April 10, 2026
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Chapter 9: Heredity and Evolution
Complete NCERT Solutions (Intext + Exercise) | CBSE Class 10 Science | Questions & Marks
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Class 10 (CBSE) - SYLLABUS
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Intext Questions (Page 143)
12 marks
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?
Answer: Trait B (60%) is likely to have arisen earlier because it has had more time to spread through the population via asexual reproduction.
22 marks
How does the creation of variations in a species promote survival?
Answer: Variations help individuals adapt to changing environments. Those with advantageous variations survive and reproduce, increasing the species' overall survival.
Intext Questions (Page 147)
33 marks
How do Mendel's experiments show that traits may be dominant or recessive?
Answer: In crosses between pure tall and pure dwarf pea plants, the F₁ generation were all tall. This shows tallness is dominant. In F₂, both tall and dwarf appeared in 3:1 ratio, confirming dwarfness is recessive.
43 marks
How do Mendel's experiments show that traits are inherited independently?
Answer: Mendel's dihybrid cross (e.g., seed shape and colour) showed that traits are inherited independently of each other, as the F₂ ratio was 9:3:3:1, not the parental combination only.
52 marks
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?
Answer: No. Without knowing the man's genotype (AA or AO), we cannot determine dominance definitively. If the man is AO, O can appear in offspring.
Intext Questions (Page 150)
63 marks
How is the sex of a child determined in human beings?
Answer: Females have XX chromosomes; males have XY. The father contributes either X or Y sperm. If X sperm fertilizes the egg (X), girl (XX) is produced. If Y sperm fertilizes, boy (XY) is produced.
NCERT Exercise Questions (Page 156)
Ex 11 mark
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 (a) TTWW (b) TTww (c) TtWW (d) TtWw.
Answer: (c) TtWW – tall (Tt) and violet (WW).
Ex 21 mark
An example of homologous organs is (a) our arm and a dog's foreleg (b) our teeth and an elephant's tusks (c) potato and runners of grass (d) all of the above.
Answer: (d) all of the above.
Ex 32 marks
In evolutionary terms, can we say which among bacteria, spiders, fish and chimpanzees have a 'better' body design? Why or why not?
Answer: No. Each is adapted to its environment. Simpler designs (bacteria) are also highly successful. 'Better' depends on survival in a given niche, not complexity.
Ex 42 marks
A basket of vegetables contains carrot, potato, radish and tomato. Which of them represent the homologous structures?
Answer: Carrot, radish, and potato are modified roots/stems, but carrot and radish are homologous (modified roots), while potato is a modified stem (homologous to stem).
Ex 52 marks
What are the different ways in which individuals with a particular trait may increase in a population?
Answer: (i) Natural selection – trait provides survival advantage. (ii) Genetic drift – random change in gene frequency. (iii) Migration – individuals with trait move into population.
Ex 63 marks
Why are human beings who look so different from each other in terms of size, colour and looks said to belong to the same species?
Answer: They can interbreed and produce fertile offspring, which is the biological definition of a species. Physical differences are variations within the species.
Ex 72 marks
In evolutionary terms, we have more in common with (a) a Chinese school-boy (b) a chimpanzee (c) a spider (d) a bacterium.
Answer: (a) a Chinese school-boy – same species, most recent common ancestor.
Ex 82 marks
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?
Answer: Not definitively. Both parents having light eyes could mean the trait is recessive (both homozygous) or dominant (if both are heterozygous with light dominant). Without more data, we cannot conclude.
Ex 93 marks
How are the areas of study – evolution and classification – interlinked?
Answer: Classification groups organisms based on similarities, which reflect evolutionary relationships. Evolutionary history explains why certain groups share characteristics.
Ex 102 marks
Explain the terms analogous and homologous organs with examples.
Answer: Homologous organs have same basic structure but different functions (e.g., forelimbs of humans and whales). Analogous organs have different structures but similar functions (e.g., wings of birds and insects).
Ex 113 marks
Outline a project which aims to find the dominant coat colour in dogs.
Answer: Cross pure-breeding dogs of two coat colours (e.g., black × white). F₁ offspring all show one colour – that is dominant. Interbreed F₁ to get F₂; ratio confirms dominance.
Ex 122 marks
Explain the importance of fossils in deciding evolutionary relationships.
Answer: Fossils provide direct evidence of past organisms, showing transitional forms and timelines. They help trace evolutionary lineages and understand common ancestry.
Ex 132 marks
What evidence do we have for the origin of life from inanimate matter?
Answer: Miller-Urey experiment showed that organic molecules (amino acids) could form from inorganic precursors under simulated early Earth conditions (sparks, gases).
Ex 143 marks
Explain how sexual reproduction gives rise to more viable variations than asexual reproduction. How does this affect the evolution of those organisms that reproduce sexually?
Answer: Sexual reproduction combines DNA from two parents, producing more genetic variation. This increases adaptability and speeds up evolution, allowing sexually reproducing organisms to better survive environmental changes.
Ex 152 marks
How is the equal genetic contribution of male and female parents ensured in the progeny?
Answer: Each parent contributes one set of chromosomes to the zygote through gametes (sperm and egg), each with half the genetic material. Offspring receive 50% from each parent.
Ex 162 marks
Only variations that confer an advantage to an individual organism will survive in a population. Do you agree with this statement? Why or why not?
Answer: Not always. Some neutral variations persist by chance (genetic drift). Also, a variation may become advantageous later when the environment changes.
Additional Important Questions
A12 marks
What is a gene? Where are genes located?
Answer: A gene is a unit of heredity that controls a specific trait. Genes are located on chromosomes inside the nucleus.
A22 marks
Define evolution. What are the key factors that drive evolution?
Answer: Evolution is the gradual change in heritable characteristics of populations over generations. Factors: natural selection, genetic drift, mutation, and gene flow.
A32 marks
Who proposed the theory of natural selection?
Answer: Charles Darwin and Alfred Russel Wallace.
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