NCERT Solutions for Class 10 Science Chapter 9 Heredity and Evolution

NCERT Intext Questions

Page Number 143

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 ?

Ans. In asexual reproduction traits of parent are carried over offspring with minimal or no variation. In the given situation trait B has higher percentage than trait A. So trait B is likely to have arisen earlier.

2. How does the creation of variations in a species promote survival ?

Ans. The variations provide stability to the population of various species by preventing them from getting wiped out during adverse conditions by the process called natural selection.

Hence variations in a species promote survival.

Page Number 147

1. How do Mendel’s experiments show that traits may be dominant or recessive?

Ans. When Mendel crossed tall and dwarf plant of pea. In F1 generation all plants were tall. When he self-crossed the F1 generation he observed that 75% of plants are tall and 25% plants are dwarf. Thus he concluded that tallness is a dominant trait and dwarfness is a recessive trait. This experiment proves Mendel’s first law of inheritance.

2. How do Mendel’s experiments show that traits are inherited independently?

Ans. Mendel did a dihybrid cross between pea plants with round seeds and yellow pea plants with wrinkle seeds. In F1 progeny all plants are yellow with round seeds. This shows that yellow plants with round seeds was dominant trait.
When F1 plants were self pollinated, the F2 progeny consists of the phenotypic 9 (round yellow) : 3 (round green) : 3 (wrinkled yellow) : 1 (wrinkled green). Thus it can be concluded that green and yellow trait and round and wrinkled shaped are inherited independently. This experiment proves Mendel’s second law of independent assortment.
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3. 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?

Ans. Here the father’s blood group is A, hence it can either be homozygous (AA) or heterozygous (AO).

The mother group is O and this is only possible if it is homozygous (OO). For the daugther to have blood group O, the father must be heterzygous (AO) with mother (OO). So it is not enough to tell that which of the traits of blood group A or
O is dominant.

Homozygous mother (OO) with Homogyzous father (AA)

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All progeny of blood group A.

Homozygous mother (OO) with heterozygous father (AO)

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50% progeny with blood group A and 50% progeny with blood group O.

4. How is the sex of the child determined in human beings?

Ans. In humans, the female gamete is having two X chromosomes (XX) and male gamete has one X chromosome and one Y chromosome. When X chromosome of male gamete is fertilized with X chromosome of female gamete, the resulting zygote develops into female (XX). When Y chromosome of male is fertilized with X chromosome of female gamete, the resulting zygote develops into male (XY). So the sex of child is determined by male parent.

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Page Number 150

1. What are the different ways in which individuals with a particular trait may increase in a population?

Ans. The different ways in which individuals with a particular trait may increase in a population are natural selection, genetic drift, mutation and geographical isolation.

2. Why are traits acquired during the life-time of an individual not inherited?

Ans. This happens because an aquired trait involves change in non-reproductive tissues (somatic cells) which cannot be passed on to the germ cells or the progeny. Therefore these traits can not be inherited.

3. Why are the small numbers of surviving tigers a cause of worry from the point of view of genetics?

Ans. A small number of surviving tigers is a cause of worry because tigers do not allow large number of variations to occur, which are essential for survival, and it can result in the loss of genetic variability. This sudden extinction of the tigers and their genes will affect the diversity of the nature. This might even lead to extinction of species.

Page Number 151

1. What factors would lead to the rise of a new species?

Ans. The factors that would lead to the rise of a new species are the following:

  1. Geographical isolation of a population caused by various types of barriers (such as mountain ranges, rivers and sea). The geographical isolation leads to reproductive isolation due to which there is no flow of genes between separated groups of pupulation.
  2. Genetic drift caused by drastic changes in the frequencies of particular genes by chance alone.
  3. Variations caused in individuals due to a natural selection.

2. Will geographical isolation be a major factor in the speciation of a self-pollinating plant species? Why or why not?

Ans. Geographical isolation cannot be major factor in the speciation of a self pollinated plant because single parent is involved in it. There is no gene flow between two geographically isolated species.

3. Will geographical isolation be a major factor in the speciation of an organism that reproduces asexually ? Why or why not?

Ans. Geographical isolation cannot be a major factor in the speciation of an asexually reproducing organism because asexually reproducing organisms pass on the DNA from single parent to offspring that leave no chance of speciation.

Page Number 156

1. Give an example of characteristics being used to determine how close two species are in evolutionary terms.

Ans. If similar characteristics are shown in different organisms, then these are considered to be inherited from common ancestry. It also shows the closeness of the species.

For example, bats and birds have some similarity in their wings, so they are closely related, while lizard and squirrel do not have wings so these are not closely related to the birds and bats.

2. Can the wing of a butterfly and the wing of a bat be considered homologous organs? Why or why not?

Ans. The wings of a butterfly and the wings of a bat cannot be considered to be homologous organs because they have different structure and origin but have the same function of flying. So they are analogous organ.

