Heredity and Evolution Exercise Solutions | KalviGavani

How to Use This Page

These are complete NCERT Solutions for CBSE Class 10 Science Chapter 9 — Heredity. For every question:

1. Attempt the answer yourself first — then compare with the solution. 2. For genetics problems, always draw the Punnett square step by step before reading the explanation. 3. Pay special attention to Key Concepts and Common Mistakes sections — these are the areas where students lose marks in board exams. 4. Use the Exam Checklist before your test to verify you have covered every important topic.

Key Concepts (Quick Recap)

• Heredity: The transmission of traits (characteristics) from parents to offspring through genes.
• Gene: A specific segment of DNA on a chromosome that codes for a particular protein/trait.
• Allele: Alternative forms of the same gene (e.g., T for tall, t for short).
• Dominant allele: Expressed in both homozygous and heterozygous conditions; represented by a capital letter (e.g., T).
• Recessive allele: Expressed only in the homozygous condition; represented by a small letter (e.g., t).
• Homozygous: Two identical alleles for a trait — TT (homozygous dominant) or tt (homozygous recessive).
• Heterozygous: Two different alleles for a trait — Tt (also called hybrid).
• Phenotype: The observable physical expression of a trait (e.g., tall or short).
• Genotype: The genetic makeup for a trait (e.g., TT, Tt, or tt).
• Monohybrid cross: A cross between two individuals differing in one trait. F2 phenotypic ratio = 3:1.
• Dihybrid cross: A cross between two individuals differing in two traits. F2 phenotypic ratio = 9:3:3:1.
• Punnett square: A grid diagram used to predict the genotypes of offspring from a cross.
• Sex chromosomes: In humans, females have XX and males have XY. The sex of the child is determined by the father's sperm (X or Y).
• Law of Segregation: During gamete formation, the two alleles of a gene separate so each gamete receives only one allele.
• Law of Independent Assortment: Alleles of different genes assort independently of one another during gamete formation (applies to genes on different chromosomes).
• Evolution: The change in the inherited characteristics of a population over successive generations through natural selection.
• Speciation: The formation of new species from existing ones, often due to geographical isolation, genetic drift, and natural selection over long periods.
• Homologous organs: Organs with similar basic structure and origin but different functions (e.g., forelimbs of human, bat, whale). Evidence of common ancestry and divergent evolution.
• Analogous organs: Organs with different structure and origin but similar functions (e.g., wings of bat and butterfly). Evidence of convergent evolution.
• Fossils: Preserved remains or traces of organisms that lived in the past. Provide direct evidence for evolution and help establish evolutionary relationships.
• Molecular phylogeny: Studying evolutionary relationships by comparing DNA, RNA, and protein sequences across species.

Intext Questions

Page 143

In asexual reproduction, a single parent produces genetically identical offspring (clones). A trait that arose earlier has had more generations to spread through the population. Since trait B is present in a much larger proportion (60%) compared to trait A (10%), trait B must have originated earlier and been passed on to more offspring over time. Trait A, being present in only 10% of the population, is a relatively recent variation.

• If the environment changes (e.g., new disease, climate shift, new predator), some individuals with favourable variations will survive while others may not.
• The surviving individuals reproduce and pass the favourable variation to the next generation.
• Over many generations, the favourable trait becomes common in the population — this is natural selection.
• Without variation, a single adverse environmental change could wipe out the entire species since all individuals would be equally vulnerable.

Variation is the raw material for natural selection and evolution. It acts as a pre-adaptation to future environmental changes.

Page 147

Cross diagram: Parent 1 (TT, tall) x Parent 2 (tt, short)

• F1 generation: All offspring were tall (genotype Tt). The short trait disappeared completely.
• Mendel then self-pollinated the F1 plants:

Cross diagram: F1 (Tt, tall) x F1 (Tt, tall)

• F2 generation: Tall and short plants appeared in a 3:1 ratio (3 tall : 1 short).

Conclusions: 1. The tall trait that appeared in F1 is dominant — it masks the expression of the short allele. 2. The short trait that disappeared in F1 but reappeared in F2 is recessive — it is expressed only when both alleles are recessive (tt). 3. The F1 plants (Tt) carried both alleles, but only the dominant one was expressed.

