Class 10 Science Chapter 9 Heredity and Evolution
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NCERT Solutions For Class 10 Science Chapter 9 Heredity and Evolution
Class 10 Science Chapter 9 Heredity and Evolution
Q.1 . A Mendelian experiment consisted of breeding tall pea plants bearing violet flowers with short pea plants bearing white flowers . The progeny all bore violet flowers , but almost half of them were short . This suggests that the genetic make – up of the tall parent can be depicted as
( a ) TTWW
( b ) TTww
( c ) TtWW
( d ) TtWw
Ans . Correct option : ( c )
Explanation : The genetic make – up of the tall parent can be depicted as TtWW . Since , all the progeny bore violet flowers , it means that the tall plant having violet flowers has WW genotype for violet flower colour .
Since the progeny is both tall and short , the parent plant was not a pure tall plant . Its genotype must be Tt . Therefore , the cross involved in the given question is
Therefore , half the progeny is tall , but all of them have violet flowers .
Q. 2. An example of homologous organs is
( a ) our arm and dog’s fore – leg .
( b ) our teeth and an elephant’s tusks .
( c ) potato and tunners of grass .
( d ) all of the above
Ans . ( d )
Q. 3. In evolutionary terms , we have more in common with
( a ) a Chinese school – boy
( b ) a chimpanzee .
( c ) a spider .
( d ) a bacterium .
Ans . ( a )
Q. 4. A study found that children with light – coloured eyes are likely to have parents with light coloured eyes . On this basis , can we say anything about whether the light eye colour trait is dominant or recessive ? Why or why not ?
Ans . Let us assume that children with light coloured eyes can either have LL or Ll or ll genotype . If the children have LL genotype , then their parents will also be of LL genotype .
If the children with light – coloured eyes have ll genotype , then their parents will also have ll genotype .
Therefore , it cannot be concluded whether light eye colour is dominant or recessive .
Q. 5. How are the areas of study – evolution and classification interlinked ? [ CBSE Board , All India Region , 2017 ]
Ans . Classification involves grouping of organism into a formal system based on similarities in internal and external structure or evolutionary history .
Two species are more closely related if they have more characteristics in common . And if two species are more closely related , then it means they have a more recent ancestor . For example : in a family , a brother and sister are closely related and they have a recent common ancestor i.e. , their parents . A brother and his cousin are also related but less than the sister and her brother . This is because the brother and his cousin have a common ancestor i.e. , their grandparents in the second generation whereas the parents were from the first generation . With subsequent generations , the variations make organisms more different than their ancestors . This discussion clearly proves that we classify organisms according to their resemblance which is similar to creating an evolutionary tree .
Q. 6. Explain the terms analogous and homologous organs with examples .
Ans . Homologous organs are similar in origin ( or are embryologically similar ) but perform different functions . For example , the forelimbs of humans and the wings of birds look different externally but their skeletal structure is similar . It means that their origin is similar ( as wings in birds are modifications of forearm ) but functions are different the wings help in flight whereas human forearm helps in various activities .
Analogous organs , on the other hand , have different origin but perform similar functions . For example , the wings of a bird and a bat are similar in function but this similarity does not mean that these animals are more closely related . If we carefully look at these structures , then we will find that the wings of a bat are just the folds of skin that are stretched between its fingers whereas the wings of birds are present all along the arm . Therefore , these organs are analogous organs .
Q. 7. Outline a project which aims to find the dominant coat colour in dogs .
Ans . Dogs have a variety of genes that govern coat colour . There are at least eleven identified gene series ( A , B , C , D , E , F , G , M , P , S , T ) that influence coat colour in dog . A dog inherits one gene from each of its parents . The dominant gene gets expressed in the phenotype . For example , in the B series , a dog can be genetically black or brown . Let us assume that one parent is homozygous black ( BB ) , while the other parent is homozygous brown ( bb ) .
In this case , all the offsprings will be heterozygous ( Bb ) .
Since black ( B ) is dominant , all the offsprings will be black . However , they will have both B and b alleles . If such heterozygous pups are crossed , they will produce 25 % homozygous black ( BB ) , 50 % heterozygous black ( Bb ) , and 25 % homozygous brown ( bb ) offsprings .
