SAQ for Heredity and Evolution Class 10 Science NCERT
Important Questions1
Answer
Mendel conducted breeding experiences with garden peas:
(i) He studied (pure) plants of a tall/short varieties
(ii) He crossed them and obtained F1 progeny.
(iii) He found that F progenies were all tall plants.
(iv) He selfed the (hybrid) plants of progeny.
(v) He found that in F2 progeny there were tall as well visible contrasting character as short plants.
(vi) The three quarter plants were tall and one quarter were short.
2
Answer
(i) When a cross was made between a tall pea plant with round seeds and a short pea plant with wrinkled seeds, the F1 progeny plants are all tall with round seeds. This indicates that tallness and round seeds are the dominant traits.
(ii) When the F1 plants are self-pollinated, the F2 progeny consist of some tall plants with round seeds and some short plants with wrinkled seeds which are the parental traits.
(iii) There were also some new combinations like tall plants with wrinkled seeds and short plants with round seeds.
(iv) Thus, it may be concluded that tall and short traits and round and wrinkled seed traits have been inherited independently.
Note:
Genotype: 9:3:3:1
9 = Tall Round
3 = Tall Wrinkled
3 = Short Round
1 = Short Wrinkled
3
If we cross-bred tall (dominant) pea plant with pure-bred dwarf (recessive) pea plant, we will get plants of F1 generation, we obtain pea plants of F2 generation.
(i) What do the plants of F1 generation look like?
(ii) State the ratio of tall plants to dwarf plants in F2 generation.
(iii) State the type of plants not found in F1 generation but appeared in F2 generation. Write the reason for the same.
Answer
(i) Tall
(ii) 3: 1
(iii) Dwarf
Reason: Being a recessive trait, dwarfness can only be expressed in the recessive homozygous condition or in the absence of dominant trait.
4
Answer
75% of the plants were with round seeds.
5
Answer
6
Answer
DNA replication or DNA copying is the process of producing two identical replicas from one original DNA molecule during cell division.
Importance of DNA Copying:
(i) DNA replication needs to occur so that during cell division, new cells will also have a copy of organism's DNA.
(ii) DNA is necessary to make all the RNA and proteins needed for cells to carry out necessary reactions and cellular processes in order to survive.
7
'It is possible that a trait is inherited but may not be expressed.' Give a suitable example to justify this statement.
OR
'A trait may be inherited, but may not be expressed.' Justify this statement with the help of a suitable example.
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 traits in F2 generation shows that the dwarf trait/character was present in individuals of F1 but it did not not expressed (due to the presence of tallness, a dominant trait/character).
8
Answer
Mendel conducted a dihybrid cross; and observed that though he started with two types of parents, he obtained four types of individuals in F2. The appearance of new recombination in F2 generations along with parental type characters shows that traits are inherited independently.9
(i) Name the unit of inheritance. What is its functions?
(ii) How are inherited traits different from acquired traits? Give example.
Answer
(i) Gene.
It is the carrier of genetic information from one generation to another.
(ii) The traits that are obtained from parents are inherited traits.
e.g., Fused and free ear lobes.
The traits that develop during lifetime of an individual are acquired traits.
10
Answer
11
(i) Define Genetics.
(ii) Who is regarded as the 'Father of Genetics'? Name the plant on which he performed his experiment.
(iii) Why did he selected that specific plant for his experiments?
Answer
(i) Genetics is the branch of biology that deals with the study of heredity and variations
(ii) Gregor Johann Mendel, garden pea.
(iii) Garden pea plants were easily available / they grow in one season/fertilisation was easy.
12
In one of his experiments with pea plants Mendel observed that when a pure tall pea plant is crossed with a pure dwarf pea plants, in the first generation, F, only tall plants appear.
(i) What happens to the traits of the dwarf plants in this case?
(ii) When the F, generation plants were self-fertilised, he observed that in the plants of second generation, F., both tall plants and dwarf plants were present. Why it happened ? Explain briefly.
Answer
(i)Â 
The dwarf traits of the plants is not expressed due to the presence of the dominant tall trait.
Tall:Dwarf = 3 : 1 ratio
(ii) In the F2 generation, both the tall and dwarf traits are present in the ratio of 3 : 1. This showed that the traits for tallness and dwarfness are present in the F1 generation, but the dwarfness, being the recessive trait, does not express itself in the presence of tallness, the dominant trait.
13
Answer
Mendel conducted breeding experiments on Pea plants.
(i) He selected pure breed tall and dwarf plants.
(ii) He cross-pollinated these plants.
