NCERT Exemplar Class 10 Science Chapter 9 Heredity and Evolution are part of NCERT Exemplar Class 10 Science. Here we have given NCERT Exemplar Class 10 Science Chapter 9 Heredity and Evolution.

NCERT Exemplar Class 10 Science Chapter 9 Heredity and Evolution
Short Answer Questions

Multiple Choice Questions (MCQs)

Question. 1 Exchange of genetic material takes place in

(a) Vegetative reproduction

(b) Asexual reproduction

(c) Sexual reproduction

(d) Budding

Answer. (c)

Explanation:  Sexual reproduction involves meiosis in gamete mother cells to form gametes and fusion of male and female gametes into zygote.

Question. 2 Two pink coloured flowers on crossing resulted in 1 red, 2 pink and 1 white flower progeny. The nature of the cross will be

(a) Double fertilization

(b) Self pollination

(c) Cross fertilization

(d) No fertilization

Answer. (c)

Explanation:  Cross fertilization is transfer of pollen from one plant to the stigma of flower borne on different plant of same species.

Question. 3 A cross between a tall plant (TT) and short pea plant (tt) resulted in progeny that were all tall plants because

(a) Tallness is the dominant trait

(b) Shortness is the dominant trait

(c) Tallness is the recessive trait

(d) Height of pea plant is not governed by gene ‘T’ or ‘t’

Answer. (a)

Explanation: According to law of dominance, a trait is represented by two contrasting factors of a gene in a heterozygous individual; the allele/factor that can express itself in heterozygous individual is called as dominant trait. The other factor whose effect is masked by presence of dominant factor, is called recessive factor.

Question. 4 Which of the following statement is incorrect?

(a) For every hormone there is a gene

(b) For every protein there is a gene

(c) For production of every enzyme there is a gene

(d) For every molecule of fat there is a gene

Answer. (d)

Explanation:  Fat biosynthesis occurs through metabolic reaction.

Question. 5 If a round, green seeded pea plant (RR yy) is crossed with wrinkled, yellow seeded pea plant, (rr YY) the seeds production in F1 generation are

(a) Round and yellow

(b) Round and green

(c) Wrinkled and green

(d) Wrinkled and yellow

Answer. (a)

Explanation:  The cross between RR yy and rr YY seeds will obtain RrYy offspring which will exhibit round and yellow phenotype as these traits are dominant one.

Question. 6 In human males all, the chromosomes are paired perfectly except one. This/these unpaired chromosome is/are

(i) Large chromosome

(ii) Small chromosome

(iii) Y-chromosome

(iv) X-chromosome

            (a) (i) and (ii)

            (b) (iii) only

            (c) (iii) and (iv)

            (d) (ii) and (iv)

Answer. (c)

Explanation:  Human males are hemizygous for X and Y chromosomes.

Question. 7 The maleness of a child is determined by

(a) The X-chromosome in the zygote

(b) The Y-chromosome in zygote

(c) The cytoplasm of germ cell which determines the sex

(d) Sex is determined by chance

Answer. (b)

Explanation:  The maleness of a child is determined by presence of Y-chromosome in zygote.

Question. 8 A zygote which has an X-chromosome inherited from the father will develop into a

(a) Boy

(b) Girl

(c) X-chromosome does not determine the sex of a child

(d) Either boy or girl

Answer. (b)

Explanation:  Humans follow XX- XY mechanism of sex determination.

Question. 9 Select the incorrect statement

(a) Frequency of certain genes in a population change over several generations resulting in evolution.

(b) Reduction in weight of the organism due to starvation is genetically controlled.

(c) Low weight parents can have heavy weight progeny.

(d) Traits which are not inherited over generations do not cause evolution.

Answer. (b)

Explanation:  The weight reduction due to starvation is environmentally determined factor.

Question. 10 New species may be formed if

(i) DNA undergoes significant changes in germ cells.

(ii) Chromosome number changes in the gamete.

(iii) There is no change in the genetic material

(iv) Mating does not take place

            (a) (I) and (ii)

            (b) (I) and (iii)

            (c) (ii), (iii) and (iv)

            (d) (i), (ii) and (iii)

Answer. (a)

Explanation:  Change in genetic material leads to reproductive isolation of population into a new species.

