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Heredity and Evolution - Class 10 Science Textbook Solution

 Heredity and Evolution
 Heredity and Evolution Class 10 Extra s with Answers Science Chapter 9
1. What is meant by characteristics?
It is the detail of appearance or behaviour; in other words, a particular form or a particular functions.
Example: Four limbs of human beings is a characteristic and that plant can perform photosynthesis is also a characteristic.
2. Who is known as the father of genetics?
Gregor Johann Mendel is known as the father of genetics.
3. Define chromosome.
Chromosome is a thread-like structure that bears genes and are enclosed within a nucleus. It is composed of DNA and protein.
4. Name the plant which was selected by Mendel for his experiments why he selected these plants?
Mendel opted garden pea (Pisum satirum) to conduct his experiment.
Advantages of Selecting pea plant:
1.    It is annual with short life cycle. So, several generations can be studied in short period.
2.    It contains bisexual flowers, which are self pollinating.
3.    It is easy to cross pollinate it.
5. Define a gene.
: Gene is a segment of a DNA molecule which carries the code for the synthesis of a specific protein.
6. Define alleles.
Genes which code for a pair of contrasting traits are known as alleles, i.e., they are the different forms of the same gene.
7. Write the expanded form of DNA.
Deoxyribonucleic acid.
8. Where is DNA found in a cell?
DNA is found in genes or chromosomes.
9. What is the function of a gene?
Gene is the carrier of the genetic information from one generation to the next.
10. Who proposed the theory of natural selection?
Charles Darwin proposed the theory of natural selection.
11. Which of the following traits are recessive in pea plant? Dwarfness, violet flower, wrinkled seed.
Dwarfness and wrinkled seeds are the recessive characters.
12. How many pairs of chromosomes are found in human beings?

23 pairs of chromosomes are found in human beings.
13. In humans, the gene for black hair colour is B and gene for brown hair colour is b. What will be the hair colour of person having the genetic constitution?
(a) BB
(b) bb
(c) Bb

(a) Black hair
(b) Brown hair
(c) Black hair (Heterozygous)
14. Write the names of two types of chromosomes found in an organism.
Autosomes and sex chromosomes.
15. How many chromosomes are present in a sperm and an ovum?
Sperm and ovum have 23 chromosomes each.
16. What is a sex chromosome?
The chromosomes which are associated with sex determination of an organism are called sex chromosomes.
17. Which sex chromosomes are found in male and female human beings?
Males have one X and one Y-chromosomes, whereas females have two X-chromosomes.
18. Who gave the theory of inheritance of acquired characters?
19. Define mutation.
Sudden changes in the genetic form of an organism which are passed on to the next generation are called mutations. Mutations lead to variations in an organism.
20. What are fossils?
Fossils are the remains or impressions of the dead animals and plants that lived in the past.
21. What is the significance of Archaeopteryx in evolution?
Archaeopteryx provides evidence of evolution of birds from reptiles and its study shows that:
1.    birds and reptiles had a common ancestor.
2.    birds have evolved from reptiles.
22. Why are acquired characters not inheritable?
Acquired characters are not inherited because they affect the somatic cells and these changes are not incorporated in the chromosomes of cells that will form germ cells.
23. What is genetic drift?
The elimination of the genes of certain traits when a section of a species population migrates or dies due to natural calamity. It alters the gene frequency of the remaining population.
24. How do we know how old a fossil is?
Age of fossils can be estimated by the depth of the layer of rocks in which they are found. Age of fossils can also be detected from the ratio of isotopes in the fossils containing rocks.
25. What is organic evolution?
Evolution is the sequence of gradual changes of the living organisms, which takes place in the primitive organisms over millions of years resulting in the formation of new species.
26. What are the basic events in evolution?
A gradual genetic change in a group of living beings to produce new forms brought about by changes in DNA during reproduction are the basic events in evolution.
27. When a cell reproduces, what happens to its DNA?
When a cell reproduces, copy of DNA is created.
28. Newly formed DNA copies may not be identical at times. Give one reason.
Newly formed DNA copies may not be identical at times if there is error or inaccuracies in DNA copying.
29. Why is variation important for a species?
Variation is important for species to survive.

