CBSE Class 10 Science Heredity Assignment Set 01

MCQ Questions for NCERT Class 10 Science Heredity and Evolution

Question. In a monohybrid cross done by Mendel
(a) F1 had 75% tall plants and 25% dwarf plants
(b) F1 had all tall plants
(c) F1 had all dwarf plants
(d) none of the above was observed
Answer : B

Question. Which of the following constitutes an example of Homologous organs?
(a) Wings of bat and wings of butterfly
(b) Forelimbs of horse and wings of bat
(c) Eyes of insects and eyes of man
(d) Legs of cockroach and hindlegs of frog
Answer : B

Question. Mendel was
(a) an Englishman
(b) an American
(c) a German
(d) an Austrian
Answer : D

Question. What was used by Mendel for his experiments?
(a) Fruit flies
(b) Pea plant
(c) Rice plant
(d) Rabbits
Answer : B

Question. What does a dominant character mean?
(a) It is expressed first.
(b) It suppresses the recessive character.
(c) It remains unaffected by the recessive character.
(d) All of the above
Answer : D

Fill in The Blank

Question. In a monohybrid cross, one of the alleles is ...... and another, ...... .
Answer : dominant, recessive

Question. DNA is hereditary material because it is ...... and ...... .
Answer : replicated, transmitted

Question. As it stands today, characters ...... are not inherited, and in this regard theory of ...... is not sustainable.
Answer : acquired, Lamarck

Question. Mutation is generally ...... to animals.
Answer : beneficial

Match The Columns

                A                                             B
(i) Natural selection                            (a) Lamarck
(ii) Inheritance of acquired characters  (b) Phenotype
(iii) Homozygous condition                  (c) Darwin
(iv) Physical traits                              (d) Molecular phylogeny
(v) Study of DNA sequence                 (e) TT

Answer : (i)(c)  (ii)(a)  (iii)(e)  (iv)(b)  (v)(d)

Very Short Answer Question

Question. Who gave the first scientific theory of organic evolution?
Answer : Lamarck

Question. Define allele.
Answer : Unit factor responsible for genetic character, named by Mendel

Question. Name the book in which Darwinism is described.
Answer : Origin of Species

Assertion and Reason

DIRECTION : In the following questions, a statement of assertion (A) is followed by a statement of reason (R). Mark the correct choice as:
(a) Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).
(b) Both assertion (A) and reason (R) are true but reason (R) is not the correct explanation of assertion (A).
(c) Assertion (A) is true but reason (R) is false.
(d) Assertion (A) is false but reason (R) is true.
(e) Both Assertion and Reason are false.

Question. Assertion : Although living organism always arise from other living organism,, life should certainly have had a beginning.
Reason : The study of the conditions and the
mechanisms involved in the creation of most primitive living structures on earth is actually the problem of origin of life.
Answer : A

Question. Assertion : Speciation is the process of formation of a new species from a pre-existing one.
Reason : Mutation plays a role in speciation.
Answer : A

Question. Assertion : Ear muscles of external ear in man are poorly developed.
Reason : These muscles are useful which move external ear freely to detect sound efficiently.
Answer : C

Question. Assertion : Wings of butterfly and wings of bat are analogous organs.
Reason : Analogous organs have different origin and structural plan but same function.
Answer : A

Question. Assertion : Fossils are remains of dead organisms.
Reason : It is helpful in study of evolution.
Answer : B

Question. Assertion : We have lost all the direct evidence of origin of life.
Reason : The persons responsible for protecting evidences were not skilled.
Answer : C

Question. Assertion : The genetic complement of an organism is called genotype.
Reason : Genotype is the type of hereditary properties of an organism.
Answer : A

Question. Assertion : Forelimbs of vertebrates are homologous organs.
Reason : Analogous organs have same origin but different functions.
Answer : C

Question. Assertion : Variations are seen in offspring produced by asexual reproduction.
Reason : DNA molecule generated by replication is not exactly identical to original DNA.
Answer : A

Question. Assertion : A geneticist crossed two pea plants and got 50% tall and 50% dwarf in the progeny.
Reason : One plant was heterozygous tall and the other was dwrf.
Answer : A

Heredity and Evolution

Question. How are fossil studies important in working out evolutionary relationships. How would the age of fossils be determined?
Answer : Fossils are impressions of the body/ body parts or the remains of organisms living in the past, which got preserved in sediments of earth. Any remains of an organism that has been preserved in the earth’s crust.Study of fossils helps in working out evolutionary relationships. Age of fossils can be determined by time dating using isotopes of carbon (carbon dating).

Question. a. How many gene sets should a germ cell have?
b. Mention two factors causing evolution.
Answer : 
a. One gene set.
b. (i) Reproduction isolation (ii) Natural selection

Question. Tails of some mice were amputated and they were allowed to breed. The mice that were produced also had their tails amputated and it was repeated for many generations. What is the reason of tail appearing again and again?
Answer :Such traits are acquired traits. Traits acquired during the life-time of an individual are not inherited as these changes are not in the genes of reproductive tissues.
The changes in the non-reproductive tissues cannot be passed on to the DNA of the germ cells, hence such acquired changes are not inherited by the progeny.
Example: tailless mice (if tail is removed by surgery) cannot produce tailless progeny.