Q. 8. Explain the importance of fossils in deciding evolutionary relationships .
Ans . Fossils are the remains of the organism that once existed on earth . They represent the ancestors of the plants and animals that are alive today . They provide evidences 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 . Let us explain the importance of fossils in deciding evolutionary history with the help of the following example .
About 100 million years ago , some invertebrates died and were buried in the soil in that area . More sediment accumulated on top of it turning it into sedimentary rock . At the same place , millions of years later , some dinosaurs died and their bodies were buried on top of the sedimentary rock . The mud containing dinosaurs also turned into a rock . Then , millions of years later , some horse – like creatures died in that area and got fossilized in rocks above the dinosaur fossils .
Sometime later , due to soil erosion or floods in that area , the rocks containing horse – like fossils are exposed . If that area is excavated deeper , then the dinosaur and invertebrate’s fossils can also be found . Thus , by digging that area , scientists can easily predict that horse – like animals evolved later than the dinosaurs and the invertebrates .
Thus , the above example suggests that the fossils found closer to the surface of the earth are more recent ones than the fossils present in deeper layers . Let us start 100 million years ago , Some invertebrates on the sea – bed die , and are burled in the sand . More sand accumulates , and sandstone forms under pressure .
Million of years later , dinosaurs living in the area die and their bodies , too , are buried in mud . This mud is also compressed into rock , above the rock containing the earlier invertebrate fossils .
Again millions of years later , the bodies of horse like creatures dying in the area are fossilised in rocks above these earlier rocks .
Much later , by erosion or water flow wears away some of the rock and exposes the horse – like fossils . As we dig deeper , we will find older and older fossils .
Q. 9. What evidence do we have for the origin of life from inanimate matter ?
Ans . A British scientist , J.B.S. Haldane , suggested that life originated from simple inorganic molecules . He believed that when the earth was formed , it was a hot gaseous mass containing elements such as nitrogen , oxygen , carbon , hydrogen , etc. These elements combined to form molecules like water ( H₂O ) , carbon dioxide ( CO₂ ) , methane ( CH4 ) , ammonia ( NH3 ) , etc. After the formation of water , slowly the earth surface cooled and the inorganic molecules interacted with one another in water to form simple organic molecules such as sugars , fatty acids , amino acids , etc. The energy for these reactions was provided by solar radiations , lightning , volcanic eruptions , etc. This was proved by the experiment of Stanley L. Miller and Harold C. Urey in 1953. They took a mixture of water ( H₂O ) , methane ( CH4 ) , ammonia ( NH3 ) , and hydrogen gas ( H₂ ) in a chamber and sparks were passed through this mixture using two electrodes . After one week , 15 % of the carbon from methane was converted into amino acids , sugars , etc. These organic molecules are polymerized and assembled to form protein molecules that gave rise to life on earth .
Q. 10. Explain how sexual reproduction gives rise to more viable variations than asexual reproduction . How does this affect the evolution of those organisms that reproduce sexually ?
Ans . In sexual reproduction , two individuals having different variations combine their DNA to give rise to a new individual . Therefore , sexual reproduction allows more variations , whereas in asexual reproduction , chance variations can only occur when the copying of DNA is not accurate . Additionally , asexual reproduction allows very less variations because if there are more variations , then the resultant DNA will not be able to survive inside the inherited cellular apparatus .
However , in sexual reproduction , more variations are allowed and the resultant DNA is also able to survive , thus making the variations viable .
Variation and Evolution : Variants help the species to survive in all the conditions . Environmental conditions such as heat , light , pests , and food availability can change suddenly at only one place . At that time , only those variants resistant to these conditions would be able to survive . This will slowly lead to the evolution of a better adapted species . Thus , variation helps in the evolution of sexually reproducing organisms .
Q. 11. How is the equal genetic contribution of male and female parents ensured in the progeny ?
Ans . In human beings , every somatic cell of the body contains 23 pairs of chromosomes . Out of these 23 pairs , the first 22 pairs are known as autosomes and the remaining one pair is known as sex chromosomes represented as X and Y. Females have two X chromosomes and males have one X and one Y chromosome . The gamete receives half of the chromosomes . Therefore , the male gametes have 22 autosomes and either X or Y chromosome . The female gamete , on the other hand , has 22 autosomes and X chromosome . During reproduction , the male and female gametes fuse and thus the progeny receives 22 autosomes and one X or Y chromosome from male parent and 22 autosomes and one X chromosome from the female parent .
