Heredity — Class 10 Biology NCERT Solutions (Free)
Free step-by-step NCERT solutions for Class 10 Biology chapter "Heredity" — 7 important questions with detailed answers for CBSE board exam preparation.
TL;DR: Free step-by-step NCERT solutions for Class 10 Biology chapter "Heredity" — 7 important questions with detailed answers for CBSE board exam preparatio…
Written & reviewed by the Syllab.in Academic Team (CBSE/NCERT subject experts) · Updated
NCERT Solutions for Class 10 Biology — Heredity. Step-by-step answers to all 7 textbook questions from this chapter, written for CBSE board preparation and free to use.
Heredity — All 7 Questions Solved
Q1. What is heredity? Define the terms gene, allele, and trait.
Heredity is the passing of characteristics from parents to offspring through genes. It is the basis of inheritance.
Key definitions:
1. Gene:
- A unit of heredity
- A segment of DNA that codes for a specific protein
- Responsible for a particular trait
- Located on chromosomes
- Humans have about 20,000-25,000 genes
2. Allele:
- Different forms or variants of a gene
- Example: For eye color gene, alleles might be for brown eyes or blue eyes
- Each organism has two alleles for each gene (except in gametes which have one)
- Dominant allele: expressed when present; usually represented by capital letter (A)
- Recessive allele: expressed only when homozygous; usually represented by lowercase letter (a)
3. Trait (character):
- Observable characteristic of an organism
- Examples: height, eye color, hair color, skin color
- Determined by genotype (genetic makeup) and environment
- Genotype: actual alleles present (e.g., AA, Aa, aa)
- Phenotype: observable appearance (e.g., brown eyes, tall)
Example: For pea plant height
- Gene: height gene
- Alleles: T (tall) and t (short)
- Trait: height of plant
- Genotype: TT (homozygous tall), Tt (heterozygous tall), tt (homozygous short)
- Phenotype: tall or short based on genotype
Q2. Who was Gregor Mendel? What are Mendel's laws of inheritance?
Gregor Mendel (1822-1884):
- Augustinian friar and scientist
- Conducted experiments with pea plants in monastery garden
- Discovered fundamental laws of inheritance
- Work was ignored in his lifetime; rediscovered in 1900
- Now called the "Father of Genetics"
Mendel's Laws of Inheritance:
1. Law of Segregation:
- During gamete formation, alleles of a gene separate so each gamete receives only one allele
- During fertilization, pairs of alleles are restored
- Each offspring inherits one allele from each parent
- Example: Tall (T) × Short (t) → Offspring Tt (appears tall because T is dominant)
- In next generation, Tt × Tt → 1 TT : 2 Tt : 1 tt (3 tall : 1 short phenotypic ratio)
2. Law of Independent Assortment:
- Inheritance of one trait does not affect inheritance of another trait
- Alleles of different genes assort independently during gamete formation
- Applies when genes are on different chromosomes
- Explains why offspring can have new combinations of traits
- Example: Tall plant with green seeds can produce short plant with yellow seeds
3. Law of Dominance (implied):
- Dominant alleles mask recessive alleles in heterozygous organisms
- Dominant trait appears in F1 generation even if crossed with recessive
- Recessive trait reappears in F2 generation
- Example: Tall (T) is dominant over short (t); Tt appears tall
Mendel's experiments proved inheritance follows mathematical patterns.
Q3. What is a monohybrid cross? Explain Mendel's monohybrid cross with a Punnett square.
A monohybrid cross is a cross between two organisms that differ in only one trait, controlled by one gene with two alleles.
Example: Cross between tall pea plant (TT) and short pea plant (tt)
Setup:
- TT plant produces gametes: all T
- tt plant produces gametes: all t
- Cross: TT × tt
Punnett square:
Gametes of tt parent
t t
T | Tt | Tt |
T | Tt | Tt |
Gametes of TT parent
F1 Generation Results:
- Genotype: All Tt (100% heterozygous)
- Phenotype: All tall (100%)
- Ratio: 1:1 genotypic (Tt:Tt is same), phenotypic all tall
Mendel's explanation:
- Tall (T) is dominant
- Short (t) is recessive
- Even though t allele is present in Tt, tall phenotype appears
- Tall trait segregates from short trait in parents but reappears in next generation
F2 Generation (if F1 self-fertilizes: Tt × Tt):
Gametes of Tt parent
T t
T | TT | Tt |
t | Tt | tt |
Gametes of Tt parent
F2 Results:
- Genotype: 1 TT : 2 Tt : 1 tt (1:2:1 ratio)
- Phenotype: 3 tall : 1 short (3:1 ratio)
- Short trait reappears in F2
This 3:1 ratio proved Mendel's law of segregation. The recessive allele is hidden in F1 but expressed when homozygous in F2.
