Chapter 9 Heredity and Evolution

Question 1.

All the variations in a species do not have equal chances of survival. Why? (Foreign 2014)
Answer:
All the variations do not have equal chances of survival in the environment in which they live. Depending on the nature of variations, different individuals would have different kinds of advantages. The organisms which are most adapted to the environment will survive.

Question 2.
“Only variations that confer an advantage to an individual organism will survive in a population.” Justify this statement. (Foreign 2011)
Answer:
Variations are the structural, functional or behavioural changes from the normal characters developed in the living organisms. Inheritable variations participate in evolution. According to Darwin, natural selection sorts out individuals with favourable variations. Such organism will survive, reproduce more and thus, will leave more progenies. Hence, useful variations get established in nature.

Question 3.
Assertion (A) : The sex of a child in human beings will be determined by the type of chromosome he/she inherits from the father.
Reason (R) : A child who inherits ‘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).
(a) Both (A) and (R) are true and (R) is the correct explanation of the assertion (A).
(b) Both (A) and (R) are true, but (R) is not the correct explanation of the assertion (A).
(c) (A) is true, but (R) is, false.
(d) (A) is false, but (R) is true. (2020)
Answer:
(a) Both (A) and (R) are true and (R) is the correct explanation of the assertion (A).

Question 4.
A Mendelian experiment consisted of breeding pea plants bearing violet flowers with pea plants bearing white flowers. What will be the result in F₁ progeny? (2018)
Answer:
According to the Mendelian experiment, violet coloqr (VV) is a dominant trait while white colour (ww) is a recessive trait. Hence, the colour of the flower in F₁ progeny will be violet (Vw).

Question 5.
Name the information source for making proteins in the cells. (Delhi 2014)
Answer:
Deoxyribonucleic acid (DNA) present in the chromosomes of cell nucleus is the information source for making proteins.

Question 6.
What is a gene? (AI 2014)
Answer:
A gene is a unit of DNA on a chromosome which governs the synthesis of particular protein that controls specific characteristics (or traits) of an organism.

Question 7.
What is heredity? (AI 2014)
Answer:
The inheritance of characters (or trails) from the parents to their offsprings is called heredity.

Question 8.
Why is the progeny always tall when a tall pea plant is crossed with a short pea plant? (Foreign 2014)
Answer:
When a tall pea plant is crossed with a short pea plant, the resultant progeny is always tall because tall is dominant trait and short is recessive trait. Therefore, dominant trait expresses itself in the progeny.

Question 9.
Write a difference between inherited traits and acquired traits giving one example of each. (Delhi 2013C)
Answer:
A trait (or characteristic) of an organism which is ‘not inherited’ but develops in response to the environment is called an acquired trait. For example, if a group of mice are normally bred, all their progeny will have tails. Now, if the tails of these mice are cut by surgery in each generation, tail-less mice will not be produced. This is so because removal of tail is an acquired character.

A trait (or characteristic) of an organism which is caused by a change in its genes (or DNA) is called an inherited trait. Inherited traits can be passed on to the progeny of the organism because they have produced changes in the genes (or DNA) of the organism. For example, skin colour in human beings.

Question 10.
(a) Why did Mendel carry out an experiment to study inheritance of two traits in garden pea?
(b) What were his findings with respect to inheritance of traits in F₁ and F₂ generation?
(c) State the ratio obtained in the F₂ generation in the above mentioned experiment. (2020)
Answer:
(a) Mendel carried out crosses with two traits to see the interaction and basis of inheritance between them. In a dihybrid cross given by Mendel, it was observed that when two pairs of characters were considered each trait expressed independent of the other.

(b) For example, a cross between round yellow and wrinkled green parents.

In F₁ generation, all plants are with round yellow seeds. But in F₂ generation, we find all types of plants : Round yellow, Round green, Wrinkled yellow, Wrinkled green.
F₂ generation ratio : Round-yellow = 9 : Round- green = 3 : Colour of stem in F₁ progeny Wrinkled- yellow = 3 : Wrinkled-green = 1

Question 11.
A green stemmed rose plant denoted by GG and a brown stemmed rose plant denoted by gg are allowed to undergo a cross with each other.
(a) List your observations regarding :
(i) Colour of stem in their F₁ progeny
(ii) Percentage of brown stemmed plants in F₂ progeny if plants are self pollinated.
(iii) Ratio of GG and Gg in the F₂ progeny.
(b) Based on the findings of this cross, what conclusion can be drawn? (2020)
Answer:
(a) (i) Colour of stem in F₁ progeny:

The colour in the F₁ progeny is green stemmed as green stem colour is dominant.

(ii) F₁ progeny on self pollination:

F₂ generation Green stemmed: Brown stemmed
\(\frac { 1 }{ 4 }\) or 25% of F₂ progeny are brown stemmed rose plant.

(iii) Ratio of GG and Gg in F₂ progeny:
Genotype of F₂ progeny – GG : Gg
1 : 2

(b) This is a monohybrid cross. This shows that out of two contrasting traits only one dominant trait appears in F₁ generation and the trait which does not express is recessive. On selfing the F₁ plants, both the traits appear in next generation but in a definite proportion.

Question 12.
(a) Why is the F₁ progeny always of tall plants when a tall plant is crossed with a short pea plant?
(b) How is F₂ progeny obtained by self-pollination of F₁ progeny different from F₁ progeny? Give reason for this observation.
(c) State a conclusion that can be drawn on the basis of this observation. (2020)
Answer:
(a) When a tall plant (TT) is crossed with a short tea plant (tt), only tall plants are obtained in F₁ progeny. It is because out of two contrasting traits only one appears in the progeny of first generation. This means that the trait which appears in F₁ generation is dominant and the trait which does not express is recessive. The character TT for tall plant is dominant, so all the plants are tall.

(b) On selfing F₁ progeny

In F₂ generation we obtained both tall and dwarf plants. Appearance of suppressed recessive trait in individuals of F₂ generation in Mendelian cross indicates that characters of recessive traits are not lost. When the F₁ generation plants were allowed to self-fertilise both the parental trait were expressed in definite proportion in F₂ generation.

(c) Above observation shows that only one dominant allele is expressed. This is called law of dominance. In a heterozygous individual, two dissimilar alleles remain together and do not get mixed up. At the time of gamete formation, they separate so that each gamete receives only one allele is always pure. This is called “law of purity of gametes”.

Question 13.
Name the plant Mendel used for his experiment. What type of progeny was obtained by Mendel in F₁ and F₂ generations when he crossed the tall and short plants? Write the ratio he obtained in F₂ generation plants. (Delhi 2019)
Answer:
Mendel selected garden pea (Pisum sativum) for his series of hybridisation experiments.
He first selected two pureline plants (tall plant having gene TT and short plant having gene tt) and then crossed such plants having contrasting characters. In the F₁ generation, he observed that only one of the two contrasting character appeared, he called this character as dominant and the one which does not get expressed in F₁ was called as recessive. He later selfed the F₁ plants and observed that both the traits appear in next generation but in a definite proportion. This can be explained by the following cross :

So, the plants of F₁ generation will be all tall plants and after selfing the ratio of tall and dwarf plants that Mendel obtained in F₂ generation plants is 3 : 1.