Question. In an area A, the leaf materials available to beetles were very less. What are the two consequences seen in case of beetles?
Answer : a. Due to poor nutrition, the average weight of adult beetles decreases.
b. The number of beetles (population) decreases due to starvation.

Question.“The chromosome number of the sexually producing parents and their offspring is the same”. Justify this statement.
Answer :  In sexual reproduction, both the gametes (male and female) contain half the number of chromosomes (haploid or n) and by the fusion of these gametes, the zygote have full set (diploid 2n) chromosomes.

Question. What is the importance of DNA copying in reproduction? Why is variation beneficial to the species but not necessary for the individual? Explain.
Answer : DNA copying is essential part of reproduction because it ensures that same blueprint of the body design is maintained. Variation for the species is beneficial for
adaptation and better survival. It may result in new species formation.

Question. How did Mendel explain that it is possible that a trait is inherited but not expressed in an organism?
Answer : Yes, it is possible.
Example - When pure tall pea plants are crossed with pure dwarf pea plants, only tall pea plants are obtained in F1 generation.
On selfing tall plants of F1 both tall and dwarf plants are obtained in F2 generation in the ratio 3:1.
Reappearance of the dwarf character, a recessive trait in F2 generation shows that the dwarf trait/character was present in individuals of F1 but it did not express (due to the present of tallness, a dominant trait / character)

Question. What is evolution? How does it occur?
Answer : Organic Evolution: Gradual unfolding of organisms from pre-existing organisms through change is called evolution.
a. Evolution occurs in the form of genetic drift and natural selection combined with geographical separation.
b. Speciation - evolution of a new species from preexisting species-occurs.
c. Fossils are impressions of the body/body parts or the remains of organisms living in the past, which got preserved in sediments of earth.
d. Study of not living species but also fossils helps in working out evolutionary relationships.
e. Fossils provide one of the most acceptable evidences in support of evolution, because we can study the evolutionary past of individuals in the form of their fossils.

Question.How can we say that change in genes can be brought about the change in DNA?
Answer : A gene is a segment of DNA on a chromosome occupying a specific position.
a. It provides informations from one protein to another and is responsible for its alteration.
b. It ensures the stability of the DNA of the species.
So, we can say that change in gene segment can bring about change in DNA.

Question.What is micro evolution? Does it explain speciation?
Answer :
a. In micro evolution, the changes are small which occurs in lower categories and change the common characteristics of a particular species.
b. It does not properly explain speciation. 

Question. Where are the genes located? What is the chemical nature of gene?
Answer :Genes are located on chromosomes in linear sequence and at fixed positions. Chemically, genes are acidic in nature since they are nucleic acids which constitute DNA.

Question. What is F₂ generation?
Answer : The generation produced by the offsprings of F1
generation i.e., first generation as parent is called F2

Question. a. Write full form of DNA.
b. Why are variations essential for the species?
Answer :
a. Deoxyribonucleic acid
b. Survival 

Question. What are sex chromosomes? Which sex chromosomes are found in male and female human beings? State the chromosome responsible for the development of male child in human beings?
Answer : Sex chromosomes are set of chromosomes present in human beings which help in sex determination.
XX and XY.
Y chromosome. 

Question. Variations are important for the survival of species overtime. Justify this statement with reasons.
Answer : 
a. It causes adaptations,
b. It promotes natural selection.

Question. a. Do eyes of octopus, insect and human show relation in their evolutionary process?
b. How can we say that birds have evolved from reptiles?
Answer :
a. No, eyes of these animals do not show any evolutionary relationship as they are analogous organs.
b. The presence of feathers both in dinosaurs and birds indicate birds have evolved from reptiles.Some organs may be adapted to perform new functions during evolution, e.g., feathers which were meant for providing insulation in cold weather, slowly became adapted for flight. Hence birds are closely related to reptile as dinosaurs had feathers, though could not fly. 

Question.Explain how gene expresses itself in a cell? Why are we somewhat similar to our parents yet not identical to them?
Answer :Genes are functional segments of DNA. They are units of heredity that gets passed on through reproduction from parents to progeny. It is the blue print of life. DNA expresses itself through synthesis of proteins.Proteins make structures in our body and also controls the functioning. In other words cells, tissues, organs and hence a living body expresses the traits inherited as genes/ DNA. Since we inherit the DNA, half from one and half from other parents, hence we somewhat resemble them. At the same time we show mixed traits of both so can not be identical to either of them.

Question. a. What is the genetic constitution of human sperm?
b. Mention the chromosomes pair present in zygote determining the sex of a male child.
Answer : a. 22 + Y or 22 + X
b. (22 + Y + 22 + X)

Question. “Natural selection and speciation lead to evolution”. Justify the statement.
Answer : Natural selection: Some variations are advantageous for an organism to adapt better in the prevailing conditions of habitat. It makes it easy to obtain food and mating partner by them. In this manner they are able to propagate more, transmitting their genes to next generation and producing more individuals with similar genetic makeup and phenotype.
a. It leads to change in frequency of some genes in a population which give survival advantage to a species from elimination.
b. When most of the members of a population possess this variation, it is called its adaptation.
c. Over a period of time large numbers of adaptations are accumulated in the various populations of a species which may be physically segregated from each other.
d. Geographical barriers like mountains, rivers etc., lead to incapability to reproduce amongst themselves in the population.
e. By processes like genetic drift and natural selection combined with geographical separation, when the populations of a species become incompatible/ fail to reproduce with each other, speciation- evolution of a new species from pre-existing species - occurs.