Cross diagram: Parent 1 (RRYY, round-yellow) x Parent 2 (rryy, wrinkled-green)

• F1 generation: All offspring were round-yellow (RrYy), confirming round and yellow are dominant.
• Mendel self-pollinated F1 plants:

Cross diagram: F1 (RrYy) x F1 (RrYy)

• F2 generation: Produced a 9:3:3:1 ratio — 9 round-yellow : 3 round-green : 3 wrinkled-yellow : 1 wrinkled-green.

Key observation: Two new combinations (round-green and wrinkled-yellow) appeared in F2 that were not present in either parent. This could only happen if the alleles for seed shape (R/r) and seed colour (Y/y) assorted independently during gamete formation.

This demonstrates the Law of Independent Assortment — alleles of different genes separate independently of one another during gamete formation.

Page 150

Explanation:

• The mother has blood group O, so her genotype must be ii (homozygous recessive).
• The father has blood group A, so his genotype is either I^A I^A (homozygous) or I^A i (heterozygous).
• The daughter has blood group O (ii). She must receive one i allele from each parent.
• Since the daughter received an i allele from the father, the father's genotype must be I^A i (heterozygous).
• The father has the A allele but passed the i allele to his daughter. The daughter did not express blood group A even though she received an A allele from... wait — she received i from father and i from mother, giving ii (blood group O).
• This confirms that blood group O (ii) is recessive — it is expressed only when both alleles are i.
• Blood group A (I^A) is dominant — it is expressed even when only one A allele is present.

Cross diagram: Father (I^A i, blood group A) x Mother (ii, blood group O)

Offspring: 50% I^A i (blood group A) : 50% ii (blood group O). This matches the observation.

Page 151

Page 155

• Female: XX sex chromosomes
• Male: XY sex chromosomes

During reproduction: 1. The mother's egg always carries an X chromosome (since she has XX). 2. The father's sperm carries either X or Y (since he has XY). 3. If an X-bearing sperm fertilises the egg → XX → female child. 4. If a Y-bearing sperm fertilises the egg → XY → male child.

Cross diagram: Father (XY) x Mother (XX)

Offspring: 50% XX (girl) : 50% XY (boy)

The sex of the child is determined by the type of sperm (X or Y) that fertilises the egg — i.e., the father determines the sex of the child.

The mother can only contribute an X chromosome (since females are XX). It is the father who contributes either X or Y, which determines the sex of the child. Therefore, the father's sperm determines whether the child is male or female. Blaming the mother is biologically wrong and socially unjust. This misconception has led to discrimination against women in some societies and must be corrected through education.

Page 156

1. Natural selection: If a trait provides a survival advantage in a given environment, individuals with that trait survive longer and produce more offspring. Over generations, the trait becomes common. 2. Genetic drift: In small populations, chance events can cause certain traits to become more or less common regardless of their survival value (e.g., a natural disaster wiping out a large portion of the population). 3. Gene flow: Migration of individuals between populations can introduce new traits or increase the frequency of existing ones. 4. Sexual selection: Traits that improve mating success (even if they reduce survival) can increase in frequency.

Page 158

• Father: Blood group A → genotype I^A i (heterozygous, not I^A I^A)
• Mother: Blood group B → genotype I^B i (heterozygous, not I^B I^B)

Cross diagram: Father (I^A i) x Mother (I^B i)

Offspring: I^A I^B (AB) : I^A i (A) : I^B i (B) : ii (O) in a 1:1:1:1 ratio. This confirms the child with blood group O is possible only if both parents are heterozygous.

Example: A bodybuilder develops large muscles through training (acquired trait), but his child will not be born with large muscles because the genes in his sperm were not changed by the training.

1. Reduced genetic variation — fewer alleles in the population. 2. Inbreeding depression — mating between closely related individuals increases the frequency of harmful recessive traits. 3. Vulnerability to diseases — less variation means less ability to fight new pathogens. 4. Risk of extinction — a single environmental change or disease could wipe out the entire population. 5. Ecological imbalance — tigers are apex predators; their loss disrupts the food chain.