Question 14.
How did Mendels experiments show that different traits are inherited independently? Explain. (Delhi 2017)
Answer:
In a dihybrid cross given by Mendel, it was observed that when two pairs of traits or characters were considered, each trait expressed independent of the other. Thus, Mendel was able to propose the Law of Independent Assortment which says about independent inheritance of traits. This could be explained clearly from the given cross:

F₂ generation ratio : Round-yellow = 9 : Round- green = 3: Wrinkled-yellow = 3: Wrinkled-green = 1

Question 15.
How did Mendel explain that it is possible that a trait is inherited but not expressed in an organism? (AI 2017)
Answer:
Mendel first selected two pure line plants. He then crossed such plants having contrasting characters. In the F₁ generation, he observed that only one of the two contrasting character appeared, he called it dominant and the one which does not get expressed in F₁ was recessive. He later selfed the F₁ plants and observed that both the traits appear but in a definite proportion. It can be explained by the following cross :

This is how Mendel explained that a trait may be inherited but not expressed in the plant.

Question 16.
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, F₁ 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, F₂ both tall plants and dwarf plants were present. Why it happened? Explain briefly. (Delhi 2016)
Answer:
(a) Mendel’s monohybrid cross indicated that out of two contrasting traits only one appears in the progeny of first generation. This implies that the trait which appears in F₁ generation is dominant and the trait which does not express is recessive. We can also say that gene controlling the dominant trait is dominant gene or allele and gene controlling the recessive trait is recessive gene or allele.

In F₁ progeny although the dominant trait is expressed but genes for both dominant and recessive traits are present in a heterozygous condition. The recessive trait has a chance to express in next generation only if recessive genes come in homozygous condition. This can be illustrated by the given cross:

(b) Appearance of suppressed recessive trait in individuals of F₂ generation in Mendelian cross indicates that the characters of recessive traits are not lost even when they are not expressed. When the F₁ generation plants were allowed to self- fertilise, both the parental traits were expressed in definite proportion in F₂ generation. This could be explained by the given cross by selfing the gametes obtained in F₁ generation.

Question 17.
How did Mendel interpret his result to show that traits may be dominant or recessive? Describe briefly. (Delhi 2016)
Answer:
Mendel crossed the pea plant for two contrasting characters under consideration. The trait that expressed itself in F₁ generation was dominant and the one not expressed in F₁ generation was recessive. He later selfed the plants of F₁ generation and recovered, both parental traits in a definite proportion in F₂ generation. Mendel interpreted his results as, the trait that expressed itself in F₁ was dominant and the one that reappeared in F₂ generation was recessive. It can be demonstrated by the following cross:

Question 18.
In a monohybrid cross between tall pea plants (TT) and short pea plants (tt) a scientist obtained only tall pea plants (Tt) in the F₁ generation. However, on selfing the F₁ generation pea plants, he obtained both tall and short plants in F₂ generation. On the basis of above observations with other angiosperms also, can the scientist arrive at a law? If yes, explain the law. If not, give justification for your answer. (Delhi 2016)
Answer:
In the situation discussed in the question the scientist can arrive at two different laws, i.e., law of dominance and law of segregation (or law of purity of gametes). This can be explained with the help of following crosses:

In F₁ hybrid two dissimilar alleles are present for one character, i.e., height T is for tallness and t is for dwarfness, out of which only one allele called dominant allele expresses itself and the one which remains unexpressed is called recessive allele. This is called “law of dominance”.

Also the two dissimilar alleles that remain together in a heterozygous individual do not get mixed up and keep their distinct identity. Hence, at the time of gamete formation they separate so that each gamete receives only one allele and is always pure which enables reappearance of recessive trait in F₂ progenies when the two recessive alleles come together. This is called “law of purity of gametes.”

Question 19.
How do Mendel’s experiment show that traits are inherited independently? (AI 2016)
Answer:
Refer to answer 14.

Question 20.
With the help of an example justify the following statement: “A trait may be inherited, but may not be expressed.” (AI 2016)
Answer:
A trait may be inherited but may not be expressed, this could be explained by the given example. When a tall pea plant was crossed with a dwarf pea plant, then F₁ generation plants were all tall. When F₁ plants were selfed, then F₂ generation plants were both tall and dwarf. This shows that the F₁ plants had inherited both the parental traits but did not express dwarfness or recessive trait in the presence of the trait for tallness or dominant trait. This could be explained by the given cross :

Question 21.
List two differences in tabular form between dominant trait and recessive traits. What percentage/proportion of the plants in the F2 generation/progeny were round, in Mendel’s cross between round and wrinkled pea plants? (Foreign 2016)
Answer:
Differences between dominant traits and recessive trait are given below:

Dominant trait	Recessive trait
(i) It is the trait controlled by dominant allele.	It is the trait controlled by recessive allele.
(ii) It is the trait which is expressed in F1 generation.	It is the trait which remains suppressed in F1 generation and appears in F2 generation.

Out of total 4 genotypes possible in F₂ generation 31 genotypes result in phenotypic expression of round seeds. So, the percentage of plants with round seeds will be 75%. This can be illustrated as follows:

Question 22.
Explain Mendel’s experiment with peas on inheritance of characters considering only one visible contrasting character. (Foreign 2016, 2014)
Answer:
Mendel crossed a pure tall pea plant with pure dwarf pea plant. All the plants obtained in F₁ generation were tall. When Mendel selfed plants from F₁ generation then he obtained both tall and dwarf plants in F₂ generation in the ratio of 3 : 1. This can be illustrated as follows :

This explains that for each pair of contrasting characters there are two alleles. The trait which is expressed in F₁ is dominant trait and is controlled by dominant allele and the trait which remains unexpressed in F₁ is the recessive trait and is controlled by recessive gene. When both the contrasting alleles are present together in F₁ individuals, no mixing of alleles occurs and they again segregate at the time of gamete formation Therefore, when the recessive alleles come together they result in reappearance of recessive trait in F₂ generation.

Question 23.
“It is a matter of chance whether a couple will have a male or a female child.” Justify this statement by drawing a flow chart. (Foreign 2015)
Answer:
Sex is determined at the time of fertilisation when male and female gametes fuse. Male produces two types of gametes, i.e., having X or Y chromosome and female produces same type of gametes containing X chromosomes. The sex of the child is determined at the time of fertilisation when male and female gametes fuse to form zygote.

If a sperm (male gamete) carrying X chromosome fertilises an egg or ovum (female gamete) carrying X chromosome, then the offspring will be a girl (female). This is because the offspring will have XX combination of sex chromosomes.

If a sperm (male gamete) carrying Y chromosome fertilises an egg or ovum (female gamete) which has X chromosome, then the offspring will be a boy (male). This is because the offspring will have XY combination of sex chromosomes.

Therefore, there are 50% chance of a male child and 50% chance of a female child.

Question 24.
“It is possible that a trait is inherited but may not be expressed.” Give a suitable example to justify this statement. (Foreign 2015)
Answer:
Refer to answer 20.