Q4. What is sex determination? Explain how sex is determined in humans.
Sex determination is the biological process that determines whether an organism develops as male or female.
In humans, sex is determined by chromosomes:
Chromosome system:
- Humans have 23 pairs of chromosomes (46 total)
- 22 pairs are autosomes (non-sex chromosomes)
- 1 pair is sex chromosomes: XX or XY
Sex chromosomes:
- Females: XX (two X chromosomes)
- Each parent contributes one X
- Homozygous for X chromosome
- Males: XY (one X and one Y chromosome)
- Mother contributes X
- Father contributes Y
- Heterozygous for sex chromosome
Sex determination during reproduction:
Parents:
- Father: XY (produces two types of sperm: 50% X, 50% Y)
- Mother: XX (produces only X eggs)
Possible fertilizations:
- Sperm with X + Egg with X = XX (female baby)
- Sperm with Y + Egg with X = XY (male baby)
- Ratio: 1:1 (50% male, 50% female)
Why males are sometimes called heterogametic:
- Father's sperm determines sex of baby
- X sperm produces female; Y sperm produces male
- Punnett square shows 50% chance of each sex
Sex-linked traits:
- Traits controlled by genes on X chromosome
- Males (XY) need only one recessive allele to express recessive trait
- Females (XX) need two recessive alleles to express recessive trait
- Example: Color blindness is more common in males (XcY) than females
Q5. Explain the difference between dominant and recessive traits with examples.
Dominant and Recessive Traits:
Dominant trait:
- The trait that appears in the organism's phenotype when at least one dominant allele is present
- Represented by capital letter (e.g., A, T, B)
- Expressed in both homozygous (AA) and heterozygous (Aa) genotypes
- Dominant allele: stronger or masks the recessive allele
- Example: Brown eyes (B) is dominant over blue eyes (b)
- BB person: brown eyes
- Bb person: brown eyes (blue is hidden)
Recessive trait:
- The trait that appears only when two recessive alleles are present (homozygous recessive)
- Represented by lowercase letter (e.g., a, t, b)
- Expressed only in homozygous (aa) genotype
- Cannot appear in heterozygous (Aa) genotype because dominant allele masks it
- Example: Blue eyes (b) is recessive
- bb person: blue eyes
- Bb person: brown eyes (blue is hidden)
- bb person: blue eyes
Common examples in humans:
1. Eye color:
- Brown (dominant) vs. blue (recessive)
2. Hair texture:
- Curly (dominant) vs. straight (recessive)
3. Widow's peak:
- Present (dominant) vs. absent (recessive)
4. Earlobe attachment:
- Free earlobes (dominant) vs. attached earlobes (recessive)
5. Tongue rolling:
- Ability to roll (dominant) vs. inability (recessive)
6. Color blindness:
- Normal vision (dominant) vs. color blind (recessive)
- X-linked recessive; more common in males
Key point:
- Recessive trait can skip generations
- Two brown-eyed parents (BB and Bb) can have blue-eyed child (bb) if both carry b allele
- This shows the difference between genotype (genetic makeup) and phenotype (observable trait)
Q6. What is the difference between homozygous and heterozygous organisms? Give examples.
Homozygous:
- An organism having two identical alleles for a particular gene
- Both alleles are the same (either both dominant or both recessive)
- Represented as: AA (homozygous dominant) or aa (homozygous recessive)
- Homozygous organisms are called pure breeds
- Produce only one type of gamete for that gene
- Example: TT (both alleles for tallness) or tt (both alleles for shortness)
Heterozygous:
- An organism having two different alleles for a particular gene
- One dominant allele and one recessive allele
- Represented as: Aa or Bb (capital and lowercase letter)
- Heterozygous organisms are called hybrids
- Produce two types of gametes for that gene (50% with one allele, 50% with other)
- Example: Tt (one allele for tall, one for short)
Comparison:
Genotype:
- Homozygous: AA or aa
- Heterozygous: Aa
Alleles:
- Homozygous: identical
- Heterozygous: different
Gametes produced:
- Homozygous: one type only
- Heterozygous: two types
Dominant allele expression:
- Homozygous dominant (AA): dominant trait always expressed
- Homozygous recessive (aa): recessive trait always expressed
- Heterozygous (Aa): dominant trait expressed (recessive masked)
Example with flower color (R = red, r = white):
- RR: homozygous dominant; red flowers; produces only R gametes
- rr: homozygous recessive; white flowers; produces only r gametes
- Rr: heterozygous; red flowers (dominant expressed); produces R and r gametes in equal amounts
In breeding:
- Homozygous parents produce consistent offspring
- Heterozygous parents produce variable offspring
Q7. Explain how variation occurs in organisms. What are the sources of genetic variation?