Question. If YYRR is round yellow, what do the following represent?
yyrr yyRR
Answer : yyrr - Wrinkled, green seeds
yyRR — Round, green seeds

Question. “Two areas of study namely evolution and classification are interlinked”. Justify this statement.
Answer : All living things are identified and categorised on the basis of their body design in form and function. After a certain body design comes into existence, it will shape the effects of all other subsequent design changes simply because it already exists. So, characteristics which came into existence earlier are likely to be more basic than characteristics which have come into existence later.
This means that the classification of life forms will be closely related to their evolution. On connecting this idea of evolution to classification, it is seen that some groups of organisms with ancient body designs have not changed very much. However, other groups of organisms have acquired their particular body designs relatively recently. Because there is a possibility that complexity in design will increase over evolutionary time, it may not be wrong to say that older organisms are simpler, while younger organisms are more complex.

Question. In a study it was found that fused ear lobes were found in more numbers within a population rather than free ear lobes. What can you infer from the above observation with respect to dominant / recessive trait?
Answer : It can be inferred from the observation that fused ear lobes were found in more numbers within a population that fused ear lobes is a dominant trait whereas free ear lobes is a recessive trait.

Question. (A) Why did Mendel carry out an experiment to study inheritance of two traits in garden-pea?
(B) What were his findings with respect to inheritance of traits in F₁ and F₂ generation?
(C) State the ratio obtained in the F₂ generation in the above mentioned experiment. 
Answer : (A) Mendel carried out an experiment to study inheritance of two traits in garden pea to see the interaction and basis of inheritance between them. He also concluded that also
traits segregate during gamete formation and finally he gave three laws.
(1) Law of segregation
(2) Law of Dominance
(3) Law of Independent Assortment
(B) Mendel observed that—In F₁ generation, feature of only one parental type appear. The features of other parents were not expressed. He called the first one which appeared as dominant features/ character and the other features which did not appear called them as recessive. The characters are not lost even when they are not expressed. When F₁ off springs were allowed to be self pollinated, both the parental traits were expressed in definite proportion in F₂ generation. He had started with two combinations of characteristics and two new combinations of characteristics and appeared in F₂ generation.

Question. Mention that total number of chromosomes along with the sex chromosomes that are present in a human female and a human male. Explain how in sexully producing organisms the number of chromosomes in the progeny remains the same as that of the parents.
Answer : The total number of chromosomes in humans is 23 pairs (or 46 chromosomes). Out of this, the 23rd pair is sex chromosome. The equal genetic contribution of male and female parents in a progeny is ensured through special type of reproductive cells called gametes. Gametes have only half the amount of DNA as compared to other body cells. So, when the gametes from male and female parents combine during sexual reproduction to form a zygote, they contribute equal amount of DNA. Let us further understand it with the help of
an example. The normal body cells of human beings contain 46 chromosomes each. Now, the human male gamete has 23 chromosomes and the human female gamete has also 23
chromosomes.
So, during sexual reproduction the combination of 23 chromosomes from male and 23 chromosomes from female ensures equal genetic contribution in the progeny, i.e., 23 + 23
= 46 chromosomes.

Question. How is the sex of child determined in human beings?
Answer : In human beings, sex of the child depends upon which kind of male gamete fertilizes the female gamete. If sperm carrying X chromosomes fertilizes the ovum carrying X chromosome, then the child born will be a girl. If a sperm carrying Y chromosome fertilizes the ovum which carries X chromosome, then the child born will be a boy.

Question. How did Mendel’s experiments show that different traits are inherited independently? Explain
Answer : Mendel crossed pea plants having round green seeds (RRyy) with pea plants having wrinkled yellow seeds (rrYY).
Since the F1 plants are formed after crossing pea plants having green round seeds and pea plants having yellow wrinkled seeds, F1 generation will have both these characters in them.However, as we know that yellow seed colour and round seeds are dominant characters, therefore, the F1 plants will have yellow round seeds. Then this F1 progeny was self-pollinated and the F2 progeny was found to have yellow round seeds, green round seeds, yellow wrinkled seeds and green wrinkled seeds in the ratio of 9:3:3:1

Question. In pea plant, round seed is dominant over the wrinkled. If a cross is carried out between these two plants, the genotype ratio of F2 progeny obtained from this cross is
(a)1:1
(b)3:1
(c)1:2:1
(d)9:3:3:1
Answer : Correct option(c)

Question. Newly formed DNA copies may not be identical at times. Give one reason.
Answer : If there is error in DNA copying/inaccuracies in DNA copying.