Question 25.
A cross was made between pure breeding pea plants, one with round and green seeds and the other with wrinkled and yellow seeds.
(a) Write the phenotype of F₁ progeny. Give reason for your answer.
(b) Write the different types of F₂ progeny obtained along with their ratio when F₁ progeny was selfed. (Delhi 2014, Delhi 2013C)
Answer:
(a) The given cross was made between pure breeding pea plants, one with round and green seeds and the other with wrinkled and yellow seeds.
Yellow seed colour and round seed shape is dominant over green seed colour and wrinkled seed shape. In F₁ generation, dominant traits express itself, whereas recessive traits get suppressed.

Therefore, the phenotype of F₁ progeny is round and yellow.

(b) The different types of F₂ progeny obtained along with their ratio when F₁ progeny was selfed could be illustrated by the given cross.

Phenotypic ratio : 9 : 3 : 3 : 1
Round yellow seeds – 9 ; Round green seeds – 3;
Wrinkled yellow seeds – 3; Wrinkled green seeds – 1

Question 26.
(a) Mendel crossed tall pea plants with dwarf pea plants in his experiment. Write his observations giving reasons on the F₁ and F₂ generations.
(b) List any two contrasting characters other than height that Mendel used in his experiments in pea plants. (Delhi 2014)
Answer:
(a) The possible cross of Mendel’s experiment is:

Hence, tall (T) is dominant whereas dwarf (t) is recessive. In F₁ generation, only dominant trait expresses itself, whereas recessive trait gets suppressed.

In F₂ generation, both traits, i.e., dominant and recessive express themselves. In this way, Mendels experiment showed that the traits (tall and dwarf) are inherited independently.

(b) The two contrasting characters other than height that Mendel used in his experiment in pea plants are round/wrinkled seeds and violet/white flowers.

Question 27.
“A trait may be inherited, but may not be expressed”. Justify this statement with the help of a suitable example. (AI 2014)
Answer:
Refer to answer 20.

Question 28.
“The sex of a newborn child is a matter of chance and none of the parents may be considered responsible for it”. Justify this statement with the help of flow chart showing determination of sex of a new born. (Delhi 2013)
Answer:
Refer to answer 23.

Question 29.
A blue colour flower plant denoted by BB is cross-bred with that of white colour flower plant denoted by bb.
(a) State the colour of flower you would expect in their F₁ generation plants.
(b) What must be the percentage of white flower plants in F₂ generation if flowers of F₁ plants are self-pollinated?
(c) State the expected ratio of the genotypes BB and Bb in the F₂ progeny. (Delhi 2012)
Answer:
(a) The colour of the flower in F₁ generation will be blue.
(b) If the flowers of F₁ generation are self pollinated, then the percentage of white flowers in F₂ generation must be 25%.
(c) The expected ratio of the genotypes BB and Bb in the F₂ generation progeny is 1 : 2.
The above results could be depicted by the given cross:

Question 30.
If we cross pure-breed tall (dominant) pea plant with pure-breed dwarf (recessive) pea plant we will get pea plants of F₁ generation.
If we now self-cross the pea plant of F₁ generation, then we obtain pea plants of F₂ generation.
(a) What do the plants of F₁ generation look like?
(b) State the ratio of tall plants to dwarf plants in F₂ generation.
(c) State the type of plants not found in F₁ generation but appeared in F₂ generation, mentioning the reason for the same. (AI 2012)
Answer:
(a) The plants of F₁ generation will be all tall plants.
(b) The ratio of tall plants to dwarf plants in F₂ generation is 3 : 1.
(c) Dwarf plants are not found in F₁ generation but appeared in F₂ general ion. This is so because in F₁ generation only dominant trait (tall) expresses itself and recessive trait (dwarf) gets suppressed. The dwarf plants appeared in F₂ generation, because the traits whether dominant or recessive are independently inherited. In others words, a single copy of (T) is enough to make the plant tall, while both copies have to be (t) for the plant to be dwarf.

Question 31.
With the help of a flow chart explain in brief how the sex of a newborn is genetically determined in human beings. Which of the two parents, the mother or the father, is responsible for determination of sex of a child? (Foreign 2012)
Answer:
Refer to answer 23.

Question 32.
How is the sex of the child fixed during the fertilisation, step in human beings? Explain. (Foreign 2011)
Answer:
Human beings have 23 pairs chromosomes (22 pairs of autosomes +1 pair of sex chromosome). A male has one X chromosome and one Y chromosome whereas a female has two X chromosomes. Sex of a child depends on the two conditions which takes place during fertilisation. The two conditions are given below:

·         If a sperm carrying X chromosome fertilises an ovum which carries X chromosome, then the child born will be girl.

·         If a sperm carrying Y chromosome fertilises an ovum which carries X chromosome, then the child born will be a boy.

Question 33.
How do Mendels experiments show that
(a) traits may be dominant or recessive?
(b) inheritance of two traits is independent of each other? (Delhi 2017)
Answer:
(a) Mendel first crossed pure-bred tall pea plants with pure-bred dwarf pea plants and found that only tall pea plants were produced in the first generation (F₁). He then self crossed the tall pea plants of the F₁ generation and found that tall plants and dwarf plants were obtained in the second generation or (F₂) in the ratio of 3 : 1. Mendel said that the trait of dwarfness of one of the parent pea plant had not been lost, it was merely concealed or suppressed in the first generation to re-emerge in the second generation. He called the suppressed trait of ‘dwarfness as ‘recessive trait’ and the expressed trait of ‘tallness’ as the ‘dominant trait’. In this way, Mendel’s experiments with tall and dwarf pea plants showed that the traits may be dominant or recessive.
Hence this could be explained by the given cross :

(b) Mendel observed two combinations of characteristics in seeds, round-yellow and wrinkled-green, and two new combinations of characteristics had appeared in the F₂ generation, round-green and wrinkled- yellow. According to Mendel’s second law of inheritance more than one pair of traits are considered in a cross simultaneously, the factors responsible for each pair of trait are distributed independently to the gametes.

The cross given below showing dihybrid cross explains that the inheritance of two traits is independent of each other.

Question 34.
(a) Why did Mendel choose garden pea for his experiments ? Write two reasons.
(b) List two contrasting visible characters of garden pea Mendel used for his experiment.
(c) Explain in brief how Mendel interpreted his results to show that the traits may be dominant or recessive. (Foreign 2016)
Answer:
(a) Mendel choose garden pea for his experiments because:
(i) It was easy to grow and it shows some clear contrasting traits like some pea plants are tall whereas others are dwarf.
(ii) Pea plants are self pollinating and many generation of pea plants can be produced in comparatively less time.

(b) The contrasting characters of garden pea plant studied by Mendel are:

Character	Plant
	Dominant	Recessive
1. Plant height	Tall	Yellow
2. Colour of the seed	Dwarf	Green

(c) Refer to answer 17.

Question 35.
Give the respective scientific terms used for studying:
(a) the mechanism by which variations are created and inherited and
(b) the development of new type of organisms from the existing ones. (Delhi 2014)
Answer:
(a) Heredity is the mechanism by which variations are created and inherited.
(b) Evolution is the process in which development of new type of organisms takes place from the existing ones.

