NCERT Solutions For Class 10 Science Chapter 9

NCERT Science Textbook for Class 10

How Is The Equal Genetic Contribution Of Male And Female Parents Ensured In The Progeny?

Genetic material in most organisms is present in pair of chromosomes. Gametes in sexually reproducing organisms are formed by the process of meiosis (a type of cell division occurring mainly in reproductive cells for the formation of gametes) 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 female and half from the male. Thus, process of meiosis is necessary to ensure equal genetic contribution of male and female parents through gametes.

Differentiate Between Homologous Organs And Analogous Organs.

Homologous organs
1. Organs with similar basic plan of origin but different in function.
2. Fundamental plan of structure is similar e.g., fore-limb of man, horse and whale.
Analogous organs
1. Organs with similar function but different in origin.
2. Fundamental plan of structure is different e.g., wings of butterfly and bat.

What Are Fossils? State Their Importance In The Study Of Evolution With The Help Of A Suitable Example.

Fossil: A fossil is the record of an organism that lived in the geologic past. Remains, impressions, cast , mould and traces of organisms of past have been preserved in the earth’s crust as fossils. Fossils help us to build up broad historical sequence of biological evolution. For example, the study of fossils of Equidae (horse) family provides reliable records of evolution. The dawn horse-five-toed Echippus, gave rise to Mesohippus which in turn developed into several lines of evolution. The modern Equus (one-toed horse) evolved from Pliohippus.

What Is Organic Evolution?

Evolution can be defined as the changes in the genetic composition of a population with the passage of each generation. Evolution can be defined as the gradual unfolding of the organism from pre-existing organism through change since the beginning of life. The origin of life is very much associated with evolution. Evolution or organic evolution is the study of how complex organisms of today evolved (formed) from the simpler forms of the past.

How Are Fossils Formed? State Two Methods Of Determining The Age Of Fossils.

Fossil: Remains, impression, casts or traces of organisms of past geologic ages have been preserved in earth’s crust called fossils.
Determining age of a fossil: There are two methods:
(i) This is relative, the fossils we find closer to the surface are more recent than the fossils we find in deeper layers of earth’s crust.
(ii) The second way of dating fossils is by the ratios of different isotopes of same element in the fossil material.

Questions For Practice

Question:
Define the following:
(a) Analogous Organs
(b) Evolution
(c) Genetic Drift
Answer:
(a) Analogous: Organs which look similar because they have common use but differ in their structure and components are called analogous organs
(b) Evolution: The gradual unfolding of events by which new organisms evolved from pre-existing organisms through changes since the beginning of life, is said to be evolution.
(c) Genetic Drift: The random change in gene frequency and gene number by chance only, irrespective of its being advantageous or not in a population is called genetic drift. The effect of genetic drift is very small in a large if population is small.

Questions From Board Papers

Question 1:
Difference between Inherited and Acquired traits.
Answer 1:
Inherited Traits
1. The traits which are inherited by the parents (father and mother) by the off springs (progeny) are called inherited traits.
2. These traits cannot be acquired in the life time but are inherited from parent to the progeny.
3. These traits are due to genetic make-up of the progeny.
Acquired Characters
1. The traits which an individual does not inherit from its parent.
2. These traits are acquired by the individuals due to life time experiences. They are not transmitted
3. These traits are not inherited over generations. Because these traits or experiences are not incorporated in the germ cells (genome or DNA make up) of the individual.

Question 2:
Experiences of an individual during its lifetime cannot be passed on to its progeny and cannot direct evolution. Justify this statement giving an example.
Answer 2:
Experiences of an individual during its life time are in the somatic cells of the body. That is, in non-reproductive tissues that cannot be passed to the next generation. Thus, they cannot contribute directly in evolution.
For inheritance of traits over generations and involvement in direct evolution, they must be passed to the DNA of the germ cells (reproductive tissues). The characteristics of DNA are passed to the progeny and also can play direct role in evolution.

Important Questions On 10th Science Chapter 9

How are the areas of study − evolution and classification − interlinked?

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.

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?

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.

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

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.

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?

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.

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

Sometimes for a species, the environmental conditions change so drastically 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.
However, not all variations are useful. Therefore, these are not necessarily beneficial for the individual organisms.

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?

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.

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

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.

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

This happens 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.

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

Small numbers 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.

What factors could lead to the rise of a new species?

Natural selection, genetic drift and acquisition of traits during the life time of an individual can give rise to new species.

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

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.

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

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.

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

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.

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?

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.

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

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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