Variation is the difference in traits among individuals of the same species.
Sources of genetic variation:
1. Sexual reproduction:
a) Random segregation of alleles
- During meiosis, alleles separate randomly
- Each gamete gets random combination of alleles
- Different combinations in different gametes
b) Independent assortment
- Different chromosomes assort independently
- Creates new combinations of genes
c) Random fertilization
- Any sperm can fertilize any egg
- Many possible combinations of alleles in zygote
- Result: Each offspring (except identical twins) is genetically unique
2. Mutation:
- Change in DNA sequence
- Creates new alleles
- Source of all new genetic variation
- Types: substitution, insertion, deletion
- Can be spontaneous or induced (radiation, chemicals)
- Most mutations are neutral or harmful; rarely beneficial
3. Gene flow (migration):
- Movement of genes between populations
- When individuals migrate and breed in new population
- Introduces new alleles to population
4. Recombination (crossing over):
- Exchange of genetic material between homologous chromosomes during meiosis
- Creates new combinations of alleles on same chromosome
- Increases variation in gametes
Environmental factors:
- Also cause phenotypic variation (non-genetic)
- Example: Same plant genotype grows taller with more sunlight
- Does not change genetic makeup
- Called phenotypic variation
Types of variation in population:
- Continuous variation: height, skin color (polygenic, influenced by many genes and environment)
- Discontinuous variation: blood type, ability to roll tongue (single gene, not influenced by environment)
Importance of variation:
- Allows natural selection and evolution
- Provides raw material for adaptation
- Increases population fitness and survival chances
- Different organisms have different traits to fill different ecological niches
Heredity — Practice MCQs with Answers
Attempt these 20 multiple-choice questions after working through the solutions above. Each carries the correct option and the reasoning behind it, so a wrong answer tells you which idea to revisit.
MCQ 1. The process by which characteristics are transmitted from parents to offspring is called:
- A. Variation
- B. Adaptation
- C. Heredity
- D. Evolution
Answer: C. Heredity — Heredity is the biological process of transmitting traits and characteristics from one generation to the next.
MCQ 2. Which of the following is responsible for carrying genetic information from one generation to the next?
- A. Proteins
- B. Lipids
- C. DNA
- D. Carbohydrates
Answer: C. DNA — Deoxyribonucleic acid (DNA) is the primary genetic material that stores and transmits hereditary information in most organisms.
MCQ 3. In a monohybrid cross, what is the phenotypic ratio of tall to dwarf plants in the F2 generation when a pure tall plant is crossed with a pure dwarf plant?
- A. 1:2:1
- B. 3:1
- C. 1:1
- D. 9:3:3:1
Answer: B. 3:1 — When F1 generation (all tall, Tt) is self-pollinated, the F2 generation shows a phenotypic ratio of 3 tall: 1 dwarf.
MCQ 4. Which of Mendel's laws states that factors (alleles) for different traits are inherited independently of each other?
- A. Law of Dominance
- B. Law of Segregation
- C. Law of Independent Assortment
- D. Law of Purity of Gametes
Answer: C. Law of Independent Assortment — The Law of Independent Assortment states that alleles for different genes segregate independently of one another during gamete formation.
MCQ 5. What are the alternative forms of a gene called?
- A. Chromosomes
- B. Loci
- C. Alleles
- D. Traits
Answer: C. Alleles — Alleles are different versions or alternative forms of a gene that occupy the same locus on homologous chromosomes.
MCQ 6. If an organism has a genotype 'Tt', what does 'T' represent?
- A. A recessive allele
- B. A dominant allele
- C. A homozygous trait
- D. A hybrid trait
Answer: B. A dominant allele — In genetics, uppercase letters conventionally represent dominant alleles, which express their trait even when only one copy is present.
MCQ 7. A cross between a pure tall pea plant (TT) and a pure dwarf pea plant (tt) resulted in all tall plants in the F1 generation. This demonstrates:
- A. Law of Segregation
- B. Law of Independent Assortment
- C. Law of Dominance
- D. Incomplete dominance
Answer: C. Law of Dominance — The appearance of only the tall trait in the F1 generation signifies that the allele for tallness is dominant over the allele for dwarfness.
MCQ 8. In humans, the presence of a 'Y' chromosome in a zygote typically indicates the development of:
- A. A female
- B. A male
- C. Twins
- D. An intersex individual
Answer: B. A male — The presence of a Y chromosome (XY sex chromosomes) is the genetic determinant for male sex in humans.