Heredity and Evolution Extra s Short Answer Type
1. Why offsprings differ from parents in certain characters?
It is due to biparental percentage. The genes on chromosomes which pass over to the next generation is partly derived from both the parents (mother and father). During fertilisation of egg by the sperm, new combination of chromosomes enter the zygote, due to which certain variations occur in the offsprings. Thus, brothers and sisters show variations in their complexion habits and behaviour.
2. What are the causes of variations?
Following are the causes of variations:
1.    Dual percentage: Offsprings inherit some features from mother and some from father hence no offspring will exactly resemble to either of the parents or each other.
2.    Mutation in gene or chromosomal pattern also causes variations.
3. 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

(i). yellow – dominant
green – recessive
(ii) round – dominant
wrinkled – recessive
4. What is the contribution of Mendel to genetics?
Mendel observed the occurrence of contrasting characters of garden pea in various generations. On this basis, he interpreted that these contrasting characters are controlled by factors. He considered each and every character as a unit, which is controlled by a ‘factor’. Factors are carriers of hereditary information. Now, factors are known as genes…
5. Do genetic combination of mothers play a significant role in determining the sex of a newborn?
No, because mothers have a pair of X-chromosomes. All children will inherit an ‘X’ chromosome from their mother regardless of whether they are boys or girls.
6. How does use and disuse of an organ help in evolution of a new species?
According to Lamarck, those organs which are used regularly become strong and more developed. On the other hand, those organs which are not used regularly become weak and degenerate. Such characters are inherited to the offsprings and so on. In the due course of time, such characters become permanent in later generations and become quite different from their ancestors, resulting in the formation of a new species.
7. A very small population of a species faces a greater threat of extinction than a larger population. Provide a suitable genetic explanation.
Fewer individuals in a species impose extensive inbreeding among them. This limits the appearance of variations and puts the species at a disadvantage it there are changes in the environment. Since the individuals fail to cope up with the environmental changes, they may become extinct.
8. Does the occurrence of diversity of animals on earth suggest their diverse ancestry also? Discuss this point in the light of evolution.
Though animals have a vast diversity in structures they probably do not have a common ancestry, because common ancestry may greatly limit the extent of diversity. As many of these diverse animals are inhabiting the same habitat, their evolution by geographical isolation and speciation is also not likely. Thus, a common ancestry for all the animals is not the likely theory.
9. 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.
All human races have evolved from a common ancestor because everybody has:
1.    Common body design, structure, physiology and metabolism.
2.    Constant chromosome number.
3.    Common genetic blue print.
4.    Freely inter-breeding species.
10. A change in DNA that is useful for one property to start with, can become useful later for a different function. Explain.
A change/feature/property of an organism that may have helped in to adopt to an environmental condition can also become useful for a completely different function in the future. For example, feathers in birds, a character developed and selected during natural selection for providing insulation in cold weather, became useful in later stages for flight.
Some dinosaurs had feathers, but they could not fly. Birds later adapted the feathers to fly.
11. List two differences in tabular form between dominant trait and recessive trait. What percentage/proportion of the plants in the F, generation/progeny were round, in Mendel’s cross between round and wrinkled pea plants ?