(iii) In the F1 generation, he obtained only tall plants. Tallness is the dominant trait.
(iv) Then, he produced F2 generation by self-cross of hybrids/F1.
(v) He found that 3/4th of the plants were tall and 1/4th were dwarf.
(vi) The trait which remains hidden in F1 generation plants is the recessive traits.
14
Answer
Yes, the scientist may arrive at the law of dominance according to which the trait that is expressed in the F1 generation is the dominant trait, although both the dominant and recessive traits are present in the F1 generation. In the F2 generation the recessive traits is also expressed along with dominant traits.15
Answer
Environmental Cue :
(i) In some animals, the temperature at which fertilised eggs are kept determines whether the developing anima in egg is male or female.
(ii) In some animals like snail, individual can change sex.
Genetical Cue: A child who inherits an X-chromosome from her father will be a girl and one who inherits a Y-chromosome from the father will be boy.
16
A pea plant with blue colour flower denoted by BB is cross-bred with a pea plant with white flower denoted by ww.
(i) What is the expected colour of the flowers in their F1 progeny?
(ii) What will be the percentage of plants bearing white flower in F2 generation, when the flowers of F1 plants were selfed?
(iii) State the expected ratio of the genotype BB and BW in the F2 progeny.
Answer
(i) All the flowers in F1 generation will have dominant blue colour.
(ii) When F1 plants were selfed, the percentage of white flower in F2 generation will be 25%.
(iii) Genotype of BB and BW in the F2 progeny are 1:2.
17
A cross was made between pure breeding pea plants one with round and green seeds and the other with wrinkled and yellow seeds.
(i) Write the phenotype of F1 progeny. Give reason for your answer.
(ii) Write the different types of F2 progeny obtained along with their ratio when F1 progeny was self-crossed.
Answer
(i) The cross was made between round shape is dominant over green colour and wrinkled-yellow seeds.
In the given cross, two traits were taken into account, which is a dihybrid cross.
Yellow colour and round shape is dominant over green colour and wrinkled shape.
The above cross shows round and yellow seeds in the F1 generation. It occurs because dominant traits (round and yellow) express itself, whereas recessive traits (wrinkled and green) get suppressed.
(ii) On selfing of F1 generation:
Round yellow : Wrinkled yellow : Round green : Wrinkled green
Phenotypic ratio â 9:3:3:1
Round yellow â 9
Round green â 3
Wrinkled yellow â 3
Wrinkled green â 1
18
(a) Mention crossed tall pea plants with dwarf pea plants in his experiment. Write his observation giving reason on the F1 and F2 generations.
(b) List any two contrasting characters other than height that Mendel used in his experiments in pea plants.
Answer
(a) Mendel Crossed tall pea plants with dwarf pea plants.
Mendelâs Observation:
The F1 generation contained all tall plants due to principle of dominance. When F1 generation underwent selfing, the trait that was unexpressed in F1 (dwarf) was observed in some F2 progeny. Thus, both traits, tall and dwarf, although present in F1 hybrid (Tt) were segregated and expressed in F2 generation in the ratio of 3:1.
Mendel proposed that something was being passed unchanged from generation to generation. He called these things factors (presently called genes). Factors contain and carry hereditary information. He also observed that traits might not show up in an individual but were passed to the next generation.
(b) The two contrasting traits other than height used by Mendel were:
(i) Pod colour: Green pod colour dominant over yellow colour
(ii) Seed shape: Round seed shape dominant over wrinkled seed shape.
19
A cross was carried out between a pure bred tall pea plant and a pure bred dwarf pea plant and F1 progeny was obtained. Later, the F1 progeny was self-crossed to obtain F2 progeny. Answer the following questions:
(i) What is the phenotype of the F1 progeny and why?
(ii) Give the phenotypic ratio of the F1 progeny.
(iii) Why is the F2 progeny different from the progeny ?
Answer
(i) The phenotype of F1 progeny is tall pea plant (Tt) because it contained a factor/gene for tallness which is a dominant factor.
(ii) The phenotypic ratio of F1 progeny is 3:1.
(iii) F1 progeny resembled only one parent which is dominant in nature but, in F2 progeny, phenotypically three plants are tall and one plant is dwarf. But according to genotypic ratio, one plant is pure tall, two are hybrid tall and one plant is pure dwarf.
20
The genotype of green-stemmed tomato plants is denoted by CG and that of purple-stemmed tomato plants as egg. When these two plants are crossed :
(i) What colour of stem would you expect in their F1 progeny?
(ii) Give the percentage of purple-stemmed plant if F2 plants are self-pollinated.