Q. 12. Only variations that confer an advantage to an individual organism will survive in a population . Do you agree with this statement ? Why or why not ?
Ans . In species , variations that offer survival advantages are naturally selected . Individuals adjust to their environments with the help of these selected variations and consequently these variations are passed on to their progeny . Evolution of organisms occurs as a result of this natural selection . However , there can be some other variations , which do not offer any survival advantage and arise only accidentally . Such variations in small populations can change the frequency of some genes even if they are not important for survival . This accidental change in the frequency of genes in small populations is referred to as genetic drift .
Thus , genetic drift provides diversity ( variations ) without any survival advantage .
Q. 1. If a trait A exists in 10 % of a population of an asexually reproducing species and a trait B exists in 60 % of the same population , which trait is likely to have arisen earlier ? [ NCERT Q. 1 , Page 143 ]
Ans . In asexual reproduction , the reproducing cells produce a copy of their DNA through some chemical reactions . However , this copying of DNA is not accurate and therefore , the newly formed DNA has some variations .
It can be easily observed in the above figure that in asexual reproduction , very few variations are allowed . Therefore , if a trait is present in only 10 % of the population , it is more likely that the trait has arisen recently . Therefore , it can be concluded that trait B that exists in 60 % of the same population has arisen earlier than trait A.
Q. 2. How does the creation of variations in a species promote survival ? [ NCERT Q. 2 , Page 143 ]
Ans . The environmental conditions change so drastically for a species that their survival becomes difficult . For example : if the temperature of water increases suddenly , most of the bacteria living in that water would die . Only few variants resistant to heat would be able to survive . If these variants were not there , then the entire species of bacteria would have been destroyed . Thus , these variants help in the survival of the species . Though , not all variations are useful . So , these are not necessarily beneficial for the individual organisms .
Q. 3. How do Mendel’s experiments show that traits may be dominant or recessive ? [ NCERT Q. 1 , Page 147 ]
Ans . Mendel selected true breeding tall ( TT ) and dwarf ( tt ) pea plants . Then , he crossed these two plants . The seeds formed after fertilization were grown and these plants that were formed represent the first filial or F₁ generation . All the F₁ plants obtained were tall .
Then , Mendel self – pollinated the F₁ plants and observed that all plants obtained in the F₂ generation were not tall . Instead , one – fourth of the F₂ plants were short .
From this experiment , Mendel concluded that the F₁ tall plants were not true breeding . They were carrying traits of both short height and tall height . They appeared tall only because the tall trait is dominant over the dwarf trait .
Q. 4. How do Mendel’s experiments show that traits are inherited independently ? [ NCERT Q. 2 , Page 147 ]
Ans . 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 .
In the above cross , more than two factors are involved , and these are independently inherited .
Q. 5. A man with blood group A marries a woman with blood group O and their daughter has blood group O. Is this information enough to tell you which of the traits – blood group A or O – is dominant ? Why or why not ? [ NCERT Q. 3 , Page 147 ]
Ans . No. This information is not sufficient to determine which of the traits – blood group A or O – is dominant . This is because we do not know about the blood group of all the progeny . Blood group A can be genotypically AA or AO . Hence , the information is incomplete to draw any such conclusion .
Q. 6. How is the sex of the child determined in human beings ? [ NCERT Q. 4 , Page 147 ]
Ans . In human beings , the females have two X chromosomes and the males have one X and one Y chromosome . Therefore , the females are XX and the males are XY . The gametes , as we know , receive half of the chromosomes . The male gametes have 22 autosomes and either X or Y sex chromosome . Type of male gametes : 22 + X or 22+ Y.
However , since the females have XX sex chromosomes , their gametes can only have X sex chromosome .
Type of female gamete : 22 + X
Thus , the mother provides only X chromosomes . The sex of the baby is determined by the type of male gamete ( X or Y ) that fuses with the X chromosome of the female .
Q. 7. What are the different ways in which individuals with a particular trait may increase in a population ? [ NCERT Q. 1 , Page 150 ]
Ans . Individuals with a particular trait may increase in a population as a result of the following :
( i ) Natural selection : When that trait offers some survival advantage .