75% of the plants were with round seeds.
12. How many pairs of chromosomes are present in human beings? Out of these how many are sex chromosomes? How many types of sex chromosomes are found in human beings?
23 pairs of chromosomes are present in human beings. One pair of these are sex chromosomes.
Two types of sex chromosomes are there:
XX and XY.
What is DNA copying? State its importance.
A process where a DNA molecule produces two similar copies of itself in a reproducing cell is called DNA copying
Its importance are:
•    It makes the transmission of characters from parents to the next generation possible.
•    It causes variation in the population.
14. “We cannot pass on to our progeny the experiences and qualifications earned during our life time.” Justify the statement giving reason and examples.
Or With the help of two suitable examples, explain why certain experiences and traits earned by people during their lifetime are not passed on to their next generations. When can such traits be passed on?
We acquire knowledge and skills in our lifetime such as learing dance, music, physical fitness, etc. But these skills cannot be passed to our progenies because:
1.    Such characters or experiences acquired during one’s lifetime do not bring any change in the DNA of the germ cell.
2.    Only germ cells are responsible for passing on the characters from the parents to the progemy. These traits can be passed to the next generation when the changes are in the DNA of the germ cell.
15. ‘Different species use different strategies to determine sex of a newborn individual. It can be environmental cues or genetically determined’. Explain the statement by giving example for each strategy.
Environmental cure: In some animals, the temperature at which fertilised eggs are kept determines whether the developing animal in egg is male or female. In some animals like snail, individual can change sex.
Genetic củe: A child who inherits an X chromosome from the father will be a girl and one who inherits a Y chromosome from the father will be a boy.:
16. List the two types of reproduction. Which one of the two is responsible for bringing in more variations in its progeny and how?
The two types of reproduction are sexual reproduction and asexual reproduction. Sexual reproduction is responsible for bringing in more variations because of the process of DNA copying which may result in some error in it. Also, it involves fusion of male and female gametes from two different parents.
17. How do variations occur in an offspring?
Dissimilarities between members of the same species is called variations. Two offsprings of the same parents show certain variations. Variations occur due to sex chromosomes. Variations arising in germplasm (genes) of the organism are heritable. Mother and father contribute to the gene pattern of the offsprings through their chromosomes, in which recombination occurs at the time of gametogenesis. In zygote formation, gene pattern of both parents come together, that causes some variations between parents and offsprings and amongst offsprings also.
18. Why is variation beneficial to the species but not necessary for the individual?
The importance of variation in organism introduced during reproduction is that it helps the species of various organisms to survive and flourish even in adverse environment. If all the organisms of a population living in that habitat are exactly identical, then there is a danger that all of them may die and no one would survive under those conditions.
This will eliminate the species from the habitat completely. However, if some variations are present in some individual organism to tolerate excessive heat or cold, then ther is a chance for them to survive and flourish even in adverse excessive heat or cold. Thus, variation is useful for the survival of a species over time.
19. What is speciation ? List four factors responsible for speciation.
Speciation is the formation of new species from the pre-existing population. Factors responsible for speciation:
1.    Genetic drift
2.    Natural selection
3.    Geographical isolation
4.    Mutation
20. List in tabular form, two distinguishing features between the acquired traits and the inherited traits with one example of each.
List three distinguishing features, in tabular form, between acquired traits and the inherited traits.
Distinguish between the acquired traits and the inherited traits in tabular form, giving one example for each
Differentiate between inherited and acquired characters. Give one example for each type.


21. In one of his experiments with pea plants Mendel observed that when a pure tall pea plant is crossed with a pure dwarf pea plant, in the first generation, F, only tall plants appear.
(a) What happens to the traits of the dwarf plants in this case ?
(b) 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.

(a) The dwarf traits of the plants is not expressed in the presence of the dominant tall trait.
(b) In the F, 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 F, generation, but the dwarfness, being the recessive trait does not express itself in the presence of tallness, the dominant trait.
22. What do you mean by sex chromosomes ? Explain the process of sex-deter mination in man.
Sex-Determination: A Person can have either a male sex or female sex, The process by which sex of a new born individual is determined is called sex-determination.
In human beings, there are 23 pairs of chromosomes, out of which 22 pairs are auto somes and one pair is Sex Chromosomes. A male has one x and one y chromosome and female has two X-chromosomes.
23. Mention the total number of chromosomes along with th sex chromosomes that are present in a human female and a human male. Explain how is sexually producing organisms the number of chromosomes in the progeny remains the same as that of the parents.
Human male has 22 pairs of chromosomes along with XY sex chromosome. Human female has 22 pairs of chromosomes along with XX sex chromosomes.
The original number of chromosomes (the amount of DNA) becomes half during gamete formation. When the gametes fuse, the original number of chromosomes (the amount of DNA) is restored in the progeny.
24. “Two areas of study namely ‘evolution’ and ‘classification’ are interlinked”. Justify this statement.
“Evolution and classification of organisms are interlinked.” Give reasons to justify this statement.
Different forms of organisms have evolved during the course of evolution. Classification deals with grouping of these organisms into groups and subgroups based on their similarities and differences. The more characteristics any two species have in common more closely they are related. In other words, they will have a more recent ancestor.
Thus, classification helps is tracing the evolutionary relationships between the two organisms. Hence classification and evolution are interlinked.
25. Explain the following:
(a) Speciation,
(b) Natural Selection.