Loss of tigers also means loss of biodiversity, which ultimately affects the entire ecosystem that humans depend on.

Page 158 (continued)

For example, a person who loses a finger in an accident will not have children born without that finger. Similarly, a person who learns to swim cannot pass this skill genetically to their child.

Page 158 (Speciation)

1. Geographical isolation: A population is split into two groups by a physical barrier (mountain, river, ocean). The two groups can no longer interbreed. 2. Genetic drift: In isolated populations, random changes in gene frequencies occur, especially in small populations. 3. Natural selection: Each isolated population faces different environmental conditions, so different traits are favoured in each group. 4. Accumulation of differences: Over many generations, the two populations accumulate so many genetic differences that they can no longer interbreed even if brought together. 5. Reproductive isolation: When the two populations can no longer produce fertile offspring together, they have become separate species.

Factors that increase the rate of speciation: geographical barriers, small population size, different environmental pressures, and long periods of isolation.

Chapter-End Exercise

Q1. 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) TTVV (b) TTvv (c) TtVV (d) TtVv

• All progeny bore violet flowers → the tall parent must be homozygous for violet (VV). If it were Vv, some white-flowered progeny would appear.
• About half the progeny were short → the tall parent must be heterozygous for height (Tt). If it were TT, all progeny would be tall.

Cross diagram: Tall parent (TtVV) x Short parent (ttvv)

F1: 50% TtVv (tall, violet) : 50% ttVv (short, violet) — matches the observation.

Q2. An example of homologous organs is:

(a) Our arm and a dog's fore-leg (b) Our teeth and an elephant's tusks (c) Potato and runners of grass (d) All of the above

• (a) Our arm and a dog's fore-leg: Both have the same basic skeletal plan (humerus, radius, ulna, carpals, metacarpals, phalanges) derived from a common ancestor, though used for different functions.
• (b) Our teeth and an elephant's tusks: Tusks are modified incisor teeth — same origin, different function.
• (c) Potato and runners of grass: Both are modified stems — same origin (stem), different function (potato stores food; grass runner spreads the plant).

All three are homologous organs — similar structure and origin but different functions.

(Note: Some textbooks mark (a) as the answer. Check with your teacher for the accepted answer in your exam context.)

Q3. In evolutionary terms, we have more in common with:

(a) A Chinese school-boy (b) A chimpanzee (c) A spider (d) A bacterium

Q4. A study found that children with light-coloured eyes are more likely to have parents with light-coloured eyes. Based on this, can we say anything about whether the light eye colour trait is dominant or recessive? Why or why not?

• If light eye colour is recessive: Both parents are homozygous recessive (ll). All children will be ll → light eyes. This fits the data.
• If light eye colour is dominant: Both parents could be homozygous dominant (LL). All children will be LL → light eyes. This also fits the data.

To determine dominance, we would need additional data — for example, the eye colour of children from a cross between a light-eyed parent and a dark-eyed parent. If the trait is dominant, such a cross could produce both light-eyed and dark-eyed children. If recessive, all children of a homozygous dark-eyed (dominant) parent would have dark eyes.

Q5. Outline a project which aims to find the dominant coat colour in dogs.

Project: Determining Dominant Coat Colour in Dogs

Step 1 — Select pure-breeding parents:

• Choose a black-coated dog that has produced only black puppies in previous matings (homozygous BB).
• Choose a brown-coated dog that has produced only brown puppies in previous matings (homozygous bb).

Step 2 — Perform the cross:

• Mate the black dog (BB) with the brown dog (bb).

Cross diagram: Black (BB) x Brown (bb)

Step 3 — Observe F1 generation:

• If all F1 puppies are black → black is dominant.
• If all F1 puppies are brown → brown is dominant.
• If a mix appears → the parents were not pure-breeding; repeat with verified pure lines.

Step 4 — Confirm with F2 generation:

• Inter-cross F1 puppies (Bb x Bb).

• If black is dominant: F2 ratio will be approximately 3 black : 1 brown.
• The colour appearing in 3/4th of F2 is dominant; the one in 1/4th is recessive.

