answer: No, on this basis alone, we CANNOT determine whether the light eye colour trait is dominant or recessive.

Scientific Reason: The given study only shows a correlation (that light-eyed parents tend to have light-eyed children). A trait can be consistently passed down in a family line regardless of whether it is dominant or recessive:
- If Light Eye Colour is Recessive (ee): Both parents would have genotype ee and can only pass e alleles to their children, resulting in 100% light-eyed offspring (ee), matching the observation.
- If Light Eye Colour is Dominant (E): Both parents could have homozygous dominant (EE) or heterozygous (Ee) genotypes and still produce light-eyed children.
Conclusion: To scientifically determine dominance, we require data from crosses between contrasting eye colours (e.g., dark eyes crossed with light eyes) across multiple generations (F₁ and F₂ ratios).

Question 3 Outline a project which aims to find the dominant coat colour in dogs. [CBSE 2024, 2023]

Answer: To determine the dominant coat colour in dogs, we can design a Mendelian breeding experiment across two generations:

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EXPERIMENTAL PROJECT DESIGN: DETERMINING DOMINANT COAT COLOUR IN DOGS
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1. Selection of Pure-Breeding Parents (P Generation):
   • Select a pure-breeding (homozygous) Black coat dog (BB) and a pure-breeding 
     homozygous White/Brown coat dog (bb).

2. First Generation Cross (F₁ Generation):
   • Cross the pure-breeding Black dog with the White/Brown dog.
   • Observe the coat colour of all puppies in the resulting F₁ litter.
   • Hypothesis: The coat colour that appears in 100% of the F₁ puppies is the 
     DOMINANT trait (e.g., if all puppies are Black, Black coat is Dominant).

3. Second Generation Cross (F₂ Generation Confirmation):
   • Inter-breed the heterozygous F₁ dogs (Bb × Bb).
   • Count and record the coat colour of all puppies in the F₂ generation.
   • If the F₂ generation yields approximately a 3 : 1 ratio (3 Black : 1 White), 
     it confirms that Black coat colour is dominant and White coat is recessive.
================================================================================

Question 4 How is the equal genetic contribution of male and female parents ensured in the progeny? [CBSE 2024, 2020]

Answer: Equal genetic contribution from both parents is ensured through the specialized process of Meiosis (Reduction Division) during gamete formation and subsequent Fertilisation:

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MECHANISM OF EQUAL GENETIC CONTRIBUTION:
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1. Diploid Somatic Cells (2n):
   Human body cells contain 23 pairs (46 chromosomes) consisting of 22 pairs of 
   autosomes and 1 pair of sex chromosomes.

2. Haploid Gamete Formation via Meiosis (n):
   During gametogenesis, meiosis halves the chromosome number:
   • Father's Sperm receives: 22 Autosomes + 1 Sex Chromosome (X or Y) = 23 chromosomes.
   • Mother's Ovum receives : 22 Autosomes + 1 Sex Chromosome (X)     = 23 chromosomes.

3. Restoration of Diploid Number (2n) at Fertilisation:
   When the haploid sperm (23 chromosomes) fuses with the haploid egg (23 chromosomes):
   23 (Paternal DNA) + 23 (Maternal DNA) = 46 Chromosomes (23 Pairs in Zygote)
================================================================================

Conclusion: The zygote receives exactly one set of 23 chromosomes from the father and one matching set of 23 chromosomes from the mother. Thus, each gene in the offspring exists in two copies—one maternal and one paternal—guaranteeing exact 50% genetic contribution from each parent.

Frequently Asked Questions (FAQs) – Class 10 Biology Chapter 8

Question 1: Who is known as the Father of Genetics? [CBSE 2024] Answer: Gregor Johann Mendel is universally recognized as the Father of Genetics for discovering the fundamental principles and statistical laws of inheritance through hybridization experiments on pea plants.

Question 2: Why did Mendel select the garden pea plant (Pisum sativum) for his experiments? [CBSE 2024, 2022] Answer: Mendel chose garden pea plants because:

They exhibit clear, easily distinguishable contrasting traits (e.g., tall/short, round/wrinkled seeds).
They have a short life cycle, allowing results to be analyzed across multiple generations quickly.
They are naturally self-pollinating but can be easily cross-pollinated manually.
A single cross produces a large number of seeds for statistical analysis.