(iii) In what ratio would you find the green and purple colour in the F1 progeny ?
Answer
(i) Green colour
(ii) 25%
(iii) 3:1
21
Answer
The appearance of all Tall plants in the F1 generation shows that tallness is the dominant character while dwarfness is the recessive character.
The ratio of Tall to Dwarf in F2 generation is 3:1:1
22
Answer
Such traits are called acquired traits are experienced an individual during his lifeline. These traits involve changes in non-reproductive cells (somatic cells) which are not transferred to germ cells. So, these traits cannot be passed on to the next generation.
E.g., Learning skills like swimming, dancing, cooking, body building, etc., are acquired traits and cannot be passed on to the next generation.
23
Answer
24
Answer
Chromosomes are long thread-like structures which contain hereditary information of the individual and are thereby the carriers of genes and hereditary characteristics.
Male individual have 46 chromosomes, but because the gametes are always haploid, i.e., they have half the number of chromosomes; sperms will be haploid (23 chromosomes). Female individual also contains 23 chromosomes in egg. It is the fusion of the sperm and egg which leads to an offspring with 46 chromosomes.
25
Answer
Sex of a child is dependent on the type of the male gamete that fuses with female gamete. Human beings possess 23 pairs of chromosomes. Out of these, 22 pairs are known as autosomes, while the remaining one pair comprises sex chromosomes (XX in females and XY in males). At the time of fertilization. The egg cell fuses with the sperm cell, resulting in the formation of the zygote. If the egg cell carrying an X chromosomes fuses with the sperm carrying an X chromosome fuses with the sperm carrying a Y chromosome, the resulting child would be a boy.
26
Answer
A child who inherits a X chromosome from his father would be a girl (XX) while a child who inherits a Y chromosome from the father would be a boy (XY).
Since, the sex of the child in determined by what he/she inherit from father, therefore the probability of getting male or female child is 50%.
27
Answer
Sex of a child depends on what happens during fertilization.
(i) If a sperm carrying X-chromosome fertilises an ovum which carries X-chromosome, Then the child born will be a girl.
(ii) If a sperm carrying Y-chromosome fertilises an ovum which Carries X- chromosome, then the child born will be boy.
Thus, sperm (from father) determines the sex of the child.
28
Answer
(i) Speciation: Origin of new species from pre-existing ones.
Role of Genetic drift and natural selection: Genetic drift is flow genes from one population to another by chance factor or randomly. Over generation, it will accumulate different changes in different population.
In addition, natural selection operates differently in different population selecting the fittest/favourable feature in both the population.
Over a long period of time, the differences in the two population may become so drastic that they no longer reproduce with each other and thus, give rise to new species.
29
Answer
Speciation is the evolution of reproductive isolation among once interbreeding population.
Factors which can lead to speciation are:
(i) Over generation, genetic drift may accumulate which lead to speciation.
(ii) Natural selection may work differently in different location which may give rise to speciation.
âą Severe DNA change
âą A variation may occur which does not allow sexual act between two groups.
Detailed Answer:
Speciation: Speciation is the formation of new and distinct species in the course of evolution. It involves the splitting of a single evolutionary lineage into two or more genetically independent lineages.
30
Explain the following:
(a) Speciation
(b) Natural Selection
Answer
(a) Speciation: It is the evolution of reproductive isolation among once interbreeding population.
(b) Natural Selection:
(i) There is natural variation within any population and same individuals have more favourable variations than others.
(ii) Population remains fairly constant even though all species produce a large number of offsprings.
(iii) This is due to 'competition' or 'struggle for existence between same and different species.
(iv) The struggle for survival within population eliminates the unfit individuals and those with 'favourable variations survive and pass on these variations to their progeny to continue. This is called natural selection. The favourable variations are accumulated over a long time period leading to the origin of a new species.
31
Answer
(a) Natural selection is defined as the change in frequency of some genes in a population, which gives survival advantage to a species.
(b) Whereas speciation is the development of a new species from pre-existing ones.
(c) This leads to a sequence of gradual change in the primitive organisms over millions of years, to form newer species which are very different from older ones. This is called evolution.
32
Answer
Characters that a person acquires during oneâs life time are known as acquired characters/traits.
Such changes do not occur in the reproductive tissues.
Changes in the non-reproductive tissues are not passed on to the DNA of the germ cells and therefore not inherited by the next generation.
33
Answer
Factors responsible for speciation are:
(i) Gene flow must be stopped so that sub-populations become genetically isolated.
(ii) Mutation plays a role in speciation. Changes in allele and genotype frequencies within the populations, by natural selection.