Question. 11 Two pea plants, one with round green seeds (RRyy) and another with wrinkled yellow (rrYY) seeds produce F₁ progeny that have round, yellow (RrYy) seeds. When F₁ plants are selfed, the F₂ progeny will have new combination of characters. Choose the new combination from the following.

(i) Round, yellow

(ii) Round, green

(iii) Wrinkled, yellow

(iv) Wrinkled, green

            (a) (i) and (ii)

            (b) (i) and (iv)

            (c) (ii) and (iii)

            (d) (i) and (iii)

Answer. (b)

Explanation:  The new combination in F₂ progeny will be offspring showing phenotype other than aren’t ones (round and green; wrinkled yellow). Hence, the recombinant progeny will be one with round yellow and wrinkled green seeds.

Question. 12 A basket of vegetables contains carrot, potato, radish and tomato. Which of them represent the correct homologous structures?

(a) Carrot and potato

(b) Carrot and tomato

(c) Radish and carrot

(d) Radish and potato

Answer. (c)

Explanation:  Homologous organs perform different functions and have different appearance but share common basic structural plan and origin.

Question. 13 Select the correct statement

(a) Tendril of a pea plant and phylloclade of Opuntia are homologous

(b) Tendril of a pea plant and phylloclade of Opuntia are analogous

(c) Wings of birds and limbs of lizards are analogous

(d) Wings of bird and wings of bat are homologous

Answer. (d)

Explanation:  Homologous organs perform different functions and have different appearance but share common basic structural plan and origin. Analogous organs perform similar function and have similar appearance but have different basic structure and origin.

Question. 14 If the fossil of an organism is found in the deeper layers of earth, then we can predict that

(a) The extinction of organism has occurred recently

(b) The extinction of organism has occurred thousands of years ago

(c) The fossil position in the layers of earth is not related to its time of extinction

(d) Time of extinction cannot be determined

Answer. (b)

Explanation:  Depth of each stratum signifies the relative age of fossils present in it. The deeper the stratum, the older the rock and the fossils present in it.

Question. 15 Which of the following statements is not true with respect to variation?

(a) All variations in a species have equal chances of survival.

(b) Change in genetic composition results in variation.

(c) Selection of variants by environmental factors forms the basis of evolutionary processes.

(d) Variation is minimum in asexual reproduction.

Answer. (a)

Explanation:  Variations that impart reproductive or survival advantage to organisms are being favored during evolution.

Question. 16 A trait in an organism is influenced by

(a) Paternal DNA only

(b) Maternal DNA only

(c) Both maternal and paternal DNA

(d) Neither by paternal nor by maternal DNA

Answer. (c)

Explanation:  An organism develops from zygote which in turn is product of fusion of male and female gamete.

Question. 17 Select the group which shares maximum number of common characters

(a) Two individuals of a species

(b) Two species of a genus

(c) Two genera of a family

(d) Two genera of two families

Answer. (a)

Explanation:  Species form the lowest taxonomic category and includes all the organisms that share maximum number of common characters. While moving up in taxonomic hierarchy, number of shared characters get reduced.  

Question. 18 According to the evolutionary theory, formation of a new species is generally due to

(a) Sudden creation by nature

(b) Accumulation of variations over several generations

(c) Clones formed during asexual reproduction

(d) Movement of individuals from one habitat to another

Answer. (b)

Explanation: Accumulation of variations over several generations serve as raw material on which evolutionary force (natural selection) acts upon to create new species.

Question. 19 From the list given below, select the character which can be acquired but not inherited

(a) Colour of eye

(b) Colour of skin

(c) Size of body

(d) Nature of hair

Answer. (c)

Explanation: Environmentally determined traits are acquired. Size of body depends on nutritional status, health conditions etc.

Question. 20 The two versions of a trait (character) which are brought in by the male and female gametes are situated on

(a) Copies of the same chromosome

(b) Two different chromosomes

(c) Sex chromosomes

(d) Any chromosome

Answer. (a)

Explanation: The two versions of a trait are situated on homologous chromosomes.  