Question 36.
Write the contribution of Charles Darwin in the field of evolution. (Delhi 2014)
Answer:
‘Lhe contribution of Charles Darwin in the field of evolution is that only the organism which can adapt; to the changing environmental conditions can survive and he also stated that the natural selection is a process which plays an important role in evolution of plants and animals by selecting the organisms with traits favourable to the environment.

Question 37.
Why do mice whose tails were surgically removed just after birth for generations, continue to produce mice with tails? (Foreign 2014)
Answer:
A group of mice are normally bred and all their progeny possess tail. Now, if the tails of these mice are surgically removed in each generation, tail¬less mice will not be produced. This is so because removal of tail during the life cycle of mice is an acquired character which is not transferred to future generations. The removal of the tail cannot change the genes of the germ cells of the mice.

Question 38.
List two differences between acquired traits and inherited traits by giving an example of each. (Delhi 2019)
Answer:
Differences between the inherited traits and acquired traits are as follows :

Inherited traits	Acquired traits
(i) These are passed from the parent to offspring.	These are developed during the life of an individual.
(ii) These are genetic variations.	These are somatic variations.
(iii) These develop due to crossing over phenomenon and mutations.	These develop due to use and disuse of organs and direct effect of environment.
(iv) These are passed on from one generation to the other.	These traits die with the death of the individual.
(v) Example: Skin colour, eye colour, form of hair, polydactyly (extra fingers), free and attached ear lobes, blood groups of human beings.	Example: If a group of mice are normally bred, all their progenies will have tails. Now, if the tails of these mice are removed surgically in each generation, tailless mice will not be produced. It is so because removal of tail is an acquired character and it will not bring change in the genes of the germ cells of the mice.

Question 39.
Define genetics. Why is decrease in the number of surviving tigers a cause of concern from the point of view of genetics? Explain briefly. (AI 2019)
Answer:
Genetics is the branch of biology that deals with the study of heredity and variations. The term genetics’ was coined by William Bateson in 1906. When a population is small, the number and scope of variations is limited and hence diversity and traits are reduced. Small numbers of surviving tigers are a cause of worry from the point of genetics because of the following reasons:

·         Their loss would cause a loss of gene pool, i.e., many genes will be eliminated from a gene pool.

·         Tigers are surviving in limited numbers, so if some natural calamity kills these small population of tigers, they will suddenly become extinct as per genetic drift phenomenon,

·         A disease may wipe out the leftover population, if the entire population is susceptible to the disease. This can cause sudden extinction of the tiger species and loss of their genes forever, thus, adversely affecting the diversity of nature.

Question 40.
Distinguish between the acquired traits and the, inherited traits in tabular form, giving one example for each. (Delhi 2017)
Answer:
Refer to answer 38.

Question 41.
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? (AI 2017)
Answer:
Certain experiences and traits earned by people during their lifetime are not passed on to their next generations because all these characters are acquired by the man during his lifetime. The man is not born with these traits and he cannot pass on these traits to his children. These could be better explained by the given examples:
(i) Child of a very good swimmer may not know how to swim. This is so because the technique of swimming is not inherited from parents but it is learnt by the person himself or herself.

(ii) A person may have a scar on the face from a cut he got in an accident. This is also an example of acquired trait which cannot be passed to the next generation.

Traits can be passed on to future generations in which changes have occurred in the genes present in the reproductive cells of the parent organisms. These traits or characters are known as inherited traits.

Question 42.
List three distinguishing features, in tabular form, between acquired traits and the inherited traits. (Delhi 2016)
Answer:
Refer to answer 38.

Question 43.
“We cannot pass on to our progeny the experience and qualifications earned during our lifetime.” Justify the statement giving reason and examples. (Delhi 2015)
Answer:
Refer to answer 41.

Question 44.
List in tabular form, two distinguishing features between the acquired traits and the inherited traits with one example of each. (Delhi 2015, AI 2012)
Answer:
Refer to answer 38.

Question 45.
Distinguish between inherited and acquired traits by giving one example of each. Give reason why the traits acquired during the lifetime of an individual are not inherited. (Foreign 2014)
Answer:
Refer to answers 38 and 41.

Question 46.
With the help of suitable examples, explain why certain traits cannot be passed on to the next generation? What are such traits called? (AI 2014)
Answer:
Refer to answer 41.

Question 47.
Tabulate two distinguishing features between acquired traits and inherited traits with one example of each. (Delhi 2013)
Answer:
Refer to answer 38.

Question 48.
“An individual cannot pass on to its progeny the experiences of its life-time”. Justify the statement with the help of an example and also give reason for the same. (Foreign 2012)
Answer:
If we breed a group of mice, all the progeny of mice will have tails just like their parents. Now, if we remove the tails surgically and again breed them, we still get new mice with tails. This is because cutting the tails of mice does not change the genes of their reproductive cells (or gametes). And since the acquired trait of‘cut tails’ does not bring about a change in the genes of mice, this trait cannot be passed on to their next generations. From this we conclude that the experiences acquired by an individual during his lifetime (called acquired traits) cannot be passed on to its progeny, and hence cannot lead to evolution because they are not caused by the change in genes.

Question 49.
Describe any three ways in which individuals with a particular trait may increase in population. (AI 2011)
Answer:
Three ways in which individuals with a particular trait may increase in population are:
(i) Genetic drift : It is the random change in gene frequency occurring by chance fluctuations. In this phenomenon, an event may increase the frequency of a particular trait having little adaptive value and survival advantage.

(ii) Natural selection : It is the phenomenon wherein nature selects traits favourable to the species in the environment. Thus, a particular trait selected by the nature increases in number.

(iii) Geographical isolation : Interbreeding populations are geographically isolated by barriers such as mountain ranges, rivers and sea. This geographical isolation leads to reproductive isolation and thus there is no gene flow between separated groups of population and therefore, population with particular character increases.

Question 50.
(a) What is the law of dominance of traits? Explain with an example.
(b) Why are the traits acquired during the life time of an individual not inherited? Explain. (2020)
Answer:
(a) Mendel’s law of dominance states that for a particular trait one gene is usually expressed over the other. The dominant allele expresses itself and the one which remains unexpressed is called recessive allele. This is called “law of dominance”. E.g., for height, there are two alleles, T for tallness and t for dwarfness.

(b) Refer to answer 41.

Question 51.
What is speciation?
Answer:
The process by which new species develop from the existing species is known as speciation.

Question 52.
What is speciation? Explain in brief the role of natural selection and genetic drift in this process. (Foreign 2016)
Answer:
‘Hie process by which new species develop from the existing species is known as speciation. Both natural selection and genetic drift play a very important role in speciation. By natural selection traits favourable to the species in the given environment are selected by the nature.

Example, beetles develop colour variation during reproduction. Consequently, in the progeny, one beetle develops green body colour instead of normal red colour. This beetle can inherit this variation in colour on to its progeny so that all its offspring are green in colour. Crows cannot easily locate green-coloured beetles on the green leaves of bushes and continue to eat red beetles which are easily located on green leaves. As a result, in population of green beetles increases in each generation.
Genetic drift is the elimination of the genes of certain traits from the small population when a section of population dies of natural calamity or migrates to other region. It alters the gene frequency of the remaining population.