(iii) Genetic isolation leads to the formation of new species.
(iv) Geographical barriers such as mountain ranges, seas or rivers may produce a barrier to gene flow and the inability of organisms or their gametes to meet, leads to reproductive isolation and hence speciation. Sometimes crossing of two unrelated species (genetic engineering) can also lead to speciation.
34
âEvolution and Classification of organisms are interlinked.â Give reasons to justify this statement.
OR
âTwo areas of study namely âevolutionâ and classificationâ are interlinkedâ. Justify this statement.
Answer
Classification is the reflection of evolutionary relationships between organisms.
More the two organisms are related to each other, more characteristics they have in common.
More characteristics they have in common, more recently they have common ancestor like a girl & her real brother.
Similarities between organisms allow us to group (classify) them & study these groups to determine how these organisms are evolutionary related.
35
Answer
(a) Evolution: Gradual accumulation of variations and its selection by nature leading to formations of new species.
(b) Variations may arise in a population due to mutations or sexual reproduction. Sub-populations are formed due to genetic drift and geographical isolation. When natural selection acts on them, most suitable variations survives leading to evolution of a new species.
(c) Fossils provide missing link between the species/ who has evolved from whom. They provide information about prehistoric organisms.
36
What is organic evolution? It cannot be equated with progress. Explain with the help of a suitable example.
OR
âEvolution should not be equated with progressâ.
Answer
Organic evolution is the process or event of change in an organism by which descendants differ from their ancestors.
Evolution cannot be equated to progress. From lower forms to higher forms, it gives rise to more complex body designs even while the simpler body designs continue to flourish. For example, human beings have not evolved from chimpanzees but both have common ancestor.
37
Answer
Creating a new copy of DNA in a reproducing cell is known as DNA copying.
Importance: Since DNA is the carrier of the blue print of the genetic characters, its copying is essential to pass on this blue print to the offsprings.
38
Answer
Homologous Organs: Organs with similar basic structure/origin but modified to perform different functions.
Example: Forelimbs of various vertebrates.
Wings of butterfly and the wings of a bat cannot be regarded as homologous organs.
Reasons: Though, they perform the similar function, they have different origin/basic structure.
39
(i) What are fossils ? How do we know how old the fossils are?
(ii) State two differences between homologous and analogous organs.
Answer
(i) Fossils : The remains and/or impressions of organisms that lived in the past.
(a) The fossils that we find close to the surface are more recent than the one we find in deeper layers.
(b) Dating fossils by detecting the ratio of different isotopes of the same elements.
(ii) (a) Homologous organs have same basic structural design.
(b) Analogous organs have different basic structural design.
(c) Homologous organs perform different functions.
(d) Analogous organs perform similar functions.
40
(i) Which of the following fossil is invertebrate and which one is vertebrate ?
(a) Dinosaur,
(b) Ammonite.
(ii) How can the age of fossil be ascertained? State in brief any two methods.
Answer
(i) (a) Vertebrate,
(b) Invertebrate.
(ii) If we dig into a rock/earth, then it is reasonable to suppose that the fossil found in the upper layers must be of more recent origin than the fossil material.
41
Answer
Formation : On certain occasions, a dead body or at least some parts may be in an environment that does not let it decompose completely and gets preserved, subsequently either the part or its impressions becomes a fossil.
Role of Fossil: Provides missing link between the species/who has evolved from whom.
They tell us about prehistoric organisms.
42
Answer
Study of homologous organs as forelimbs of mammals, birds, reptile and amphibian; show that though they perform different functions have similar basic/ internal structure; this is because they have evolved from common ancestor and help us in determining the closeness between two species in evolutionary terms.43
Homologous organs are different from analogous organs.
(i) Mention the two basic characteristics that decide about analogy and homology between the two organs.
(ii) On what basis is the classification of organisms into prokaryotic and eukaryotic done ?
Answer
(i) The two basic characteristics that decide about analogy and homology are origin, structures and function.
(ii) In prokaryotic organism, no true nucleus is found. Therefore, nucleus is absent.
In eukaryotic organisms, true nucleus is found.
44
Answer
45
Answer
(i) Freely interbreeding, producing fertile offspring.
(ii) Same number of chromosomes â 23 pairs.
(iii) Common body design, anatomy and physiology.
46
Answer
Evidence of evolution are:
(i) Analogous organs: Organisms with similar looking organs may have different origin.
(ii) Homologous organs: Organisms with apparently different looking organs may have similar origin.
(iii) Fossils: Allow us to make estimates of how far back evolutionary relationships goes. Fossils when chronologically arranged help in tracing the evolutionary history of an organism.