Step 5 — Conclusion: The coat colour that appears in all F1 offspring and three-quarters of F2 offspring is the dominant trait.

Q6. How is the sex of the child determined in human beings?

In summary: Humans have XX (female) and XY (male) sex chromosomes. The mother always contributes an X chromosome through the egg. The father contributes either X or Y through the sperm. XX = girl, XY = boy. The father's sperm determines the sex of the child.

It is scientifically wrong and socially harmful to blame the mother for the sex of the child.

Q7. A variation that is advantageous for survival is not necessarily beneficial for the individual. Explain.

• A variation that causes an organism to produce more offspring (high fecundity) is advantageous for the species' survival, but the individual may have a shorter lifespan due to the energy cost of reproduction.
• Altruistic behaviour in animals (e.g., a honeybee stinging and dying to protect the hive) benefits the colony but kills the individual.
• A mutation that increases disease resistance in a population may come with a side effect (like sickle cell anaemia — resistance to malaria but the individual may suffer from anaemia).

The key point: natural selection favours variations that increase the survival of the species (or the genes), not necessarily the welfare of the individual.

Q8. How does the creation of variations in a species promote survival?

In brief:

• Variations create differences among individuals in a population.
• When the environment changes, individuals with favourable variations survive and reproduce.
• Over generations, favourable variations spread through the population.
• Without variation, the entire population would be equally vulnerable to any change, risking extinction.
• Variation is the raw material for natural selection — it enables adaptation and evolution.

Q9. What are the differences between analogous and homologous organs? Give examples.

Examples explained:

• Homologous: Human arm, bat wing, whale flipper, and cheetah leg all have the same basic bone structure (humerus, radius, ulna) but perform different functions (grasping, flying, swimming, running). They evolved from a common ancestor and diverged.
• Analogous: Wings of a bat (modified forelimb with skin) and wings of a butterfly (outgrowth of exoskeleton) both enable flight but have completely different structures and origins. They evolved independently to serve the same function.

Q10. What are fossils? How do they provide evidence for evolution?

Fossils are the preserved remains, impressions, or traces of organisms that lived in the distant past. They are found in sedimentary rocks and include bones, shells, imprints, footprints, and even entire organisms preserved in amber or ice.

How fossils provide evidence for evolution:

1. Chronological record: Fossils in deeper (older) rock layers are simpler than those in newer layers, showing a gradual increase in complexity over time. 2. Transitional forms: Some fossils show intermediate features between major groups (e.g., Archaeopteryx — shows features of both reptiles and birds, supporting the theory that birds evolved from reptiles). 3. Extinct species: Fossils reveal organisms that no longer exist, showing that species have changed over time. 4. Evolutionary lineages: A series of fossils can trace the evolution of a group (e.g., horse fossils showing the gradual change from small, multi-toed ancestors to the modern horse). 5. Geographical distribution: Fossils of similar organisms found on different continents support continental drift and the idea that species evolved before the continents separated.

Fossil formation conditions: Organisms must be buried quickly in sediment, away from oxygen (to prevent decay), and the sediment must harden into rock over millions of years. This is why fossilisation is rare — most organisms decay before they can fossilise.

Q11. Why is the study of human evolution traced through fossil study and not by comparing species that exist today?

1. Evolution is not linear: Human evolution did not proceed in a straight line from ape to human. It was a branching process with many species existing simultaneously and most going extinct. Fossils reveal these extinct branches that modern species cannot. 2. Modern species are not our ancestors: Chimpanzees and gorillas are not our ancestors — they are our cousins who share a common ancestor with us. Comparing only living species cannot reveal the actual ancestors or the extinct intermediate forms. 3. Fossils provide a timeline: Fossils come with geological dating that tells us when each form lived. Modern species alone cannot provide a chronological sequence. 4. Missing links: Fossils of extinct hominids (like Australopithecus, Homo habilis, Homo erectus) fill the gaps between modern humans and our common ancestors with apes. These are only available through fossil records. 5. Environmental context: Fossils are found with information about the ancient environment, which helps us understand the selective pressures that drove human evolution.