Question. 21 Select the statements that describe characteristics of genes

(i) Genes are specific sequence of bases in a DNA molecule

(ii) A gene does not code for proteins

(iii) In individuals of a given species, a specific gene is located on a particular chromosome

(iv) Each chromosome has only one gene

            (a) (i) and (ii)

            (b) (i) and (iii)

            (c) (i) and (iv)

            (d) (iii) and (iv)

Answer. (b)

Explanation: Genes are stretches of DNA located on specific locus on a particular chromosome in individuals of a given species.

Question. 22 In peas, a pure tall plant (TT) is crossed with a short plant (tt). The ratio of pure tall plants to short plants in F₂ is

(a) 1 : 3

(b) 3 : 1

(c) 1 : 1

(d) 2:1

Answer. (b)

Explanation:  The monohybrid F₂ generation phenotypic ratio is 3 : 1.

Question. 23 The number of pair(s) of sex chromosomes in the zygote of humans is

(a) One

(b) Two

(c) Three

(d) Four

Answer. (a)

Explanation: Zygote is diploid structure and has one pair of sex chromosome (XX or XY).

Question. 24 The theory of evolution of species by natural selection was given by

(a) Mendel

(b) Darwin

(c) Morgan

(d) Lamarck

Answer. (b)

Explanation: Darwin proposed the theory of natural selection to explain the process of evolution.

Question. 25 Some dinosaurs had feathers although they could not fly but birds have feathers that help them to fly. In the context of evolution this means that

(a) Reptiles have evolved from birds

(b) There is no evolutionary connection between reptiles and birds

(c) Feathers are homologous structures in both the organisms

(d) Birds have evolved from reptiles

Answer. (d)

Explanation:  Development of flight feathers in reptilian ancestors of birds lead to evolution of birds (Aves).

Short Answer Questions

Question. 26 How is the sex of a new born determined in humans?

Answer.

Humans follow XX- XY mechanism of sex determination. Human females have forty-six chromosomes arranged in twenty two homologous and homomorphic autosomes and two copies of X chromosomes as sex chromosomes (44+XX). Males have twenty-two homomorphic pairs and one heteromorphic pair, the XY pair (44+XY). During meiosis, all the gametes produced by females contain only the X chromosome (22 A + X), whereas males produce two kinds of gametes, 1/2 gametes with X- and 1/2 with Y-chromosome (22 A + X and 22 A + Y). Fertilization of egg (22 + X) with sperm carrying 22+X chromosomes results in female child (44A + XX). Fertilization of egg (22 + X) with sperm carrying 22+Y chromosomes results in male child (44A + XY).

Question. 27 Do genetic combination of mothers play a significant role in determining the sex of a new born?

Answer.

Humans follow XX- XY mechanism of sex determination. Human females have forty-six chromosomes arranged in twenty two homologous and homomorphic autosomes and two copies of X chromosomes as sex chromosomes (44+XX). They do not play significant role in sex determination of child.

Males have twenty-two homomorphic pairs and one heteromorphic pair, the XY pair (44+XY). Fertilization of egg (22 + X) with sperm carrying 22+X chromosomes results in female child (44A + XX). Fertilization of egg (22 + X) with sperm carrying 22+Y chromosomes results in male child (44A + XY). Therefore, presence or absence of Y chromosome male gamete determines the sex of human child.

Question. 28 Mention three important features of fossils which help in the study of evolution.

Answer.

Fossils help in the study of evolution in following ways:

(i) Fossils exhibit mode of preservation of ancient species.

(ii) Fossils establish evolutionary relationship between organisms and their ancestors.

(iii) Fossils establish the geological time period in which organisms existed.

Question. 29 Why do all the gametes formed in human females have an X-chromosome?

Answer.

Human females have forty-six chromosomes arranged in twenty two homologous and homomorphic autosomes and two copies of X chromosomes as sex chromosomes (44+XX). Therefore, during meiosis, all the gametes produced by females contain only the X chromosome (22 A + X).

Question. 30 In human beings, the statistical probability of getting either a male or female child is 50 : 50. Give a suitable explanation.