MCQ 9. Which of the following is an example of an inherited trait in humans?
- A. Learning to ride a bicycle
- B. Eye colour
- C. A scar from an injury
- D. Developing muscles through exercise
Answer: B. Eye colour — Eye colour is determined by genes passed down from parents to offspring, making it an inherited trait.
MCQ 10. Where are genes primarily located in a eukaryotic cell?
- A. Cytoplasm
- B. Mitochondria
- C. Nucleus
- D. Ribosomes
Answer: C. Nucleus — In eukaryotic cells, the majority of genetic material, including genes, is organized into chromosomes found within the nucleus.
MCQ 11. The visible characteristic expressed by an organism is called its:
- A. Genotype
- B. Phenotype
- C. Allele
- D. Chromosome
Answer: B. Phenotype — Phenotype refers to the observable physical or biochemical characteristics of an organism, which result from the expression of its genes.
MCQ 12. A human female has the sex chromosomes:
- A. XY
- B. XX
- C. XO
- D. YY
Answer: B. XX — Human females typically possess two X chromosomes (XX) in their somatic cells.
MCQ 13. Which of these is a major source of variation in a sexually reproducing population?
- A. Mitosis
- B. Binary fission
- C. Meiosis
- D. Budding
Answer: C. Meiosis — Meiosis generates genetic variation through processes like crossing over and independent assortment of chromosomes during gamete formation.
MCQ 14. Gregor Mendel is known as the 'Father of Genetics' for his pioneering work with:
- A. Fruit flies
- B. Pea plants
- C. Bacteria
- D. Mice
Answer: B. Pea plants — Gregor Mendel conducted his groundbreaking experiments on heredity using garden pea plants (Pisum sativum).
MCQ 15. If the F1 generation of a monohybrid cross between pure tall and pure dwarf plants is self-pollinated, what percentage of F2 offspring will be homozygous dwarf?
- A. 25%
- B. 50%
- C. 75%
- D. 100%
Answer: A. 25% — In the F2 generation (Tt x Tt), the genotypic ratio is 1 TT: 2 Tt: 1 tt. Therefore, 25% of the offspring will be homozygous dwarf (tt).
MCQ 16. Which statement best describes the hierarchical relationship between DNA, genes, and chromosomes?
- A. Genes are made of chromosomes, which are made of DNA.
- B. Chromosomes are made of genes, which are made of DNA.
- C. DNA makes up genes, and genes are located on chromosomes.
- D. DNA, genes, and chromosomes are all distinct and unrelated structures.
Answer: C. DNA makes up genes, and genes are located on chromosomes. — DNA is the chemical substance that forms genes, and multiple genes are segments arranged linearly along the length of chromosomes.
MCQ 17. The genotypic ratio of the F2 generation in a monohybrid cross is:
- A. 3:1
- B. 1:2:1
- C. 9:3:3:1
- D. 1:1
Answer: B. 1:2:1 — When F1 heterozygotes (e.g., Tt x Tt) are crossed, the F2 generation typically displays a genotypic ratio of 1 homozygous dominant: 2 heterozygous: 1 homozygous recessive.
MCQ 18. What term describes a trait that is expressed only when two identical alleles are present?
- A. Dominant
- B. Recessive
- C. Codominant
- D. Incomplete
Answer: B. Recessive — A recessive trait is only expressed in an individual who inherits two copies of the recessive allele, i.e., is homozygous recessive.
MCQ 19. If a child inherits an X chromosome from the father and an X chromosome from the mother, the child will be:
- A. Male
- B. Female
- C. Either male or female
- D. Intersex
Answer: B. Female — In humans, the combination of two X chromosomes (XX) results in a female child.
MCQ 20. Why are variations beneficial to a species rather than necessarily to an individual?
- A. They always improve an individual's survival chances.
- B. They allow individuals to adapt quickly to changing environments.
- C. They help the species to survive drastic environmental changes over time.
- D. They prevent diseases in the current generation.
Answer: C. They help the species to survive drastic environmental changes over time. — Variations provide genetic diversity within a species, increasing the likelihood that some individuals will possess advantageous traits to survive and reproduce under changing environmental conditions, thus ensuring species survival.
How to Use These Solutions
Attempt each question yourself first and only then compare with the worked answer. Marks in the CBSE board exam are awarded for the METHOD as much as the final result, so reproduce the steps rather than memorising the last line. Where a solution states a law, a formula or a definition, learn that wording — examiners look for it.
Related practice for this chapter: chapter MCQs, previous-year questions, revision notes and sample papers.