(a) The process by which new species develop from the existing species is known as speciation. The factors which could lead to speciation are:
(i) Geographical isolation of population caused by various types of barriers such as mountain ranges, rivers and seas. This leads to reproductive isolation because of which there is no flow of genes between separated groups of the population.
(ii) Genetic drift caused by drastic changes in the frequencies of particular genes by chance. alone.
(iii) Variation caused in individuals because of natural selection.
(b) According to Darwin’s theory of natural selection, the individuals who are most suitable and fit are successful in struggle for existence for food, space, mate, etc. Their offsprings are also better developed and adapted to the environment. Whereas one who are less adapted to the environment may die. Thus, nature selects better-adapted organisms. This is called natural selection of the well-adapted, better-developed individuals of species.
26. “Natural selection and speciation leads to evolution.” Justify this statement.
Natural selection is defined as the change in frequency of some genes in a population, which gives survival advantage to a species. Whereas speciation is the development of a new species from pre-existing ones.
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.
27. “Fossils are related to evolution”, justify this statement. Give the two ways by which age of the fossils can be estimated?
  Fossils are the remains of the organisms that once existed on earth, i.e., they are the preserved traces of living organisms. They provide evidence of evolution by revealing the characteristics of the past organisms and the changes that have occurred in these organisms to give rise to the present organisms.
Two ways of determining age of fossils are:
•    Relative method – fossils closer to the surface are more recent.
•    Datting-finding the ratio of different isotopes of the same element.
(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.

(i) No we can not group them together because the structure of the eye in each of the organisms is different.
(ii) Fossils of certain dinosaurs and reptiles show imprints of feathers along with their bones but they could not fly presumably, using the feathers for insulation only. Later they evolved and adapted feathers for flights, thus becoming the ancestors of present-day birds.
29. Mention three important features of fossils which help in the study of evolution.

1.    Fossils represent modes of preservation of ancient species.
2.    Fossils help in establishing evolutionary traits among organisms and their ancestors.
3.    Fossils help in establishing the time period in which organisms lived.
30. Explain analogous organs and homologous organs. Identify the analogous and homologous organs amongst the following:
Wings of an insect, wings of a bat, forelimbs of frog, forelimbs of a human.
Analogous organs are those organs which have different structural designs and origin but perform similar functions.
Homologous organs are those which have the same basic structural design and origin but perform different functions.
Analogous organs: Wings of an insect, wings of a bat. Homologous organs: Forelimbs of a frog, forelimbs of a human.
31. Explain with the help of an example each, how the following provide evidences in favour of evolution:
(a) Homologous organs
(b) Analogous organs
(c) Fossils