Q12. Bacterial cell and mammoth — which is more evolved?

• Bacteria have been on Earth for over 3.5 billion years and are extraordinarily well-adapted to their environments. They reproduce rapidly, evolve quickly, and survive in extreme conditions (hot springs, deep-sea vents, frozen ice).
• Mammoths existed for millions of years and were well-adapted to cold environments before going extinct about 4,000 years ago.
• Both bacteria and mammoths have evolved through natural selection to suit their respective environments.
• Evolution is about adaptation to the environment, not about complexity. A simple organism that is perfectly adapted to its niche is just as "evolved" as a complex one.

Key concept: Evolution does not mean progress from simple to complex. It means change through natural selection. Bacteria are not "less evolved" just because they are simpler — they are incredibly successful organisms.

Common Mistakes to Avoid

1. Confusing homozygous with heterozygous: Homozygous means both alleles are the same (TT or tt). Heterozygous means different alleles (Tt). A heterozygous individual shows the dominant phenotype but carries the recessive allele.

2. Forgetting that acquired traits are NOT inherited: Changes in somatic cells (muscles, scars, learned skills) do not change germ cell DNA and therefore cannot be passed to offspring. Only genetic changes in germ cells are inherited.

3. Mixing up homologous vs analogous organs: Remember — Homologous = same structure, different function (common ancestor); Analogous = different structure, same function (no common ancestor for that organ). Use the mnemonic: Homologous = Has same Hardware; Analogous = Answer is Appearance.

4. Wrong Punnett square setup: Always write the gametes of one parent across the top and the other parent down the side. Each gamete carries only one allele per trait. For dihybrid crosses, use 4 gamete types for each heterozygous parent.

5. Thinking the mother determines the sex of the child: The mother always contributes X. Only the father can contribute X or Y. It is the father's sperm that determines the sex of the child.

6. Confusing variation with mutation: Variation refers to differences among individuals in a population (can arise from recombination during sexual reproduction). Mutation is a sudden, random change in DNA sequence. All mutations create variation, but not all variation comes from mutation.

7. Forgetting that fossils require specific conditions to form: Fossilisation is rare. It requires rapid burial, absence of oxygen, and hard body parts. Most organisms decompose before fossilising. The fossil record is incomplete — absence of fossils does not mean absence of life.

8. Misunderstanding the 9:3:3:1 ratio: This is the F2 ratio for a dihybrid cross between two heterozygous parents (AaBb x AaBb). It applies only when both genes assort independently. If the genes are linked (on the same chromosome), the ratio will be different.

9. Assuming evolution means progress toward complexity: Evolution is adaptation to environment, not a ladder of progress. Bacteria are as "evolved" as humans. Simpler does not mean less evolved.

10. Writing genotype without considering all possibilities: For blood groups, A could be I^A I^A or I^A i; B could be I^B I^B or I^B i. Always consider heterozygous possibilities unless told the parent is pure-breeding.

Exam Checklist

Before your board exam, make sure you can tick off every item:

• [ ] Define heredity, gene, allele, dominant, and recessive with examples
• [ ] Explain Mendel's Law of Segregation with a monohybrid cross and Punnett square (3:1 ratio)
• [ ] Explain Mendel's Law of Independent Assortment with a dihybrid cross and Punnett square (9:3:3:1 ratio)
• [ ] Explain sex determination in humans (XX/XY system) and why the father determines the sex
• [ ] Distinguish between inherited and acquired traits with examples
• [ ] Differentiate homologous and analogous organs with at least two examples each
• [ ] Explain how fossils provide evidence for evolution (5 points)
• [ ] Describe how speciation occurs (geographical isolation + genetic drift + natural selection)
• [ ] Explain why variations promote survival of a species
• [ ] Solve any Punnett square problem (monohybrid or dihybrid cross) correctly, showing all steps

Internal Links

• [Chapter 9 Guide — Heredity](/school/cbse/class/10/science/cbse-heredity)
• [All Class 10 Science Solutions](/school/cbse/class/10/science/solutions)
• [Class 10 Science Subject Guide](/school/cbse/class/10/science)