Suppose in sexually reproducing red beetle population, a colour variation arises wherein one beetle develops blue body colour instead of red. This beetle can also pass this colour variation to its progeny so that all its offspring are blue coloured beetles. As the population of beetles expand, initially there would be few blue coloured beetles among the majority of red coloured beetles. At this point, an elephant comes by and stamps on the bushes where the beetles live. Consequently, most of the beetles get killed. By chance, most of the survived beetles are of blue colour. This population again slowly multiply and will contain mostly blue coloured beetles over a period of time. Survival of more blue beetles in the population changed the coloured characteristic from normal red to blue over a period of time. In small population, accidents can change the frequency of some genes in a population, even if it does not give any survival advantage to the possessors.

Question 53.
What is speciation? List four factors responsible for speciation. (Delhi 2015)
Answer:
The process by which new species develop from the existing species is known as speciation. The four factors responsible for speciation are:

·         Geographical isolation of a population leads to reproductive isolation and hence speciation.

·         Genetic drift leads to the formation of new species by causing drastic changes in the frequencies of particular gene by chance alone.

·         Natural selection wherein nature in the wild selects traits favourable to the species in the given environment.

·         Random mating resulting in genetic variations in genotype frequencies within the population.

Question 54.
Explain the following:
(a) Speciation
(b) Natural selection. (Al 2015, Delhi 2011)
Answer:
(a) Speciation : Origin of new species from existing one due to reproductive isolation of a part of its population is called speciation.
Suppose a large population of beetle living in an area gets split into two sub populations due to geographical barriers like river, mountain, sea, etc. Then members of these sub-populations will not be able to interbreed. Hence, there will be no gene flow between the members of these populations. After few generations, genetic drift will accumulate different variations in each of the two geographically separated sub-populations. Natural selection may also operate simultaneously in a different way in these geographically isolated sub-populations. This makes geographically isolated sub-populations to become more and more different from each other and ultimately reproductive isolation occurs between individuals of these groups and they transform into new species.

(b) Natural selection is the phenomenon wherein nature, in the wild, selects traits favourable to the species in its environment.
Also refer to answer 52.

Question 55.
List three main factors responsible for the speciation and briefly describe each one of them. (AI 2014)
Answer:
Speciation is the process by which new species develop from the existing species.

The three important factors responsible for the speciation are:
(i) Geographical isolation of a population caused by various types of barriers such as mountain ranges, rivers and sea. Geographical isolation is the major factor in the speciation of sexually reproducing animals because it causes reproductive isolation and interrupts the flow of genes between their isolated populations through the gametes.

(ii) Genetic drift is the elimination of the genes of certain traits from the small population when a section of population dies of natural calamity or migrates to other region. It alters the gene frequency of the remaining population.

(iii) Variations introduced by random mating and hybridisation. Random fusion of gametes adds new alleles and it result in formation of new species.

Question 56.
List three main factors responsible for the rise of a new species giving a brief description about each. (Foreign 2014)
Answer:
Refer to answer 55.

Question 57.
What is meant by the term speciation? List four factors which could lead to speciation. (Delhi 2012)
Answer:
Refer to answer 53.

Question 58.
(a) Classify the following as homologous or analogous pairs:
(i) Broccoli and cabbage
(ii) Ginger and radish
(iii) Forelimbs of birds and lizard
(iv) Wings of a bat and wings of a bird
(b) State the main feature that categorises a given pair of organs as homologous or analogous. (2020)
Answer:
(a) (i) Broccoli and cabbage – Homologous organs
(ii) Ginger and radish – Analogous organs
(iii) Forelimbs of birds and lizard – Homologous organs
(iv) Wings of bat and wings of a bird – Analogous organs

(b) Homologous organs have the same fundamental structure but different in function, while analogous organs have different internal structure but similar functions.

Question 59.
“During the course of evolution, organs or features may be adapted for new functions”. Explain this fact by choosing an appropriate example. (2020)
Answer:
Evolution is the sequence of gradual changes from simple life forms to complex life form. During the evolution process, many organs are adapted for new functions.
The organs which perform different functions in different species but have similar basic structure and similar embryonic origin are called homologous organs. Homologous structures are a result of divergent evolution. Homology indicates common ancestry.

For example, the forelimbs of a frog, a lizard, a bird and a human being show similarity in basic structure. However, these organs perform entirely different functions.

The given figures showing homologous organs :

The forelimbs of a frog help the animal to prop up the front end of the body at rest, and also act as shock absorbers when the animal lands on the ground after a leap; the forelimbs of a lizard are modified for creeping movements; the forelimbs of a bird are modified for flying purposes, while the forelimbs of a human being are used for grasping. The basio similarity in the forelimbs of these different vertebrates indicates that all these vertebrates had common ancestry, i.e., they have evolved from a common ancestor who had ‘five digited’ or ‘pentadactyl’ limbs (basic plan of limbs).

This means that these vertebrates have modified according to the special needs of the subsequent generations during the course of evolution.

Question 60.
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 (Delhi 2017, AI 2015, Delhi 2011)
Answer:
(a) Homologous organs perform different functions in different species but have similar basic structure and similar embryonic origin. For example, forelimbs of a frog, lizard, bird and human being show similarity in basic structure. However, these organs perform entirely different functions but the basic similarity in the forelimbs of these different vertebrates indicates common ancestry. This means that these vertebrates have modified according to the special needs of the subsequent generations during the course of evolution.

(b) Analogous organs are different in fundamental structure and embryonic origin but have similar appearance and perform similar functions. For example, the wings of an insect and a bird have different structures but they perform the same function of flying. The presence of these organs indicates that they are not derived from common ancestors but they can still evolve to perform similar functions to survive, flourish and keep on evolving in the prevailing environment. This provides a mechanism for evolution.

(c) Fossils are the remains or impressions of the dead animals and plants that lived in the remote past. The fossils provide evidence for evolution. For example, a fossil bird called Archaeopteryx have characters of both reptiles and birds. It had feathers, fused bones and beak which are exclusive characters of birds and had teeth in the jaws, claws on fingers, a long tail, etc like reptiles. Thus, Archaeopteryx is a connecting link between the reptiles and birds, and hence suggests that the birds have evolved from the reptiles through the process of continuous evolution.

Question 61.
“Evolution and classification of organisms are interlinked”. Give reasons to justify this statement. (AI 2017)
Answer:
Evolution is the process by which newer types of organisms are developed from the pre-existing ones through modifications. Classification is the arrangement of organisms into a series of groups based on physiological, biological, anatomical or other relationships. All systems of classification are hierarchial. The more closely two species are related, the more recently they have a common ancestor. Classification of organisms necessarily involves organising them in different groups, based on the similarities and differences of characteristics. It helps in the recognisation of the basic arrangement of a hierarchical structure among diverse species. It facilitates studies or research of wide variety associated with organisms effortlessly. Thus, classification of species is infact a reflection of their evolutionary relationship or we can say that evolution and classification are interlinked.

Question 62.
“Two areas of study namely evolution’ and classification are interlinked”. Justify this statement. (AI 2016)
Answer:
Refer to answer 61.

Question 63.
List three factors that provide evidences in favour of evolution in organisms and state the role of each in brief. (Foreign 2016)
Answer:
Some of the important sources which provide evidences for evolution are homologous organs, analogous organs and fossils.
Also refer to answer 60.