Question.What is the probability that the male progeny will be a boy?
(a) 50%
(b) 56%
(c) 47.43%
(d) It varies
Answer : Correct option (a)

Question. In a pea plant, the trait of flowers bearing purple colour(PP) is dominant over white colour(pp).Explain the inheritance pattern of F1 and F2 generation with the help of a cross following the rules of inheritance of traits. State the visible characters of F1 and F2 progenies?
Answer : Let purple trait be represented by:PP and White trait be:pp

Visible characters of F1 progeny are all flowers are purple coloured and in F2 progenies 3 are purple coloured and 1 is white flower. 

MCQ Questions for Class 10 Science Heredity & Inheritance : -

Question. Define ‘evolution’. State Darwin’s theory of evolution.
Answer : Evolution is a change in the genetic composition of a population.
Darwin’s theory
1. The size of population remains the same, constant despite the fact that more offsprings are produced than needed.
2. Variations provide adaptations.
3. The best adapted organism survive in the changing environment (survival of the fittest).
4. Nature selects the best organisms with better adaptations and after many generations new species are formed (natural selection).

Question. Explain the analogous organs and homologous organs Identify the analogous and homologous organ amongst the following: Wings of an insect, wings of a bat, forelimbs of frog, forelimbs of human.
Answer : Analogous organs are those organs that have same function but have different structural design and origin. E.g., wings of birds and insects.
Homologous organs are those organs in different plants or animals which have the same basic structural design and organ but have different appearance and functions. Analogous—Wings of an insect, wings of a bat Homologous–Forelimbs of frog, forelimbs of human and wings of bat.

Question. What are various evidences in factors of evolution?
Answer : Evidences are:
(a) Homologous organs: Organs which have same structure but different function.
E.g., wings of a bat, hands of man and limbs of monkey.
(b) Analogous organ. Organs which perform similar function but are structurally different are called analogous organ. E.g., wings of bat, insects, birds.
(c) Vestigial organs: These organs are those which appear in an organism but are functionless e.g., vermiform appendix, nictitating membrane in eye is present in human beings but has no function.
(d) Embryological evidence: The study of embryos of vertebrates shows that all of them have same origin as their structure at initial stage is same. Embryo of frog, fish, man looks alike.
(e) Fossils as evidence: Archaeopteryx fossils shows a link between bird and reptiles as this fossil has some feature of bird and some of reptile.

Question. (i) What is genetics?
(ii) Give the common name of the plant on which Mendel performed its experiments.
(iii) What for did Mendel use the term factors and what are these factors called now?
(iv) What are genes? Where are the genes located?
Answer : (i) Science which deals with the study of heredity and variations is called genetic.
(ii) Pea plant.
(iii) Mendel used the term factors for ‘genes’.
(iv) Genes is the unit of inheritance. It is a part of the chromosome which controls the appearance of a set of hereditary character.
Genes are located on the chromosome.

Question. (i) What are traits?
(ii) Explain the inherited trait and acquired traits.
(iii) Define speciation. What are the factors which could lead to the rise of a new species?
Answer : (i) Traits: A characteristic feature is called trait.
(ii) Inherited and acquired trait (given in notes on page 112).
(iii) Speciation and factors (given in notes on page 112).

HOTS: (High Order Thinking Skill) Questions with Answers:

Question 1. How one is change adopted to perform different functions? Give one example
Answer: In evolution, one change occurred initially is used to perform other functions. For example, feathers were evolved for warmth, later they were adapted for flight.
In simple words: Organs or features that developed for one purpose can be modified later for a completely different function during evolution.

Exam Tip: Remember feathers as a classic NCERT example of evolutionary adaptation from thermal insulation to flight.

Question 2. What happened when Mendel crossed two traits of a character in a pea plants?
Answer: Only dominant trait appeared in \(F_1\).
In simple words: When two contrasting traits are crossed, only the dominant trait expresses itself in the first filial generation.

Exam Tip: Always mention \(F_1\) generation and the masking of the recessive trait by the dominant allele.

Question 3. Who provided experimental evidence to support theory of origin of life from inanimate matter?
Answer: Miller and Urey.
In simple words: Stanley Miller and Harold Urey proved experimentally that organic molecules could form from inorganic compounds.

Exam Tip: Recall the Miller-Urey experiment using a mixture of methane, ammonia, hydrogen, and water vapor with electric sparks.

Question 4. Why are asexually reproducing organisms capable of showing hereditary features?
Answer: Asexual reproduction tends to preserve the similarities among all the individuals belonging to a give line of descent. Therefore, asexually reproducing organisms are capable of showing hereditary features.
In simple words: Asexual reproduction copies genetic material directly, passing parental traits faithfully to the offspring.

Exam Tip: Focus on DNA copying fidelity and the absence of meiotic recombination in asexual reproduction.

Question 5. If the sperm bearing Y-chromosome fertilizes the egg, the child born will not be entirely like his father. Why is it so?
Answer: It is so because the other sex chromosome, ie the X-chromosome will also have its effects and other autosomes of the egg will also show their characteristics.
In simple words: A child inherits half its genes from the mother via the egg, combining maternal and paternal traits.

