Ncert solutions for class 10th chapter 9 Heredity and evolution science
in-textual
Question 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?
Answer: Trait B because in asexual reproduction traits which are present in the previous generation are carried over to next generation with minimal variations. Trait B have higher percentage so it is likely to have arisen earlier.
Question 2: How does the creation of variations in a species promote survival?
Answer: 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.
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Question 1: How do Mendel’s experiments show that traits may be dominant or recessive?
Answer: Mendel selected true breeding tall (TT) and dwarf (tt) pea plants. Then, he crossed these
two plants. The seeds formed after fertilization were grown and these plants that were
formed represent the first filial or F1 generation. All the F1 plants obtained were tall.
Cross-pollination of tall and short plant
Then, Mendel self-pollinated the F1 plants and observed that all plants obtained in the F2 generation were not tall. Instead, one-fourth of the F2 plants were short.
Self-pollination of F1 plants
From this experiment, Mendel concluded that the F1 tall plants were not true breeding.
They were carrying traits of both short height and tall height. They appeared tall only
because the tall trait is dominant over the dwarf trait.
in-textual
Question 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?
Answer: Trait B because in asexual reproduction traits which are present in the previous generation are carried over to next generation with minimal variations. Trait B have higher percentage so it is likely to have arisen earlier.
Question 2: How does the creation of variations in a species promote survival?
Answer: 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.
Page no. 47
Question 1: How do Mendel’s experiments show that traits may be dominant or recessive?
Answer: Mendel selected true breeding tall (TT) and dwarf (tt) pea plants. Then, he crossed these
two plants. The seeds formed after fertilization were grown and these plants that were
formed represent the first filial or F1 generation. All the F1 plants obtained were tall.
Cross-pollination of tall and short plant
Then, Mendel self-pollinated the F1 plants and observed that all plants obtained in the F2 generation were not tall. Instead, one-fourth of the F2 plants were short.
Self-pollination of F1 plants
From this experiment, Mendel concluded that the F1 tall plants were not true breeding.
They were carrying traits of both short height and tall height. They appeared tall only
because the tall trait is dominant over the dwarf trait.
Question 2: How do Mendel’s experiments show that traits are inheritedindependently?
Answer: Mendel crossed pea plants having round green seeds (RRyy) with pea plants having
wrinkled yellow seeds (rrYY).
Since the F1 plants are formed after crossing pea plants having green round seeds and
pea plants having yellow wrinkled seeds, F1 generation will have both these characters in
them. However, as we know that yellow seed colour and round seeds are dominant
characters, therefore, the F1 plants will have yellow round seeds.
Then this F1 progeny was self-pollinated and the F2 progeny was found to have yellow
round seeds, green round seeds, yellow wrinkled seeds, and green wrinkled seeds in the
ratio of 9:3:3:1.
Question 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?
Answer: 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.
Question 4: How is the sex of the child determined in human beings?
Answer: In human beings, the females have two X chromosomes and the males have one X and
one Y chromosome. Therefore, the females are XX and the males are XY.
The gametes, as we know, receive half of the chromosomes. The male gametes have 22 autosomes and either X or Y sex chromosome.
Type of male gametes: 22+X OR 22+ Y.
However, since the females have XX sex chromosomes, their gametes can only have X sex chromosome.
Type of female gamete: 22+X
Thus, the mother provides only X chromosomes. The sex of the baby is determined by
the type of male gamete (X or Y) that fuses with the X chromosome of the female.
Question 1: What are the different ways in which individuals with a particular trait may increase in a
population?
Answer: 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.
Question 2: Why are traits acquired during the life-time of an individual not inherited?
Answer: 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.
Question 3: Why are the small numbers of surviving tigers a cause of worry from the point of view of
genetics?
Answer: 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.
Question 1: What factors could lead to the rise of a new species?
Answer: Natural selection, genetic drift and acquisition of traits during the life time of an
individual can give rise to new species.
Question 2: Will geographical isolation be a major factor in the speciation of a self-pollinating plant
species? Why or why not?
Answer: 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.
Question 3: Will geographical isolation be a major factor in the speciation of an organism that
reproduces asexually? Why or why not?
Answer: 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.
Question 1: Give an example of characteristics being used to determine how close two species are in
evolutionary terms.
Answer: The presence of feathers in dinosaurs and birds indicates that they are evolutionarily
related. Dinosaurs had feathers not for flying but instead these feathers provided
insulation to these warm-blooded animals. However, the feathers in birds are used for
flight. This proves that reptiles and birds are closely related and that the evolution of
wings started in reptiles.
Question 3: What are fossils? What do they tell us about the process of evolution?
Answer: 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.
Question 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?
Answer: 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.