Question 64.
(a) 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.
(b) “Birds have evolved from reptiles”. State evidence to prove the statement. (Delhi 2015)
Answer:
(a) Planaria, insects, octopus and vertebrates cannot be grouped together on the basis of eye. Eyes of insects, octopus, Planaria and vertebrates are analogous organs which have developed over generation as an adaptation for similar function. They represent convergent evolution where distantly related groups develop similar functional structure as an adaptation for same function.

(b) Fossil bird Archaeopteryx had features like feathers, fused bones, beak which are the characteristic features of birds. It also had some features of reptiles, like, teeth in jaws, claws on free fingers, a long tail, etc. Thus, it represents a connecting link between reptiles and birds. This example provides a clue that birds have evolved from reptiles.

Question 65.
(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. (Foreign 2015)
Answer:
(a) Refer to answer 64(b).
(b) Refer to answer 64(a).

Question 66.
(a) Give the evidence that the birds have evolved from reptiles.
(b) Insects, octopus, Planaria and vertebrates possess eyes. Can we group these animals together on the basis of eyes that they possess? Justify your answer giving reason. (Delhi 2014)
Answer:
(a) Refer to answer 64(b).
(b) Refer to answer 64(a).

Question 67.
What are fossils? How do they help in the study of evolution? (Delhi 2013C, AI 2011)
Answer:
Refer to answer 60(c).

Question 68.
Distinguish between homologous organs and analogous organs. In which category would you place wings of a bird and wings of a bat? Justify your answer giving a suitable reason. (Delhi 2012)
Answer:
Those organs which have the same basic structure and similar embryonic origin but different functions are called as homologous organs.

These organs follow same basic plant of organisation during development, but in adults got modified to perform different function as an adaptation to different environments. Those organs which have different basic structure and embryonic origin but perform similar functions are called analogous organs.

The wings of bird and wings of bat are analogous organs because the basic structure or design of the wings of bird and bat are different but they perform the similar function of flying.

Question 69.
How are fossils formed? Describe, in brief, two methods of determining the age of fossils. (AI 2012)
Answer:
When organisms like plants or animals die, their bodies get decomposed by the action of microorganisms in the presence of oxygen, moisture, etc. Sometimes the conditions in the environment are such (like absence of oxygen, moisture, etc.), which do not let the body of the organism to get decompose completely. It is the body (or body part) of an organism which we get as fossil on digging the earth. In many cases the soft parts of the organisms get decomposed and we get skeleton of hard parts (like teeth, bones, etc.) as fossil. Even the soft parts of the plants and animals (which usually decompose quickly) are sometimes preserved as fossils in the form of their impressions inside the rocks. For example, if a dead leaf gets caught in mud, it will not decompose quickly. The mud around the leaf will set around it as a mould,’gradually harden to form a rock and retain the impression of the leaf. This forms a leaf fossil which can be dug out from the earth a after a long time period.

The age of fossils can be determined in two ways as given below:
(i) By the relative method: When we dig into the earth, we find fossils at different depths. The fossils which we find in layers closer to the surface of the earth are more recent and those fossils which are found in deeper layers are older; whereas the fossils found in the deepest layers of earth are the oldest ones.

(ii) Carbon dating method: When a living object dies and forms fossil, its carbon-14 radioactivity goes on decreasing gradually. By this method, the age of fossils is found by comparing the carbon-14 radioactivity left in fossils with the carbon-14 radioactivity present in living objects today.

Question 70.
(a) We see eyes in Planaria, insects, octopus and vertebrates. Can eyes be grouped together in case of the above-mentioned animals to establish a common evolutionary origin? Why?
(b) State one evidence to prove that birds have evolved from reptiles. (Foreign 2012)
Answer:
(a) Refer to answer 64 (a).
(b) Refer to answer 64 (b).

Question 71.
Explain how evolutionary relationship can be traced by the study of homologous organs. (Foreign 2011)
Answer:
Homologous organs provide morphological and anatomical evidences of evolution. There are number of organs in different groups of animals or plants which have similar basic design but are used for different purposes. These are termed as homologous organs. For example, the forelimbs of frog, lizard, bird and human beings show similarity in basic structure. The basic similarity of forelimbs of these different vertebrates indicates that all these have evolved from a common ancestor who had five digited or pentadactyle limbs which became modified according to the special needs of subsequent generations during the course of evolution. Hence, homologous organs depict divergent evolution or adaptive radiation.

Question 72.
(a) How do the following provide evidences in favour of evolution in organisms? Explain with an example for each.
(i) Homologous organs
(ii) Analogous organs
(iii) Fossils
(b) Explain two methods to determine the age of fossils. (AI 2019)
Answer:
(a) Refer to answer 60.
(b) Refer to answer 69.

Question 73.
Define evolution. How does it occur?
Describe how fossils provide us evidence in support of evolution. (AI 2016)
Answer:
Evolution is the sequence of gradual changes, from simple life forms to complex life form, i.e., from primitive organisms that lived over millions of years ago to new organisms that exist today. Evolution occurs by changes, improvement and modification of simple life forms. Fossils are the remains or impressions of organisms that lived in the remote past. Fossils provide the evidence that the present animal have originated from previously existing ones through the process of continuous evolution. Fossils can be used to reconstruct evolutionary history of an organism. The distribution pattern of fossils shows that the ancient fossils present in the bottom rocks are simple, while the most recent fossils found in the upper strata are more highly evolved. It means fossils form and become more and more complex as we proceed from earliest to recent rocks. It gives us an idea of time in history when different species were formed or became extinct. Thus, fossils provide us evidences in support of evolution.

Question 74.
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. (Foreign 2016)
Answer:
Refer to answers 60(c) and 69.

Question 75.
Define the term evolution. “Evolution cannot be equated with progress”. Justify this statement. (2020)
Answer:
Evolution is the sequence of gradual changes which take place in the primitive organisms over millions of years in which new species are produced.
Evolution should not be equated with progress because there is no real progress in the concept of evolution. Evolution is just the production of diversity of life forms and shaping of this diversity by the environmental selection. The only progress in evolution appears to be that more and more complex body designs of organisms have emerged over the ages. This will become clear from the following examples. When a new species is formed, it is not necessary that the old species will disappear or get eliminated from earth.

It will all depend on the environment. Also it is not as if the newly formed species are in any way better than the older ones. It is simply that genetic drift and natural selection processes have combined to form a population having different body design which cannot interbreed with the older population. It is a common belief that chimpanzees are the ancestors of human beings. It is, however, not true that human beings have evolved from chimpanzees. Actually, both chimpanzees and human beings had a common ancestor long time ago. The two offsprings of that ancestor evolved in their own separated ways to form the modern day chimpanzees and human beings.

Question 76.
Define the term “evolution”. Evolution should not be equated with progress.” Give reason to justify this statement. (Foreign 2014)
Answer:
Refer to answer 75.

Access Answers to Science NCERT Solutions for Class 10 Chapter 9 Heredity and Evolution

Exercise-9.1 Page: 143

1. If a trait A exists in 10% of a population of an asexually reproducing species and trait B exists in 60% of the same population, which trait is likely to have arisen earlier?