Question 6. A normal pea plant bearing colored flowers suddenly start producing white flowers. What could be the possible cause?
Answer: The appearance of white flowers is due to mutation.
In simple words: Sudden genetic changes can alter pigment production, resulting in white flowers instead of colored ones.

Exam Tip: Link sudden trait changes directly to spontaneous DNA mutations.

Question 7. Mention any two recessive traits of garden pea.
Answer: Dwarf (height of plant), wrinkled seed.
In simple words: Short plant height and wrinkled seed texture are standard recessive traits in pea plants.

Exam Tip: Memorize Mendel's contrasting pairs: tall/dwarf, round/wrinkled, yellow/green seeds.

Question 8. Write the characteristics on the basis of which duck-biled platypus is considered as a link between reptiles and mammals.
Answer: The characteristic resembling reptiles is laying of eggs and the characteristics resembling mammals is presence of mammary glands.
Answer (Alternative/Detailed): The platypus lays eggs like reptiles (oviparity) but possesses mammary glands to feed its young and has hair/fur like mammals.
In simple words: It bridges two major animal groups by sharing egg-laying traits with reptiles and milk-production traits with mammals.

Exam Tip: Highlight egg-laying and mammary glands as key transitional features.

Question 9. Why are the small number of surviving tigers a cause of worry from the point of view of genetics?
Answer: As the population of tigers is decreasing, there is loss of genes from the gene pool and there can't be recombinations and variations, thus no evolution.
In simple words: Low population size leads to inbreeding and loss of genetic diversity, making species vulnerable to extinction and unable to adapt.

Exam Tip: Use scientific terms like 'gene pool', 'genetic drift', and 'loss of variation'.

Question 10. What is called phylogenetic system of classification?
Answer: Classification based on evolutionary relationships of organisms.
In simple words: This system groups organisms based on their common ancestry and evolutionary history.

Exam Tip: Connect phylogenetic classification directly to evolutionary descent.

Question 11. Is it true that when a new species is emerged, the old species is eliminated and why?
Answer: No, it is not true that when a new species is emerged, the old species is eliminated. Because when there is a change in any species, the change is only in a part or a few members of the species population. If the newly generated species after genetic change is better in any way, it will get more opportunity to survive and if the genetic change is against the environment, it will die. Thus, unchanged members of other species may also remain and tend to live in changed environment.
In simple words: Speciation usually occurs in isolated populations; the parent species often survives alongside the new species if resources permit.

Exam Tip: Clarify that branching evolution (cladogenesis) does not necessarily cause immediate extinction of ancestral forms.

Question 12. What will be the percentage of ab gametes produced by AaBb parent?
Answer: 25 percent.
In simple words: An AaBb parent produces four equally likely gamete types (\(AB, Ab, aB, ab\)), each with a probability of 25% (\(\frac{1}{4}\)).

Exam Tip: Use the formula \(2^n\) where \(n\) is the number of heterozygous loci to find total gamete types, then calculate individual proportions.

Question 13. Mendel crossed a pure white recessive pea plant with a dominant pure red flowered plant. What will be the first generation of hybrids?
Answer: All red.
In simple words: Crossing pure dominant with pure recessive yields heterozygous offspring showing only the dominant red phenotype.

Exam Tip: State Law of Dominance: \(F_1\) generation displays 100% dominant phenotype.

Question 14. In evolutionary terms, which among-bacteria, spider, fish and chimpanzee have a “better” body design? Why or why not?
Answer: Chimpanzee have the better body design as compared to others given. They are better adapted for locomotion, communication and thinking.
In simple words: Higher complexity and advanced organ systems make primates like chimpanzees more specialized, though bacteria are equally successful survivors.

Exam Tip: Balance evolutionary complexity with biological success; bacteria survive everywhere, but complex multicellular organisms have advanced coordination systems.

Question 15. What is an offspring?
Answer: In sexual reproduction organisms raised are as a result of crossing over and exchange of gene segments and are known as offspring.
In simple words: Offspring are the new organisms produced by parents through sexual or asexual reproduction.

Exam Tip: Emphasize genetic combination and inheritance from parents.

Question 16. Why are traits acquired during life-time of an individual not inherited?
Answer: Traits acquired during life-time of an individual not inherited because change in non-reproductive tissue or somatic cells cannot be passed on to the DNA of germ cells. Thus, the acquired trait will die with the death of the individual. It is therefore non-heritable and cannot be passed on to its progeny.
In simple words: Somatic modifications do not alter germline DNA, so acquired skills or injuries cannot be transmitted to descendants.

Exam Tip: Distinguish clearly between somatic cells (non-heritable) and germ cells (heritable).

Question 17. Name the chemicals which were essential for origin of life.
Answer: Proteins and nucleic acid.
In simple words: Complex organic molecules like proteins and nucleic acids (DNA/RNA) formed the foundation of early life forms.

Question 18. Why males are called heterogametic?
Answer: Because they have dissimilar sex chromosomes.
In simple words: Human males produce two different types of sperms (one with X and one with Y chromosome), hence they are heterogametic.

Exam Tip: Contrast heterogametic males (XY) with homogametic females (XX).

Question 19. What is the percentage possibility a couple of having daughters?
Answer: 50 percent.
In simple words: Each fertilization event has an equal 1:1 chance of producing a male or female child.