Question 2: In evolutionary terms, can we say which among bacteria, spiders, fish and chimpanzees
have a ‘better’ body design? Why or why not?
Answer: 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.
Question 1: A Mendelian experiment consisted of breeding tall pea plants bearing violet flowers with
short pea plants bearing white flowers. The progeny all bore violet flowers, but almost
half of them were short. This suggests that the genetic make-up of the tall parent can be
depicted as
(a) TTWW
(b) TTww
(c) TtWW
(d) TtWw
Answer: (c) The genetic make-up of the tall parent can be depicted as TtWW
Since all the progeny bore violet flowers, it means that the tall plant having violet
flowers has WW genotype for violet flower colour.
Since the progeny is both tall and short, the parent plant was not a pure tall plant. Its
genotype must be Tt.
Therefore, the cross involved in the given question is
TtWw × ttww
↓
TtWw − ttww
Therefore, half the progeny is tall, but all of them have violet flowers.
Question 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.
Answer: (b)An example of homologous organs is our teeth and an elephant’s tusks.
Question 3: In evolutionary terms, we have more in common with
(a) a Chinese school-boy.
(b) a chimpanzee.
(c) a spider.
(d) a bacterium.
Answer:
(a) In evolutionary terms, we have more in common with a Chinese school boy.
Question 4:
A study found that children with light-coloured eyes are likely to have parents with lightcoloured
eyes. On this basis, can we say anything about whether the light eye colour
trait is dominant or recessive? Why or why not?
Answer:
Let us assume that children with light-coloured eyes can either have LL or Ll or ll
genotype. If the children have LL genotype, then their parents will also be of LL
genotype.
LL × LL
↓
LL
If the children with light-coloured eyes have ll genotype, then their parents will also have
ll genotype.
ll × ll
↓
ll
Therefore, it cannot be concluded whether light eye colour is dominant or recessive.
Question 5: How are the areas of study − evolution and classification − interlinked?
Answer: 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.
Question 6: Explain the terms analogous and homologous organs with examples.
Answer: Homologus organs are those organs which have the same basic structural design and origin but have different functions.
For example: The forelimbs of humans and the wing of birds look different externally but their skeletal structure is similar.
Analogus organs are those organs which have the different basic structural design and origin but have similar functions.
For example : The wings of birds and insects.
Question 7: Outline a project which aims to find the dominant coat colour in dogs.
Ans: Dogs have a variety of genes that govern coat colour. There are at least eleven identified
gene series (A, B, C, D, E, F, G, M, P, S, T) that influence coat colour in dog.
A dog inherits one gene from each of its parents. The dominant gene gets expressed in
the phenotype. For example, in the B series, a dog can be genetically black or brown.
Let us assume that one parent is homozygous black (BB), while the other parent is
homozygous brown (bb)
In this case, all the offsprings will be heterozygous (Bb).
Since black (B) is dominant, all the offsprings will be black. However, they will have both
B and b alleles.
If such heterozygous pups are crossed, they will produce 25% homozygous black (BB),
50% heterozygous black (Bb), and 25% homozygous brown (bb) offsprings.
Question 8: Explain the importance of fossils in deciding evolutionary relationships.
Ans: Fossil provide us evidence about
- The organisms that lived long ago such as the time period during which they lived, their structure etc.
- Evolutionary development of speciec i.e., line of their development.
- Connecting links between two groups. For example, feather present in some dinosours means that birds are very closely related to replies.
- Which organisms evolved earlier and which later.
- Development of complex body design from the simple body designs.
Question 9: What evidence do we have for the origin of life from inanimate matter?
Answer: A British scientist, J.B.S. Haldane, suggested that life originated from simple inorganic
molecules. He believed that when the earth was formed, it was a hot gaseous mass
containing elements such as nitrogen, oxygen, carbon, hydrogen, etc. These elements
combined to form molecules like water (H2O), carbon dioxide (CO2), methane (CH4),
ammonia (NH3), etc.
After the formation of water, slowly the earth surface cooled and the inorganic molecules
interacted with one another in water to form simple organic molecules such as sugars,
fatty acids, amino acids, etc. The energy for these reactions was provided by solar
radiations, lightning, volcanic eruptions, etc.
This was proved by the experiment of Stanley L. Miller and Harold C. Urey in 1953.
They took a mixture of water (H2O), methane (CH4), ammonia (NH3), and hydrogen gas
(H2) in a chamber and sparks were passed through this mixture using two electrodes.
After one week, 15% of the carbon from methane was converted into amino acids,
sugars, etc. These organic molecules are polymerized and assembled to form protein
molecules that gave rise to life on earth.
Question 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?
Answer: 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.
Question 11: How is the equal genetic contribution of male and female parents ensured in the
progeny?
Answer: 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.
Question 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?
Answer: 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.