3. What are fossils ? What do they tell us about the process of evolution ?

Ans. Fossils are the remains or traces of a dead organism. Fossils give us information about the difference and similarities between the species present in past and now and how the changes in them occured leading to evolution. They also tell us about extinct species and help in finding out the reason for their extinction.

Page Number 158

1. 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?

Ans. Human beings who look so different from each other in terms of size, colour and looks are said to belong to the same species because:

  1. They have similarities in DNA sequences.
  2. They have descended from the same ancestor.
  3. They can reproduce among themselves.

2. In evolutionary terms, can we say which among bacteria, spiders, fish and chimpanzees have a ‘better’ body design? Why or why not?

Ans. Bacteria are the primitive organisms as they came into existence very early in evolution. But these organisms are still surviving in the present conditions after millions of years. This is because they have adapted well to the changing environment over these years. Same is the case for all other organisms like spiders, fishes and chimpanzees which have adapted to their environment and have survived. Therefore, all the organisms which exist have a body design which is better as it is suited to their environment.

NCERT Exercise Questions

1. A Mendelian experiment consisted of breeding tall pea plants bearing violet flowers with short pea plants bearing white flowers. The progeny all bear 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. (c) TtWW

Explanation: This is because almost half of them were short. This suggests that the genetic make up of the tall parents can be depicted as TtWW.

2. An example of homologous organs is:

  • (a) our arm and a dog’s fore-leg
  • (b) our teeth and an elephant’s tusks
  • (c) potato and runners of grass
  • (d) all of the above

Ans. (d) All of the above

Explanation: The homologous organs are those which are same in structure and have originated from same species.

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) A Chinese school-boy

Explanation: In evolutionary terms, we have more in common with a chinese school-boy because both belong to the same species of Homo sapiens and have the same genetic organization.

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. This information is not complete. Unless we know the genotypes of these variants of the trait, it is not possible to tell which is dominant or recessive.

5. How are the areas of study-evolution and classification interlinked?

Ans. Classification and evolution are highly interlinked becuase the classification of an organism is influenced by its evolution. An organism may have a different type of look based on the morphology and are classified to a different group as compared to the other organisms because different type of adaption in organism has evolved. Hence evolution and classification are closely related.

6. Explain the terms analogous and homologous organs with examples.

Ans. Analogous organs: The organs which are different in structure but have similar functions are called analogous organs.

For example, the wings of an insect and a bird are analogous organs.

Homologous organs: The organs which are same in structure but have different functions are called homologous organs.

For example, the wing of a bat, flipper of a seal, front leg of a horse and arm of a man are homologous organs.

7. Outline a project which aims to find the dominant coat colour in dogs.

Ans. Consider two parents – one homozygous black (BB) and other homozygous brown (bb), when crossed, they produce offspring with Bb genotype. The dominant contrast can be determined by looking at the offspring. If the offspring is brown in colour than brown colour is a dominant trait and if it is black in colour than black colour is a dominant trait.

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8. Explain the importance of fossils in deciding evolutionary relationships.

Ans. Fossils provide us evidence about:

  1. The organisms that lived long ago such as the time period during which they lived, their structure etc.
  2. Evolutionary development of species i.e., the line of their development.
  3. Connecting links between two groups. For example, feathers present in some dinosaurs means that birds are very closely related to reptiles.
  4. Which organisms evolved earlier and which later.
  5. Development of complex body designs from the simple body designs.

9. What evidence do we have for the origin of life from inanimate matter?

Ans. A British scientist J.B.S. Haldane at first in 1929 suggested that life is originated from inanimate matter. According to him life must have developed from the simple inorganic molecules which were present at that time. Later, Miller and Urey in 1953 presented its evidences. They assembled an apparatus to create an early earth atmosphere which was supposed to consist of gases like methane, ammonia and hydrogen sulphide, etc. over water. This was maintained  at a temperature just below 100°C and electric sparks were then passed through the mixture of gases to stimulate lightning for about one week. At the end of one week, it was found that about 15 per cent of carbon (from methane) had been converted into simple compounds and amino acids which make up protein molecules formed in living organisms. This experiment provides the evidence that the life originated from inanimate matter (or lifeless matter) like inorganic molecules.

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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. Variations arise either because of error in DNA copying during asexual reproduction, which will be fewer. Since in sexual reproduction, gametes from two different individuals with different genetic makeup are involved so variations are much higher.

11. How is the equal genetic contribution of male and female parents ensured in the progeny?

Ans. Genetic material in most organisms is present in pairs of chromosomes. Gametes in the sexually reproducing organisms are formed by the process of meiosis during which half of the genetic material goes into each gamete. When the gametes from male and female parents fuse with each other during sexual reproduction, the normal complement is restored. Half of the genetic material comes from the female and half from the male.

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. It is agreeable that only variations that confer an advantage to an individual organism will survive in a population. All variation does not necessarily provide an equal chance of survival because it is highly depended on the nature of variation.