Question 20. Name 2 organisms in which sex determination is regulated by environmental factors.
Answer: Turtle, lizard.
In simple words: In some reptiles like certain turtles and lizards, incubation temperature of fertilized eggs determines the sex of the offspring.

Exam Tip: Note that temperature-dependent sex determination occurs in reptiles, unlike genetic sex determination in mammals.

Question 21. What are inherited traits? Give one example.
Answer: The characteristics which are transmitted from parents to their offsprings are called inherited traits. E.g. free and attached earlobes.
In simple words: Inherited traits are genetically controlled features passed down from parents to children.

Question 22. When Mendel crossed a Tall plant with a dwarf plant, no medium height plants were obtained in \(F_1\) generation. Why?
Answer: Because dominant genes express themselves and suppress the effect of recessive genes. So no medium sized plants were obtained.
In simple words: The allele for tallness is completely dominant over the allele for dwarfism, masking intermediate heights entirely in heterozygous individuals.

Exam Tip: Use the principle of complete dominance to explain why blending does not occur in monohybrid crosses.

Question 23. The gene type of green stemmed tomato plants is denoted as GG and that of purple stemmed tomato plants as GG when these two are crossed. (Note: standard textbook notation uses GG for green and gg for purple; answering based on standard Mendelian cross convention)
i. What colour of stem would you expect in \(F_1\) progeny?
ii. Give the percentage of purple stemmed plants if \(F_1\) are self pollinated.
iii. In what ratio would you find the gene types CG and Gg in the \(F_2\) progeny?
Answer:
i. Colour of \(F_1\) progeny - Green (assuming G is dominant green and g is recessive purple).
ii. Percentage of purple stemmed plants in \(F_2\) generation \(\frac{1}{4}\) or 25%.
iii. Ratio of genotypes GG and Gg is 1:2.
In simple words: Cross between GG and gg yields heterozygous Gg (all green) in \(F_1\); selfing produces a 1:2:1 genotypic ratio and 3:1 phenotypic ratio in \(F_2\).

Exam Tip: Practice monohybrid cross ratios: Genotypic ratio is 1 (GG) : 2 (Gg) : 1 (gg), Phenotypic ratio is 3 Green : 1 Purple.

Question 24. The human hand, cat paw and horse foot when studied in detail show the same structure of bones and point towards a common origin.
i. What do you conclude from this?
ii. What is the term given to such structures?
Answer:
i. They have a common ancestry.
ii. Homologous organs.
In simple words: Organs with similar structural design but different functions indicate common evolutionary descent.

Question 25. What are the causes of variations in clones?
Answer:
i. inaccuracies during DNA copying.
ii. Effect of environment termed acquired variation.
iii. Mutations are sudden stable changes which are discontinuous inheritable as produced due to changes in genetic make-up.
In simple words: Even asexual clones show minor variations due to DNA replication errors or environmental influences.

Exam Tip: List DNA copy errors and mutations as primary sources of genetic variation.

Question 26. How do we know how old is a fossil?
Answer: There are two methods:
i. Relative method when we dig into the earth, the fossils we find closer to the surface are more recent than the fossils we find in deeper layers.
ii. By detecting the ratios of different isotopes of the same element in the fossil material.
In simple words: Fossil age is determined either by the depth of the rock strata (relative dating) or by radioactive carbon dating (isotope ratios).

Question 27. Study the given data and answer the questions following the data:

i. What is the term for this type of cross?
ii. What does the data of the column marked \(F_1\) indicate?
iii. Express the gene type of the (a) parents (b) \(F_1\) progeny and (c) \(F_2\) progeny
Answer:
i. Monohybrid cross.
ii. Red colour of flower dominant over white flower.
iii.
    a. Parents - \((RR)\) and \((rr)\)
    b. \(F_1\) progeny - \(Rr\)
    c. \(F_2\) progeny - \(RR\), \(Rr\) and \(rr\)
In simple words: The data demonstrates Mendelian inheritance with a 3:1 phenotypic ratio in the \(F_2\) generation.

Exam Tip: Always verify phenotypic ratios in \(F_2\) (here 33 red : 11 white = 3:1 ratio) to identify monohybrid crosses.

Question 28. Only variation that confer and advantage to an individual organism will survive in a population. Do you agree with this statement? Why or why not?
Answer: We agree with the statement. All the variation do not have an equal chance of surviving in the environment in which they find themselves. The chances of surviving depend on the nature of variation. Different individuals have different kind of advantages. A bacteria that can withstand heat will survive better in heat wave.
In simple words: Natural selection favors advantageous variations that help organisms cope with environmental stresses like extreme temperature.

Question 1. What name is given to a sequence of gradual changes over millions of year in which new species are produced?
Answer: Evolution.
In simple words: Gradual evolutionary changes over geological time scales lead to the origin of new species.

Exam Tip: Define evolution as descent with modification over long periods.

Question 2. Which are the two processes, sexual reproduction or asexual reproduction brings about marked variations in the offspring?
Answer: Sexual reproduction brings about marked variations in offspring due to crossing over, genetic recombination, and the fusion of gametes.
In simple words: Sexual reproduction shuffles genetic material from two parents, creating diverse offspring.