(a) Homologous organs. The study of these organs suggests that these organisms with organs having same structure but performing different functions have evolved from a common ancestor, e.g., forelimbs of different vertebrates.
(b) Analogous organs. The study of these apparently similar organs suggests that the organisms with apparently similar organs do not share common ancestory. Similarity in these organs is superficial or design and the structure of these organs are very different, e.g., Wings of bird and wings of butterfly.
(c) Fossils. They provide the missing link between the species, e.g., Fossils of dinosaurs with feathers or fossils of prehistoric horse, etc.
32. Describe the contribution of Lamarck.
The gradual unfolding of organisms from pre-existing organisms through changes since the beginning of life is called evolution. The theory proposed by J. B. Lamarck is known as the theory of inheritance of acquired characters. According to this theory, the use and disuse of an organ leads to acquiring of change in that organ. These changes or variations can be passed on from one generation to the next but this idea of inheritance of acquired characters was soon discarded.
33. “It is possible that a trait is inherited but may not be expressed.” Give a suitable example to justify this statement.
With the help of an example justify the following statement: “A trait may be inherited, but may not be expressed”.
How did Mendel explain that it is possible that a trait is inherited but not expressed in an organism?
Yes, it is possible that a trait is inherited but may not be expressed. For example, when pure tall pea plants are crossed with pure dwarf pea plants, only tall pea plants are obtained in F, generation. denle of F, but it did not express.
On selfing will plants of F, both tall and dwarf plants are obtained in F, generation in the ratio 3 : 1. Reappearance of the dwarf character, a recessive trait in F, generation shows that the dwarf trait was present in individuals of F, but it did not express.
34. How did Mendel’s experiments show that different traits are inherited independently? Explain.
Mendel conducted a dihybrid cross; and observed that thought he started with two types of parents, he obtained four types of individuals in F. The appearance of new recombination in F, generations along with parental type characters show that traits are inherited undependently of each other.
35. Name two homologous structures in vertebrates. Why are they so called ? How do such organs help in understanding an evolutionary relationship?
Two homologous structures in vetebrates are:
•    limbs of birds and reptiles
•    limbs of reptiles and amphibians.
These are called so because the organs have similar structure to perform different functions in various vertebrates.
The homologous characteristics of such organs indicate common ancestory. Thus these exist an evolutionary relationship.
36. List three factors that provide evidences in favour of evolution in organisms and state the role of each in brief.
Three factors that provide evidences are:
1.    Analogous organs-organisms with similar looking organs may have different origin.
2.    Homologous organs-organisms with apparently different looking organs may have similar origin.
3.    Fossils-allow us to make estimates of how far back evolutionary relationship go. Fossils when chronologically arranged help in tracing the evolutionary history of an organism.
37. Does geographical isolation of individuals of a species lead to formation of a new species. Provide a suitable explanation.
Yes, geographical isolation gradually leads to genetic drift. This may impose limitations to sexual reproduction of the separated population. Slowly the separated individuals will reproduce among themselves and generate new variations. Continuous accumulation of those new variations through a few generations may ultimately lead to the formation of a new species.
38. What is an organic evolution ? It cannot be equated with progress. Explain with the help of a suitable example.
Organic evolution is a sequence of gradual changes which take place in the organism over millions of years resulting in the formation of new organisms or species.
Evolution is not the progress from lower form of life to higher. It has given rise to more complex body design even while simpler body designs continue to flourish. For example, human beings who have not evolved from chimpanzees, but have common ancestors.
39. Give an example of the characteristics being used to determine how close two species are in evolutionary terms.
Study of homologous organs such as forelimbs of mammals, birds, reptiles and amphibian shows that though they perform different functions but they have similar basic or internal structure. This is because they have evolved from common ancestor and help us in determining the closeness between two species in evolutionary terms.
Heredity and Evolution Extra s Long Answer Type
1. Explain with reason why giraffe has long neck.
According to Lamarck’s theory of use and disuse, giraffes had to stretch their necks and forelimbs for feeding on leaves of tall trees because of which these organs were elongated. But this theory has been completely discarded. It is now proposed that, the giraffe’s neck evolved with time because of ‘necking’ battle with time and natural selection. Only giraffes with long necks survived.
2. Give the basic features of the mechanism of inheritance.

1.    Characteristics are controlled by genes.
2.    Each gene controls one character.
3.    There may be two or more forms of gene.
4.    One form may be dominant over the other.
5.    Genes are present on chromosomes.
6.    An individual has to forms of gene whether similar or dissimilar.
7.    The two forms separate from each other at the time of gamete formation.
8.    The two forms are brought together in the zygote.
(i) Mention any two points of difference between acquired and inherited traits.
(ii) If the tail of a mouse is cut for twenty one generations, will the tail occur in the twenty second generation of that mouse ? Give reason to support your answer.
(iii) Define the term-natural selection,


(ii) The mouse will continue to have information for presence of tail in its DNA. So, it will continue to have tail because absence of tail is an acquired trait and not an inherited trait.
(iii) Natural selection means that nature selects the best trait in a species, leading to survival of fittest and evolution of species.