Answer.

Humans follow XX- XY mechanism of sex determination. Human females have forty-six chromosomes arranged in twenty two homologous and homomorphic autosomes and two copies of X chromosomes as sex chromosomes (44+XX). Males have twenty-two homomorphic pairs and one heteromorphic pair, the XY pair (44+XY). During meiosis, all the gametes produced by females contain only the X chromosome (22 A + X), whereas males produce two kinds of gametes, 1/2 gametes with X- and 1/2 with Y-chromosome (22 A + X and 22 A + Y). Therefore, possibility of fertilization of egg (22 + X) with sperm carrying 22+X chromosomes and hence chances of having a female child (44A + XX) is 50%.  Likewise, there is 50% possibility that egg (22 + X) is fertilized with sperm carrying 22+Y chromosomes to have male child (44A + XY).

Question. 31 A very small population of a species faces a greater threat of extinction than a larger population. Provide a suitable genetic explanation.

Answer.

Smaller populations are at greater threat of extinction as compared to large species. This is because less number of individual in smaller population imparts lesser changes in production of genetic variations. Lack of genetic diversity makes the smaller species more prone to extinction under any dramatic changes in prevailing environmental conditions.

On the other hand, large populations exhibit more genetic diversity on which evolutionary forces act upon to make it withstand any changes in surrounding.

Question. 32 What are homologous structures? Give an example. Is it necessary that homologous structures always have a common ancestor?

Answer.

Homologous organs perform different functions and have different appearance but share common basic structural plan and origin. Divergent evolution refers to evolution of organisms in different directions from a common ancestry and leads to development of homologous organs. For example, forelimbs of reptiles, amphibians, birds and mammals.

Yes, homologous structures are inherited from a common ancestor. In the given example, the basic structure of forelimb of mammals, birds, reptiles and amphibians is similar but they are modified to perform different functions in various vertebrates. Thus, showing they have evolved from a common ancestor.

Question. 33 Does the occurrence of diversity of animals on earth suggest their diverse ancestry also? Discuss this point in the light of evolution.

Answer.

Animals on earth exhibit vast diversity in structure and function which in turn suggest their origin from diverse ancestors. For example, Reptiles, birds and mammals exhibit vast diversity and seem to have evolved from different ancestors.

However, detailed analysis of characters of these three groups reveals their common ancestry as all of them exhibit amniotic eggs. Presence of fluid filled sac (amnion) around the embryo during its development is key feature of amniotes.  Amniotes includes three groups namely Reptilia, Aves and Mammalia. An approach to divide and sub divide each group of animals and try to find out their common characters, all diverse group seem to have evolved from a common ancestor. Diverse habitat of these groups caused them to developed different characters and to look diverse from each other as we see them today.

It can be concluded that all animals groups originated on earth from common  ancestors and exhibited divergent evolution that in turn evolve them into diverse groups.

Question. 34 Give the pair of contrasting traits of the following characters in pea plant and mention which is dominant and recessive

(i) Yellow seed

(ii) Round seed

Answer.

 (i) Seed color: yellow (Dominant); green (Recessive)

(ii) Seed shape: rounds (Dominant); wrinkled (Recessive).

Question. 35 Why did Mendel choose pea plant for his experiments?

Answer.

Mendel studied the common garden pea plant, (Pisum sativum) because

• It was easy to cultivate and had a relatively short life cycle of 3 months.
• The plant exhibited discontinuous characteristics such as flower color and pea texture.
• It was easy to control the self pollination of the plant and cross-fertilization between desired parents could be accomplished artificially.
• Presence of pure breeding varieties and easily visible contrasting characters

Question. 36 A woman has only daughters. Analyze the situation genetically and provide a suitable explanation.

Answer.

Humans follow XX- XY mechanism of sex determination. Human females have forty-six chromosomes arranged in twenty two homologous and homomorphic autosomes and two copies of X chromosomes as sex chromosomes (44+XX). They do not play significant role in sex determination of child.