Exam Tip: Contrast sexual reproduction (high variation) with asexual reproduction (cloning/low variation).

Question 3. In which gametes are present X and Y chromosomes?
Answer: Sperms (male gametes) contain either an X or a Y chromosome.
In simple words: Human male sperms carry either X or Y sex chromosomes, whereas female ova carry only X chromosomes.

Exam Tip: Remember that human females are homogametic (eggs only have X) and males are heterogametic (sperms have X or Y).

Question 4. Name two processes which change the frequency of certain genes over generation in a population.
Answer: Natural selection and genetic drift.
In simple words: Gene frequencies shift over time due to environmental selection pressures and random genetic drift.

Exam Tip: Mention natural selection, genetic drift, gene flow, and mutation as key evolutionary forces.

Question 5. Explain with examples how characteristics of a population changes over the years for the following situations.
a. To gain survival advantage
b. due to accidental survival
c. Temporary change of characteristics
Answer:
a. To gain survival advantage: Natural selection favors beneficial traits, such as pesticide resistance in insects or heat tolerance in bacteria.
b. Due to accidental survival: Genetic drift or natural disasters (bottleneck effect) randomly eliminate individuals, altering gene frequencies regardless of adaptation (e.g., beetles stepped on by an elephant).
c. Temporary change of characteristics: Somatic or acquired variations caused by environmental factors (such as weight loss due to starvation) that are not genetically inherited and disappear in subsequent generations.
In simple words: Population traits change through adaptive selection, random accidents, or non-heritable environmental adjustments.

Exam Tip: Clearly differentiate adaptive selection (natural selection), random events (genetic drift), and non-heritable changes.

Question 6. How will you substantiate the statement that heredity and its concepts were known to the ancient civilisations.
Answer: Ancient civilizations practiced artificial selection and selective breeding of domesticated plants and animals (such as wild grasses into high-yielding crops and wolves into diverse dog breeds), showing they understood that traits were inherited from parents to offspring.
In simple words: Selective breeding of livestock and crops by ancient farmers proves they understood basic principles of heredity long before modern genetics.

Question 7. How many autosomes are present in a human sperm?
Answer: 22 autosomes.
In simple words: A human sperm contains 23 chromosomes in total: 22 autosomes and 1 sex chromosome (X or Y).

Exam Tip: Distinguish total chromosomes (23 in gametes, 46 in somatic cells) from autosomes (22 pairs in somatic, 22 single in gametes).

Question 8. What are cross breed plants known as?
Answer: Hybrids.
In simple words: Plants produced by crossing two different genetic varieties are called hybrids.

Question 9. What does letter F represent in heredity?
Answer: Filial generation (from Latin *filius* meaning son/daughter).
In simple words: 'F' stands for 'Filial', representing offspring generations such as \(F_1\) and \(F_2\).

Question 10. How will you conclude that birds are closely related to reptiles?
Answer: Birds share anatomical similarities with reptiles, such as scales on their feet and the fossil discovery of *Archaeopteryx*, which possessed reptilian features (teeth, long bony tail) alongside avian features (feathers and wings).
In simple words: Fossil links like *Archaeopteryx* and shared embryonic/anatomical traits show that birds evolved from reptile ancestors.

Exam Tip: Always mention *Archaeopteryx* as the textbook example of a connecting link between reptiles and birds.

Question 11. Where did the earliest members of human beings live?
Answer: Africa.
In simple words: Fossil evidence indicates that modern humans (*Homo sapiens*) originated on the African continent.

Question 12. Which organ in man suggest that he is a descendent of herbivorous animals?
Answer: Vermiform appendix.
In simple words: The appendix is a vestigial organ associated with digesting cellulose in plant-eating ancestors.

Question 13. Why is appendix in human considered as vestigial?
Answer: Because it is a non-functional, shrivelled organ in humans that no longer plays any role in digestion, though it was large and functional in our herbivorous ancestors.
In simple words: Vestigial organs are reduced remnants of structures that were functional in evolutionary ancestors.

Question 14. Why offsprings differ from parents in certain characters?
Answer: Due to genetic recombination during sexual reproduction, crossing over, and occasional DNA mutations.
In simple words: Sexual reproduction mixes genetic material from both parents, ensuring offspring are unique rather than exact carbon copies.

Question 15. What is meant by expression TT and Tt in Mendelian terms?
Answer:
- TT represents homozygous dominant genotype (two identical dominant alleles for tallness).
- Tt represents heterozygous genotype (two different alleles, one dominant tall and one recessive dwarf allele).
In simple words: TT means pure dominant for a trait, while Tt means hybrid with one dominant and one recessive allele.

Question 16. How is sex determined in human beings?
Answer: Sex in humans is determined genetically by sex chromosomes. Females have two X chromosomes (XX) and produce eggs with an X chromosome. Males have an X and a Y chromosome (XY) and produce sperms with either an X or a Y chromosome. If an X-bearing sperm fertilizes the egg, a girl (XX) is born; if a Y-bearing sperm fertilizes the egg, a boy (XY) is born.
In simple words: The sperm chromosome (X or Y) determines whether the baby will be female (XX) or male (XY).