Solution:

Trait B is more probable to arise early as this trait has already been existing and replicating in a larger percentage of the population as compared to trait A.

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

Solution:

Genetic variations enable the species to better adapt to changes in its environment. Moreover, it is an important force in evolution as it allows the frequency of alleles to increase or decrease through natural selection. These variations will determine the difference between extinction or continuation of the species.

Exercise-9.2 Page: 147

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

Solution:

Mendel showed that the traits could either be dominant or recessive through his experiments that focused on the mono-hybrid cross. The experiment involved him crossing tall (TT) pea plants with dwarf (tt) pea plants. The resultant plants which formed after fertilisation represented the F1 (or filial) generation. All the F1 plants were tall. Mendel then proceeded to self-pollinate the filial generation plants, and the result was that 1/4th of the plants obtained in the F2 generation were dwarfs. From this experiment, Mendel concluded that the F1 tall plants were not true-breeding; instead, they carried the traits for both tall and dwarf heights. A portion of the plants was tall due to the fact that the traits for tallness were dominant over the traits for dwarfness. This cements the notion that traits can either be dominant or recessive.

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

Solution:

Mendel’s experiments show that traits are inherited independently through his dihybrid cross experiment. The experiment involved him using two traits – namely, seed shape and seed colour. The colour yellow (YY) is dominant over green (yy), while the round shape (RR) is dominant over the wrinkled shape (rr). The F2 progeny of the dihybrid cross resulted in a phenotypic ratio of 9:3:3:1; therefore, 9 plants with round yellow (RRYY) seeds, 3 plants with round green (RRyy) seeds and 3 plants with wrinkled yellow (rrYY) seeds and one with wrinkled green seeds (rryy). He further observed that the wrinkled greens and the round yellow are parental combinations while the round green and wrinkled yellow are new. A dihybrid cross between two seeds with dominant traits (RRYY) and non-dominant traits (rryy) resulted in the production of 4 types of gametes (RY, Ry, rY and ry). This means each of the gametes segregates independently of the other, and each with a frequency of 25% of the total gametes produced.

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?

Solution:

Given information is not enough to tell us which characteristics are dominant – blood group A or O. Blood type A is always dominant in ABO blood, and blood type O is always recessive. Here, the father’s blood group may be genotypically AA (homozygous) or AO (heterozygous), whereas that of the mother can be OA or OO.

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

Solution:

The sex of the child in humans is determined by the males. Males have XY chromosomes, while females have XX chromosomes. Hence, if:

• The male’s X chromosomes combine with the female’s X chromosomes, the mother gives birth to a girl
• The male’s Y chromosome combines with the female’s X chromosome, the mother gives birth to a boy

Exercise-9.3 Page: 150

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

Solution:

An individual attribute could increase a population in the following 2 ways:

(a) Natural selection: If an attribute is useful to a population, it’ll increase naturally.

For example, mosquitoes which are resilient against a particular pesticide will pass on their genes so that future generations become resistant as well. The mosquitoes which are affected by the pesticide die out.

(b) Genetic drift: If a species faces a catastrophic event where most of the population is wiped out, the surviving population can pass on their traits to the following generations. This may result in a rise in the attribute within the population.

2. Why are traits acquired during the lifetime of an individual not inherited?

Solution:

Traits acquired during a lifetime cannot be inherited by successive generations as the changes do not reflect in the DNA of the germ cells. For instance, a football player cannot pass on his skills to his offspring as they are limited to non-reproductive cells only.

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

Solution:

As the size of the tiger population decreases, the genetic pool of the species decreases too. This results in a limitation on the variations which will be introduced within the genetic makeup of the tigers. This lack of variation will result in serious implications. For example, if an illness spreads within the tiger population, it can potentially wipe out the whole population, possibly causing their extinction.

Exercise – 9.4 Page: 151

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

Solution:

Factors that would result in a new species are as follows:

(a) Mutation

(b) Genetic drift

(c) Natural selection

(d) Geographical isolation

(e) Generative isolation for prolonged periods

(f) Environmental factors on the isolated populations

(g) Quantum of genetic variant transmissible from one generation to the following generation

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

Solution:

In the pollination of plant species, geographical isolation is usually not a major factor, as no new trait will become part of the genetic makeup of a self-pollinating plant species. However, there are some possibilities of some environmental changes which could result in some variations.

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

Solution:

In the case of asexually reproducing organisms, geographical isolation can’t be considered a factor. This is due to the fact that meiosis does not occur during asexual modes of reproduction.

Exercise – 9.5 Page: 156

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

Solution:

Let us take the instance of humans and chimpanzees. Chimpanzees are able to express a wide range of emotions, such as busting out in laughter or smiling – this trait was once thought to be a feature exclusive to humans. The smile can be linked to the activation of the brain’s limbic system, where the orbicularis oculi muscle involuntarily contracts and raises the cheeks, forming wrinkles around the eyes. This implies that the smile is a true and genuine smile. Interestingly, this type of reflex has a name – the Duchenne smile. Moreover, research has shown that chimpanzees share 98.6% of our DNA – This means that humans and chimpanzees shared a common ancestor aeons ago. It is also important to note that chimpanzees are the closest living relatives to humans.

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

Solution:

The wing of a butterfly and the wing of a bat cannot be considered homologous organs as they do not share a common ancestor. Even though both structures aid in flying, they have evolved separately. To prove this, the wings of a butterfly are composed of two chitinous membranes, whereas the wings of a bat are composed of a bony skeleton, complete with blood vessels. Hence, these aren’t homologous organs but rather analogous organs.

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

Solution:

Fossils are the preserved remains of animals or plants, or other organisms that died out millions of years ago. These fossils tell us about a lot of extinct animals and also give insights into how evolution might have occurred. Fossils can be used to understand how an organism would have lived and what it may have looked like. More importantly, we can correlate with fossils as well as extant organisms to understand their relationships. For instance, scientists were able to recover protein sequences from a dinosaur called the T-rex, which confirmed its avian lineage. This means birds are the extant relatives of (avian) dinosaurs. Moreover, the pattern of fossil distribution gives us an idea of the time in history when various species were formed or become extinct.

Exercise – 9.6 Page: 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?

Solution:

While human beings do vary in colour and general appearance, their genetic makeup is identical to any other human. One of the speculations put forth for our drastic changes is due to evolutionary pressure, where the need to be easily recognised pushed us towards having widely different faces.

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

Solution:

Body designs are the result of environmental needs and pressure. Hence, we can’t conclude that one organism has a better body compared to another. For instance, fish have evolved a streamlined design as it is best suited for an aquatic environment. On the other hand, a spider or a chimpanzee might be ill-equipped to survive in such aquatic environments.

Exercises Page: 159

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 makeup of the tall parent can be depicted as

(a) TTWW

(b) TTww

(c) TtWW

(d) TtWw

Solution:

Correct answer – (c)

TtWW might be the genetic makeup of the tall parent. Since half the progenies are short, this implies that the parent plant also will have a collection of short genes; all progenies bore violet flowers, further suggesting that violet colour is dominant over white.