Males have twenty-two homomorphic pairs and one heteromorphic pair, the XY pair (44+XY). Fertilization of egg (22 + X) with sperm carrying 22+X chromosomes gives female child (44A + XX). Fertilization of egg (22 + X) with sperm carrying 22+Y chromosomes gives male child (44A + XY). Only daughters indicate fusion of egg with sperms carrying 22+X chromosomes every time.

Long Answer Questions

Question. 37 Does geographical isolation of individuals of a species lead to formation of a new species? Provide a suitable explanation.

Answer.

Yes, geographical isolation of individuals of a species leads to speciation. Splitting of existing species into two new species followed by their geographical isolation brings about reproductive isolation. Populations of a species get separated into two groups by accumulation of genetic variations in them. These newly formed groups are then isolated by any geographical barrier.

Reproductive isolation is absence of interbreeding between members of different species. Geographical barriers prevent interbreeding between two groups. This in turn causes isolation of gene pool of section of population from rest; inhibits gene flow (spread of genes between populations) and thereby leads to formation of new species.

Question. 38 Bacteria have a simpler body plan when compared with human beings. Does it mean that human beings are more evolved than bacteria? Provide a suitable explanation.

Answer.

Evolution is the process of accumulation of adaptive changes in population’s genetic makeup through successive generations. Mutation in gene pool of population leads to introduction of new / altered traits. Now, natural selection acts on these traits and favor only those traits which makes the organism more adapted to the prevailing climatic conditions. It can be said that only those mutations which give survival and reproductive advantage to organisms are favored by natural selection. Increased number of organisms having these favored traits leads to evolution of species with time.

Evolution is not related to complexity of body structure. It is not acceptable to say that all complex organisms are more evolved and all simpler organisms are less evolved. Dinosaurs were much more complex organisms which got disappear followed by appearance of birds on earth (organism with less complex body structure as compared to dinosaurs). Although bacteria are simpler organisms as compared to human but have evolved with time that enable them to withstand extremes of climatic conditions (thermophillic bacteria, halophiles etc) and to develop many adaptive traits to survive under harsh conditions (antibiotic resistance). Thus, evolutionary process does not prove the bacteria being less evolved organisms as compared to human.

Question.39 All the human races like Africans, Asians, Europeans, Americans and others might have evolved from a common ancestor. Provide a few evidences in support of this view.

Answer. 

Human races are anthropogenic concept; it has no roots in biology. Body plan, morphology, anatomy, physiology and metabolism is common to all human. They are share constant chromosome number (23 pairs) and same genetic as almost 99.9% DNA is same in all human races. And also all the human race can freely inter-breed to produce fertile offspring.

Evolutionary events also indicate origin of human from a common African ancestor. The earliest members of the human species, Homo sapiens can be traced there. Taking into account of all these evidences, all human races seem to have common ancestor.

Question. 40 Differentiate between inherited and acquired characters. Give one example for each type.

Answer.

Differences between inherited and acquired characters are:

S. No.	Inherited characters	Acquired characters
1.	Characters that are determined by genetic makeup of organisms are called as inherited characters.	Characters that are determined by environmental factors alone and genetic makeup of organisms does not play any role are called as acquired characters.
2.	These characters are passed from parents to offspring during the process of reproduction.	These characters remain confined to the individual only and are never passed from parents to offspring.
3.	Any changes in genetic makeup of an organism also alter the inherited characters.	Any changes in genetic makeup of an organism do not alter these characters.
4.	For example: Eye colour, seed colour, height	For example: Body weight, loss of finger in an accident

Question. 41 Give reasons why acquired characters are not inherited.

Answer.

Characters that are determined by environmental factors alone and genetic makeup of organisms does not play any role are called as acquired characters. These characters are developed during lifetime of an organism. Since genetic constitution does not determine these traits, they remain confined to the individual only and are never passed from parents to offspring.

For example: Body size of an individual is determined by health status, nutrition, presence or absence of any disease, severity of disease, physical activity etc. Genes do not determine the size of body as it is developed by above mentioned environmental factors during lifetime of the individual.

Question. 42 Evolution has exhibited a greater stability of molecular structure when compared with morphological structures. Comment on the statement and justify your opinion.

Answer.