Question 17. Why are flippers of whales and wings of birds are considered as homologous organs?
Answer: Because they have similar fundamental anatomical bone structures (humerus, radius, ulna, carpals, metacarpals, phalanges) inherited from a common ancestor, even though they are adapted for different functions (swimming vs flying).
In simple words: They share the same internal skeletal blueprint despite performing different external tasks.

Question 18. On the basis of the possibilities of combination of the sex chromosomes, what percentage probability does a couple have of having a son or a daughter. Show the same by making a cross.
Answer:
Parents: Mother (XX) \(\times\) Father (XY)
Gametes: Egg (X), Sperm (X or Y)
Cross/Punnett Square Results:
- XX (Daughter) = 50%
- XY (Son) = 50%
In simple words: There is an equal 50% probability for either a boy or a girl at each pregnancy.

Question 19. Explain why evolution can't be said to progress from lower forms to higher forms.
Answer: Evolution is a branching process driven by environmental adaptation rather than a linear ladder of progress. Organisms simply develop complex or simple designs that suit their specific ecological niches. For instance, bacteria are simple in design yet extremely successful survivors, whereas more complex organisms are not necessarily superior in every environment.
In simple words: Evolution creates diversity and adaptation to environments, not a straight ladder from primitive to superior species.

Question 20. How do Mendel's experiments show that traits may be dominant or recessive?
Answer: When Mendel crossed pure tall (TT) and pure dwarf (tt) pea plants, all \(F_1\) plants were tall (Tt). When these \(F_1\) plants were self-pollinated, the dwarf trait reappeared in the \(F_2\) generation in a 3:1 ratio. This proved that the recessive trait was merely suppressed, not destroyed, in the \(F_1\) generation.
In simple words: The reappearance of hidden traits in the second generation proves that traits can be dominant or recessive.

Question 21. Explain with an example how variation took place due to inheritance?
Answer: In sexually reproducing organisms, the combination of genetic material from two parents creates offspring with unique traits (variations). For example, children may inherit their mother's eye color and their father's hair texture, resulting in a combination distinct from either parent.
In simple words: Inheriting a unique mix of genes from both parents generates hereditary variations in offspring.

Question 22. Explain Urey and Miller's experiment showing evidence of the theory of origin of life.
Answer: Stanley Miller and Harold Urey set up an airtight apparatus containing a mixture of gases (\(\text{CH}_4, \text{NH}_3, \text{H}_2\), and water vapor) simulating Earth's primitive atmosphere. They passed electric sparks through it to simulate lightning and maintained it at high temperatures. After a week, they found that simple amino acids had formed from these inorganic molecules, supporting the chemical origin of life.
In simple words: Their experiment proved that life's building blocks (amino acids) could spontaneously form from inorganic gases under primordial Earth conditions.

Question 23. How does the study of fossils provide evidence in favour of organic evolution?
Answer: Fossils show sequential changes in organisms over geological time. By examining rock strata, we find that older rocks contain simpler organisms, while newer rocks contain more complex forms. Furthermore, transitional fossils like *Archaeopteryx* link distinct major groups, proving that modern species evolved from ancestral forms.
In simple words: Fossils act as a chronological record, showing how ancient simple species gradually evolved into modern complex ones.

Question 24. Can the wing of a butterfly and the wing of a bat be considered homologous organs? Why or why not?
Answer: No, they are not homologous organs; they are analogous organs. While both are used for flying, their internal structures and embryonic origins are completely different (bat wings have bony skeletal support, whereas butterfly wings are membranous extensions without bones).
In simple words: They perform the same function (flight) but have different internal structures, making them analogous rather than homologous.

Question 25. What do you understand by reproductive isolation? How is this mechanism responsible for speciation?
Answer: Reproductive isolation refers to the inability of populations of the same species to interbreed due to geographical barriers, behavioral differences, or anatomical changes. Over time, isolated populations accumulate independent genetic mutations and natural selection pressures, eventually becoming so genetically distinct that they can no longer produce fertile offspring together, resulting in the creation of a new species (speciation).
In simple words: When two groups of a species are prevented from mating, separate genetic changes accumulate until they form distinct new species.

Question 26. Though eyes are found in many organisms why can they not be grouped together?
Answer: Because the structure and design of eyes vary drastically across different animal groups (such as compound eyes in insects, simple eyes in mollusks, and camera-like eyes in vertebrates). These eyes have evolved independently through convergent or divergent pathways, representing different evolutionary origins.
In simple words: Similar functions like vision can evolve through completely different anatomical designs in unrelated species.

Question 27. Why can two sub-populations of a species not reproduce with each other? State two reasons. What will be the outcome of such a situation?
Answer:
Two reasons preventing interbreeding:
1. Geographical isolation (physical barriers like rivers, mountains, or oceans separating populations).
2. Genetic divergence (accumulation of different mutations and chromosomal changes making them incompatible).
*Outcome:* Speciation occurs, leading to the formation of two distinct new species that cannot interbreed.
In simple words: Physical separation and genetic changes stop sub-populations from mating, eventually creating two separate species.