( ii ) Genetic drift : When some genes governing that trait become common in a population .
( iii ) When that trait gets acquired during the individual’s lifetime .
Q. 8. Why are traits acquired during the life – time of an individual not inherited ? [ NCERT Q. 2 , Page 150 ]
Ans . The traits acquired during the life – time of an individual are not inherited because an acquired trait involves change in non – reproductive tissues ( somatic cells ) which cannot be passed on to germ cells or the progeny . Therefore , these traits cannot be inherited .
Q. 9. Why are the small number of surviving tigers a cause of worry from the point of view of genetics ? [ NCERT Q. 3 , Page 150 ]
Ans . Small number of tigers means that fewer variations in terms of genes are available . This means that when these tigers reproduce , there are less chances of producing progeny with some useful variations . Hence , it is a cause of worry from the point of view of genetics .
Q. 10. What factors could lead to the rise of a new species ? [ NCERT Q. 1 , Page 151 ]
Ans . Natural selection , genetic drift and acquisition of traits due to variations or mutations during the life time of an individual can give rise to new species .
Q. 11. Will geographical isolation be a major factor in the speciation of a self – pollinating plant species ? Why or why not ? [ NCERT Q. 2 , Page 151 ]
Ans . Geographical isolation can prevent the transfer of pollens among different plants . However , since the plants are self – pollinating , which means that the pollens are transferred from the anther of one flower to the stigma of the same flower or of another flower of the same plant , geographical isolation cannot prevent speciation in this case .
Q. 12. Will geographical isolation be a major factor in the speciation of an organism that reproduces asexually ? Why or why not ? [ NCERT Q. 3 , Page 151 ]
Ans . Geographical isolation prevents gene flow between populations of a species whereas asexual reproduction generally involves only one individual . In an asexually reproducing organism , variations can occur only when the copying of DNA is not accurate . Therefore , geographical isolation cannot prevent the formation of new species in an asexually reproducing organism .
Q. 13. Give an example of characteristics being used to determine how close two species are in evolutionary terms . [ NCERT Q. 1 , Page 156 ]
Ans . The presence of feathers in dinosaurs and birds indicates that they are evolutionarily related . Dinosaurs had feathers not for flying but instead these feathers provided insulation to these warm blooded animals . However , the feathers in birds are used for flight . This proves that reptiles and birds are closely related and that the evolution of wings started in reptiles .
Q. 14. Can the wing of a butterfly and the wing of a bat be considered homologous organs ? Why or why not ? [ NCERT Q. 2 , Page 156 ]
Ans . Wings of a butterfly are composed of membrane , while wings of a bat are composed of bony skeleton . Hence , these are not homologous organs as they have similar function but different origin rather analogous organs .
Q. 15. What are fossils ? What do they tell us about the process of evolution ? [ NCERT Q. 3 , Page 156 ]
Ans . Fossils are the remains of organisms that once existed on earth . They represent the ancestors of plants and animals that are alive today . They provide evidences of evolution by revealing the characteristics of the past organism and the changes that have occurred in these organisms to give rise to the present organisms .
Q. 16. Why are human beings who look so different from each other in terms of size , colour and looks said to belong to the same species ? [ NCERT Q. 1 , Page 158 ]
Ans . A species is a group of organisms that are capable of interbreeding to produce a fertile offspring . Skin colour , looks , and size are all variety of features present in human beings . These features are generally environmentally controlled . Various human races are formed based on these features . However , there is no biological basis to this concept of races . Therefore , all human beings are a single species as humans of different colour , size , and looks are capable of reproduction and can produce a fertile offspring .
Q. 17. In evolutionary terms , can we say which among bacteria , spiders , fish and chimpanzees have a ‘ better ‘ body design ? Why or why not ? [ NCERT Q. 2 , Page 158 ]
Ans . Evolution cannot always be equated with progress or better body designs . Evolution simply creates more complex body designs . However , this does not mean that the simple body designs are inefficient . In fact , bacteria having a simple body design are still the most cosmopolitan organisms found on earth . They can survive hot springs , deep sea and even freezing environment .
Therefore , bacteria , spiders , fish and chimpanzees are all different branches of evolution .
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