4. In the following crosses write the characteristics of the progeny.

(i) Round, yellow
(ii) Round, yellow
Round, green
Wrinkled, yellow
Wrinkled, green
(iii) Wrinkled, green
(iv) Round, yellow
5. How do Mendel’s experiments show that the
(i) traits may be dominant or recessive
(ii) traits are inherited independently?
How do Mendel’s experiments show that
(a) traits may be dominant or recessive ?
(b) inheritance of two traits is independent of each other?

(i) When Mendel cross-pollinated pure tall pea plants with pure dwarf pea plants, only tall plants were obtained in F generation. On self pollinating the F, progeny, both tall and dwarf plants appeared in F, generation in the ratio 3 : 1.
Appearance of tall character in both the F, and F, shows that it is a dominant character. The absence of dwarf character in F, generation and its reappearance in F, shows dwarfness is the recessive character.
(ii) When Mendel first crossed pure-breed pea plants having round-yellow seeds with pure breed pea plants having wrinkled-green seeds, he found that only round-yellow seeds were produced in the first-generation. No wrinkled-green seeds were obtained in the generation. From this, it was concluded that round shape and yellow colour of the seeds were dominant traits over the wrinkled shape and green colour of the seeds.
When the F, generation pea plants having round-yellow seeds were cross-bred by self-pollination, then four types of seeds having different combinations of shape and colour were obtained in second generation (F2). These were round-yellow, round-green, wrinkled-yellow and wrinkled green seeds. Such a cross is known as dihybrid cross as two sets of corresponding characters are considered.
Mendel observed that along with round-yellow and wrinkled-green, two new combinations of characteristics, round-green and wrinkled-yellow, had appeared in the F, generation. On the basis of this observation, Mendel concluded that though the two pairs of original characteristics (seed colour and shape) combine in the F generation, they get separated and behave independently in the subsequent generation.
6. What are the various evidences in favour of evolution?
Explain with an example for each, how the following provides evidences in favour of evolution in organisms:
(i) Homologous organs
(ii) Analogous organs
(iii) Fossils
List three factors that provide evidences in favour of evolution in organisms and state the role of each in brief.
Explain the terms analogous and homologous organs with examples
The following are the various evidences in favour of evolution:
(i) Homologous organs: Organs with a common basic structural design but with different functions are said to be homologous organ. For example, forelimbs of a frog, lizard, bird and man.
The forelimbs of man are used for grasping of lizard for running of frog for propping up and bird for flying. They have different functions but have same structural pattern.

(ii) Analogous organs: The analogous organs have different basic structure but perform similar functions. For example, the wing of insects and the wing of birds, have a totally different anatomy and origin but they perform the same function of flying of flying in air.

(iii) Evidences from fossils: The fossils also provide evidences for evolution. For example, the fossil Archaeopteryx looks like a bird but it bears a number of other features, which are found in reptiles. This observation provides a clue that birds have evolved from reptiles.
7. Explain the ways in which evolutionary relationships can be traced.
: Evolutionary relationships can be traced in the following ways:
(i) Study of homologous organs: Some organs in different organisms are similar in structure and design because they are inherited from a common ancestor. For example, forelimbs of horse, wings of bird and arms of man may be functionally different, but because of their similarity in structure, origin and design, they indicate that horse, birds and man are closely linked and had a common ancestor.
(ii) Study of fossils: Fossils are the remains or impressions of organisms that existed in the past, allow us to study organ structure of organisms that are no longer alive. Comparing their organ structure with organ structure of present-day organisms also enable us to trace evolutionary relationships.
(iii) Comparing DNA of different species: This will give us a direct estimate of how much the DNA has changed during the formation of these species. This, too, can be used as a criterion to trace evolutionary relationships.
8. Describe briefly various theories related with evolution.