47
(i) Identify the fossils A and B?
(ii) What type of fossils are these?
Answer
(i) (A) Ammonite, (B) Trilobite
(ii) Invertebrate.
48
(a) Cite the evidence on the basis of which it is concluded that birds have evolved from reptiles.
(b) Insects, Octopus, Planaria and Vertebrates also possess eyes. Can these animals be grouped together on the basis of the eyes they possess? Why or why not? Give reason to justify your answer
Answer
(a) (i) Fossils showing imprints of feathers along with the bones in dinosaur reptiles found.
(ii) They could not fly and presumably use the feathers for insulation.
(iii) Later they developed/evolved and adapted feathers for flight.
(iv) Thus, they give evidence that birds have evolved from reptiles.
(b) Yes, eyes can be grouped together, which have evolved over generation from imperfect eyes in Planaria to perfect eyes in vertebrates.
49
Answer
Useful variations give advantages to individuals in obtaining more food, reproduction, adaptation to environmental changes and higher success in the struggle for existence. They give benefit in survival and increasing the population Differential reproduction increases the useful variations in the populations. Other individuals with harmful variations will be destroyed. For example, some bacteria have ability to tolerate high temperature. But other non-resistant bacteria will be killed.49
(i) Planaria, insects, octopus and vertebrates all have eyes. Can we group eyes of these animals together to establish a common evolutionary origin? Justify your answer.
(ii) "Birds have evolved from reptiles". State evidence to prove the statement
Answer
(i) Yes, eyes can be grouped together, which have evolved over generation from imperfect eyes in Planaria to perfect eyes in vertebrates.
(ii) Dinosaur is a type of reptile which has wings. Birds also have wings, so it can be proved that birds have evolved from reptiles.
51
The modern human beings have originated in Africa.
(ii) Which evidence suggests this facts?
(iii) If an animal is similar to its ancestors, what does this imply?
Answer
(i) This was shown by the help of fossils records of Homo erectus, according to which their cranial capacity had doubled. Homo erectus were the first to leave Africa and spread through Africa, Asia, and Europe. One population of H. erectus, stayed in Africa and evolved into Homo sapiens.
(ii) This implies that form the beginning of life on earth, the inheritance of trait is going on. This has resulted in the production of existing animals that look similar to their ancestors because they have inherited traits from them and these trait that make them similar to their ancestors have not gone under evolutionary change.
52
Answer
(i) Classification is the reflection of evolutionary relationships between organisms.
(ii) More the two organisms are related to each other, more characteristics they have in common.
(iii) More characteristics they have in common, more recently they have common ancestor like a girl & her real brother.
(iv) Similarities between organisms allow us to group (classify) them & study these groups to determine how these organisms are evolutionary related.
53
Answer
(i) If any natural calamity occurs and kills these small number of surviving tigers, they can become extinct resulting in the loss of genes forever.
(ii) Small number will lead to little recombination and lesser variations that are very important for giving better survival chances to the species.
(iii) Less number of species means lesser extent of diversity and lesser number of traits which reduce the chances of adaptability with respect to the change in the environment.
54
(i) A husband has 46 chromosomes, his wife has also 46 chromosomes. Then why donât their offsprings have 46 pairs of chromosomes, which is obtained by the fusion of male and female gametes?
(ii) âGeographical isolation is not a major factor in the speciation of a self-pollinating plant species.â Justify this statement with the help of an example.
Answer
(i) Male individual has 46 chromosomes but because the gametes are always haploid i.e., they have half the no. of chromosomes, sperms will be haploid (23 chromosomes). Female individual also contains only 23 chromosomes in egg. It is the fusion of this sperm and egg which leads to an offspring with 46 chromosomes.
(ii) Geographical isolation will not be a major factor in the speciation of a self-pollinating plant species. It involves the same flower/plant in which male and female gametes are formed.
55
(a) What is variation? How is variation created in a population? How does the creation of variation in a species promote survival?
(b) Explain how, offspring and parents of organisms reproducing sexually have the same number of chromosomes.
Answer
(a) Occurrence of differences between organisms is called variation.
New variation may arise during the process of DNA copying that already has variations accumulated from previous generations.
Combining variations from two or more individuals would thus create new combinations of variations.
Species having suitable variations have more chances of survival in case of change in environment conditions.
(b) In sexually reproducing organisms, male and female gametes/reproducing cells with only half the number of chromosomes (as in the parent cell) are produced. During fertilization, when male and female gametes fuse to give rise to a zygote, original number of chromosomes are restored.