2. An example of homologous organs is

(a) Our arm and a dog’s foreleg

(b) Our teeth and an elephant’s tusks

(c) Potato and runners of grass

(d) All of the above

Solution:

Correct answer – (d)

Homologous organs have the same origin as each of the above organs but different functions. Homologous organs can be defined as the organs of various animals having similar basic structures but different functions. For example, a whale’s flippers, a frog’s forelimbs, and a man have the same basic structures but perform different functions, which is why they are called homologous organs.

3. In evolutionary terms, we have more in common with

(a) A Chinese schoolboy

(b) A chimpanzee

(c) A spider

(d) A bacterium

Solution:

Correct answer – (a)

Humans and chimpanzees are related since they belong to the identical order (Primates) and the same family (Hominidae). However, a schoolboy, regardless of ethnicity, is still a Homo sapien.

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?

Solution:

Knowledge of at least 3 generations is required to find if an attribute is dominant or recessive. Hence, it is not possible to identify if the given trait is dominant or recessive.

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

Solution:

Classification and evolution are two related fields of biology. Evolution pertains to how organisms evolve, and classification deals with finding out how two species are related to each other. For example, evolution and fossil evidence point to the fact that Australopithecus afarensis is considered one of our earliest ancestors. And classification tells us that Australopithecus afarensis belongs to the genus Homo, which is also the same genus as modern humans.

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

Solution:

Homologous organs are those organs that have the basic structural design as well as the origin; however, they serve different functions. For example, the forelimbs of humans and the wings of bats are anatomically similar.

Analogous organs are those organs that have a different structural design as well as origin; however, they perform similar functions. For example, the wings of birds and insects.

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

Solution:

Dogs have a certain set of genes that govern coat colour. There are a minimum of eleven known sequence series (A, B, C, D, E, F, G, M, P, S, T) that influence the colour of a dog. A dog inherits one copy from each of its parents. As an example, within the B series, a dog is genetically black or brown. Assume that one parent is homozygous black (BB), whereas the other parent is homozygous brown (bb).

In this case, all the offspring are going to be heterozygous (Bb).

Since black (B) is dominant, all the offspring are going to be black. However, they are going to have each B and b alleles. If such heterozygous pups are crossed, they are going to produce 25 homozygous blacks (BB), 15 heterozygous black (Bb), and 25 homozygous brown (bb) offspring.

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

Solution:

Fossils give evidence about:

(a) The organism and their paleobiology

(b) Even the behaviour of an organism can be deduced to some extent (for example, palaeontologists) have unearthed a site with more than 10,000 skeletons of a dinosaur called Hadrosaurus. This implies that the dinosaur lived in herds.

(c) Fossils also provide insight into the evolutionary history of animals and plants (for instance, palaeontologists have discovered that whales had evolved from goat-sized land-dwelling animals called Pakicetus).

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

Solution:

The evidence on the origin of life from inanimate matter was provided by Stanley L. Miller and Harold C. Urey’s experiment, which was conducted in 1953. They created an artificial environment which was reminiscent of the early earth’s atmosphere – it contained ammonia, hydrogen and other gases which were thought to have existed during primordial earth.

This concoction of gases was kept at a temperature slightly below 100 ° C. Additionally, sparks were generated to simulate lightning, which was also thought to be common during that period. At the end of the experiment, he was able to create 11 out of the 20 amino acids which are required for life.

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?

Solution:

Sexual reproduction causes a lot of viable variations because of the following reasons:

(a) Error in copying of DNA (though it was rare)

(b) Random segregation of paternal and maternal chromosomes at the time of sex cell formation.

(c) Exchange of genetic material between homologous chromosomes during the formation of gametes.

(d) Accumulation of variations occurred because of reproduction over generation after generation, and choice naturally created wide diversity.

(e) In the case of asexual reproduction, variation is severely limited as there is only one parent involved. Hence, the offspring is genetically similar to the parent

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

Solution:

Equal genetic contribution of male and female parents is ensured in progeny through the inheritance of equal numbers of chromosomes from both parents. There are 23 pairs of chromosomes, but not all are paired. The 22 pairs are called autosomes, while the remaining 1 pair is called the sex chromosomes (represented as X and Y.)

Females have two sets of X-chromosomes, while males have one X-chromosome and one Y-chromosome.

During the process of reproduction, fertilisation takes place, where the male gamete fuses with the female gamete, and it results in the formation of a diploid zygote. Furthermore, the zygote receives an equal contribution of genetic material from both parents. The male contributes 22 autosomes plus one X or Y chromosome. The female contributes 22 autosomes plus one X-chromosome.

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?

Solution:

The statement holds true – only variations provide an advantage to individual organisms that will survive in a population. For example, variations that lead to the increase in heat resistance in bacteria are very useful for survival if they find themselves in an environment where there is a sudden increase in ambient temperature. This will determine the difference between life and death for the bacteria.

NCERT Solutions for Class 10 Science Chapter 9 Heredity and Evolution

Chapter 9 – Heredity and Evolution are expected to have between 1 to 3 marks based on the yearly trends.

In 2018, however, only 1 question was asked (regarding Laws of Inheritance) in the Class 10 Science exam. But it would be wise to learn all the relevant concepts in order to avoid any unpleasant surprises.

The topics covered in this chapter are listed below:

• Laws of Inheritance
• Mendel’s Experiments
• Monohybrid Cross
• Dihybrid Cross
• Evolution and Its Theories
• Evidence of Evolution

NCERT Solutions for Class 10 Science Chapter 9 Heredity and Evolution

It is necessary to know how physical characteristics and traits are passed from a parent to his offspring. Furthermore, heredity provides a lot of insight into how genetics play a major role in organisms. Evolution is a gradual process where an organism changes and adapts to many variables. Sometimes, it is driven by the environment or competition.

Explore the definition of heredity and evolution. Discover its significance and implications. Find more learning resources on NCERT Solutions to aid your CBSE Board exam preparation and streamline your last-minute revisions.

Key Features of NCERT Solutions for Class 10 Science Chapter 9 Heredity and Evolution

• Elaborate and detailed solutions
• Tailored to meet CBSE-prescribed norms
• Use of simple and easy-to-understand language
• Nearly all jargons are explained in detail
• Access to a plethora of additional learning tools and resources ranging from sample papers to solved previous year question papers

Frequently Asked Questions on NCERT Solutions for Class 10 Science Chapter 9

Q1

What are analogous organs in Chapter 9 of NCERT Solutions for Class 10 Science?

The organs of different species having similar functions are called analogous organs. The anatomical features of these organs might not be the same, but the functions remain the same in all organisms. Students can refer to Chapter 9 of NCERT Solutions for Class 10 Science while answering the textbook questions to get a clear idea of the concepts. Both chapter-wise and exercise-wise solutions are available, which can be used by the students based on their needs.

Q2

What are the topics covered in the Heredity and Evolution of NCERT Solutions for Class 10 Science?

The topics covered in the Heredity and Evolution of NCERT Solutions for Class 10 Science are as follows:
1. Laws of Inheritance
2. Mendel’s Experiments
3. Monohybrid Cross
4. Dihybrid Cross
5. Evolution and Its Theories
6. Evidence of Evolution

Q3