(A) Darwin’s Theory:
(i) Over-production: Every organism has enormous potential to reproduce.
(ii) Struggle for existance: Population size of an organism is limited due to struggle between the members of same species as well as the members of different species. It is due to struggle for food, space and mate.
(iii) Variation: Due to struggle, the fit organisms possess some variations which are favourable and they can leave the progeny to continue the favourable variations.
(iv) Survival of the fittest: The fittest organism survive to continue the favourable variations.
(v) Formation of a new species: These variations when accumulated for a long time, leads to the origin of a new species.
(B) Lemarck’s Theory: Lemarck proposed that the evolution of life forms had occurred by the use and disuse of organs.
He gave the example of Giraffes, which initially did not had long necks. But, in order to reach the leaves on tall trees, they adapted by elongation of their necks. Bypassing this acquired character to succeeding generations over the years they came to acquire long necks.
9. What are fossils? How are they formed? Describe in brief two methods of determining the age of fossils. State any one role of fossils in the study of the process of evolution.
What are fossils? How are they formed? List two methods of determining the age of fossils. Explain in brief the importance of fossils in deciding the evolutionary relationships.
Fossils are dead remains of animals and plants from remote past.
Fossils are formed when dead organisms are not completely decomposed. The organisms may get trapped in resins of the tree, lava of volcanoes or hot mud, which when hardens retains the animal’s parts thus forming fossils. . Two methods of determining the age of fossils are:
1.    Relative method: By estimating the age of the layer of earth’s crust where the fossils is found. Fossils near the surface are recent and those in the deeper layers are more ancient.
2.    Radiocarbon dating method: By detecting the ratios of different isotopes of carbon in the fossils.
Fossils play the following roles:
1.    By determining the age of fossils we come to know the type of earth strata present at that time.
2.    We can also know the type of animals and plants present on the earth at that time.
3.    They help in establishing evolutionary relationship by providing connecting links.
10. What is speciation? List four factors that could lead to speciation. Which of these cannot be a major factor in the speciation of a self-pollinating plant species? Explain. Give reason to justify your answer.
Speciation is the formation of new species from the pre-existing population.
Factors responsible for speciation:
1.    Genetic drift
2.    Natural selection
3.    Geographical isolation
4.    Mutation
Geographical isolation cannot be a major factor in the speciation of a self-pollinating plant species because physical barrier cannot be created in self-pollinating plants.
11. How has the method of artificial selection’ by humans helped in the evolution of different vegetables?
A wild variety of a plant may show different variations. Humans have selected some such variants and grown them for generations and during the course of time, they have become totally different species.
For example, variants in wild cabbage were selected on the basis of certain features to generate different vegetables.
1.    Short distances between leaves, led to formation of green leaf buds-the common cabbage.
2.    Arrested flower development has bred broccoli.
3.    The variant with sterile flowers has made the cauliflower.
4.    Variant with swollen leaf parts-kohlrabi.
5.    Variant with larger leaves kale.
12. What are Mendel’s laws? Explain them with suitable diagram.
: Mendel’s Laws: The three major laws of inheritance proposed by Mendel are as follows:
(i) Law of Dominance (First Law): This law states that when two alternative forms of a trait or character (gene or alleles) are present in an organism only dominant trait expresses it self in F, progency. This law has its basis from the monohybrid Cross.
(ii) Law of Segregation or Law of Purity of Gametes (Second Law): This law states that when a pair of contrasting characters are brought together in hybrid union, the two charac ters stay together without mixing and separate or segregate from each other when the hybrid forms gametes. The gametes are always pure for a particular character because a gamete may carry either the dominat or the recessive character but not both because it is a haploid structure. So this law is also called ‘Law of purity of gametes’.
(iii) Law of Independent assortment (Third Law): This law states that, two factors of each character assort or separate out independent of the factors of other characters at the time of gamete formation and get randomly rearranged in the offsprings.

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