Question 3: What is a gene? [CBSE 2023] Answer: A gene is a specific functional segment of DNA located on a chromosome that contains the coded instructions for synthesizing a particular protein, thereby controlling a specific hereditary trait.

Question 4: What are alleles? Give an example. [CBSE 2024] Answer: Alleles are alternative forms or variants of the same gene that occupy the identical locus on homologous chromosomes. For example, T (allele for tallness) and t (allele for dwarfness) are alleles of the plant height gene.

Question 5: What is the difference between phenotype and genotype? [CBSE 2023, 2020] Answer:

Phenotype: The observable physical appearance or visible characteristics of an organism (e.g., Tall or Dwarf).
Genotype: The underlying genetic constitution or gene combination of an organism (e.g., TT, Tt, or tt).

Question 6: State Mendel’s Law of Segregation. [CBSE 2024] Answer: The Law of Segregation (Purity of Gametes) states that the two alleles of a gene pair separate (segregate) during gamete formation, such that each gamete carries only one allele for each trait.

Question 7: State Mendel’s Law of Independent Assortment. [CBSE 2024, 2022] Answer: The Law of Independent Assortment states that when two pairs of contrasting traits are combined in a hybrid, the segregation and inheritance of one pair of traits is completely independent of the other pair of traits.

Question 8: What is a monohybrid cross? State its F₂ phenotypic and genotypic ratios. [CBSE 2024] Answer: A monohybrid cross is a genetic cross between two parents considering the inheritance of only one single pair of contrasting traits (e.g., height).

F₂ Phenotypic Ratio: 3 : 1 (3 Tall : 1 Dwarf)
F₂ Genotypic Ratio: 1 : 2 : 1 (1 TT : 2 Tt : 1 tt)

Question 9: What is a dihybrid cross? State its F₂ phenotypic ratio. [CBSE 2023, 2020] Answer: A dihybrid cross is a genetic cross between two parents considering the simultaneous inheritance of two separate pairs of contrasting traits (e.g., seed shape and seed colour).

F₂ Phenotypic Ratio: 9 : 3 : 3 : 1 (9 Round-Yellow : 3 Round-Green : 3 Wrinkled-Yellow : 1 Wrinkled-Green).

Question 10: Why are human females referred to as homogametic and males as heterogametic? [CBSE 2024] Answer: Human females are homogametic because they produce only one type of gamete (all eggs carry an X chromosome). Human males are heterogametic because they produce two distinct types of gametes (50% sperms carry an X chromosome and 50% carry a Y chromosome).

Question 11: Can a mother be held responsible for the birth of a female child? Explain scientifically. [CBSE 2024, 2020] Answer: No, absolutely not. Mothers contribute only an X chromosome in all their ova. The sex of the child depends entirely on whether the father’s fertilizing sperm carries an X chromosome (resulting in a girl, XX) or a Y chromosome (resulting in a boy, XY).

Question 12: How do environmental factors determine sex in some non-human animals? [CBSE 2023] Answer: In certain reptiles (like turtles and crocodiles), the incubation temperature of fertilized eggs during development determines the sex of the offspring. In snails, individuals can change sex depending on environmental cues.

Question 13: What are autosomes and allosomes? [CBSE 2022] Answer:

Autosomes: The 22 pairs of non-sex chromosomes that regulate general somatic body characteristics in both males and females.
Allosomes (Sex Chromosomes): The 23rd pair of chromosomes (XX in females, XY in males) that determine the biological sex of the individual.

Question 14: What is the significance of the 1:2:1 genotypic ratio in a monohybrid cross? [CBSE 2024] Answer: The 1:2:1 ratio (1 TT : 2 Tt : 1 tt) reveals that while 75% of F₂ plants appear phenotypically tall, two-thirds of those tall plants are heterozygous carriers (Tt) that harbor the hidden recessive dwarf gene.

Question 15: How does cellular DNA control the expression of traits? [CBSE 2024, 2022] Answer: DNA sequences contain the code to synthesize specific cellular enzymes. The efficiency of these enzymes determines the production of specific proteins and growth hormones (e.g., plant growth hormone for height), thereby expressing the visible physical trait.

Mastering the NCERT Solutions for Class 10 Science Chapter 8, “Heredity”, equips students with the genetic terminology, Punnett square constructions, and Mendelian laws required for the CBSE Board Examination. Review the monohybrid and dihybrid cross ratios, the human sex determination diagram, and the 15 board-level FAQs above to secure full marks in your biology evaluations.