Evolution is the process of accumulation of adaptive changes in population’s genetic makeup through successive generations. Mutation in gene pool of population leads to introduction of new / altered morphological structures. Now, natural selection acts on modified structures and favor only those which makes the organism more adapted to the prevailing climatic conditions. Therefore, with passage of time, evolution leads to changes in morphology of organisms to impart them the survival and reproductive advantage. For example, evolution of homologous, analogous and vestigial organs.
On the other hand, biomolecules (lipids, proteins, DNA and RNA) have maintained a comparatively stable structure through evolution. The basic scaffold of these molecules has not been changed. DNA in all organisms is made up of pentose sugar, phosphate moiety and nitrogenous bases. Organisms differ only in the sequence of nitrogenous bases which in turn impart different forms of particular trait to them. Otherwise, biomolecules have maintained a stable structure.

Question. 43 In the following crosses write the characteristics of the progeny Cross.

Answer.

(a) A cross between two pure breeding dominant parents gives pure breeding dominant offspring. All progeny in this cross will have genotype RRYY and will exhibit round and yellow phenotype.

(b) A cross between two dihybrid dominant recessive parents gives phenotypic ratio of 9: 3: 3:1 in offspring.

Progeny – Round yellow : Round green : Wrinkled yellow : Wrinkled green = 9 : 3 : 3 : 1

(c) A cross between two pure breeding recessive parents gives pure breeding recessive offspring. All progeny in this cross will have genotype “rryy” and will exhibit wrinkled and green phenotype.

(d) A cross between two pure breeding dominant and recessive parents gives heterozygous dominant offspring. All progeny in this cross will have genotype RrYy and will exhibit round and yellow phenotype.

Question. 44 Study the following cross showing self pollination in F₁. Fill in the blank:

Answer.

A dihybrid cross shows inheritance of two different traits simultaneously. A cross between two pure breeding dominant and recessive parents gives heterozygous dominant offspring. All F₁ progeny in this cross will have genotype RrYy and will exhibit round and yellow phenotype. Self cross in F₁ progeny gives F₂ generation and follow law of independent assortment of characters. 

RrYy                   ×                  RrYy

Round yellow                          Round yellow

Question. 45 In Question 44, what are the combination of characters in the F₂ progeny? What are their ratios?

Answer.

A dihybrid cross shows inheritance of two different traits simultaneously. A cross between two pure breeding dominant and recessive parents gives heterozygous dominant offspring. All F₁ progeny in this cross will have genotype RrYy and will exhibit round and yellow phenotype. Self cross in F₁ progeny gives F₂ generation and follow law of independent assortment of characters. 

Combination of characters:

Phenotypic ratio= 9 round yellow, 3 round green, 3 wrinkled yellow, 1 wrinkled green. So, F₂ generation exhibits both parental and recombinant offspring.  Hence, the ratio is = 9 : 3 : 3 : 1

Question. 46 Give the basic features of the mechanism of inheritance.

Answer.

Transmission of genetic characteristic from one generation to the next is termed as inheritance. The basic features of mechanism of inheritance are following:

(i) All genetic characters are controlled by genes. There is atleast one gene for each trait.

 (ii) Each gene has two or more alternating forms called as alleles. One of the allele is dominant while other is recessive.

(iii) Genes are linearly arranged on chromosomes.

(iv) Each parent possesses a pair of genes for each characteristic on a chromosome.

(v) Meiosis during gamete formation reduces the chromosome number to half in gametes and fusion of male and female gametes during fertilization restore the normal chromosome number. This fusion also creates new combination of genes in zygote as zygote has half chromosome set from father and half from mother.

(vi) In this way, offspring inherit a pair of genes for each trait from its parents.

Question. 47 Give reasons for the appearance of new combination of characters in the F₂ progeny in question 45.

Answer.

F1 plants have round and yellow seeds. Cron between generations, they give rise to new combinations in F2 generation with round-yellow, round green, wrinkled yellow and wrinkled green in the ratio of 9 : 3 : 3 : 1.

This indicates that the chances for the pea seed to be round or wrinkled do not depend on their chances to be yellow or green. Each pair of alleles is independent of the other pair. This is known as principal of independent assortment.