CBSE Class X

Question 1:

Define the principal focus of a concave mirror.

Answer:

Light rays that are parallel to the principal axis of a concave mirror converge at a

specific point on its principal axis after reflecting from the mirror. This point is

known as the principal focus of the concave mirror.

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.

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 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 inher-

ited 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 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 8:

Explain the importance of fossils in deciding evolutionary relationships.

Answer:

Fossils are the remains of the organism that once existed on earth. They represent

the ancestors of the plants and animals that are alive today. They provide evidences

of evolution by revealing the characteristics of the past organisms and the changes

that have occurred in these organisms to give rise to the present organisms. Let us

explain the importance of fossils in deciding evolutionary history with the help of the

following example.

Around 100 million years ago, some invertebrates died and were buried in the soil in

that area. More sediment accumulated on top of it turning it into sedimentary rock.

At the same place, millions of years later, some dinosaurs died and their bodies were

buried on top of the sedimentary rock. The mud containing dinosaurs also turned into

a rock.

Then, millions of years later, some horse-like creatures died in that area and got

fossilized in rocks above the dinosaur fossils.

Some time later, due to soil erosion or floods in that area, the rocks containing

horselike fossils are exposed.

If that area is excavated deeper, then the dinosaur and invertebrates fossils can also

be found. Thus, by digging that area, scientists can easily predict that horse-like

animals evolved later than the dinosaurs and the invertebrates.

Thus, the above example suggests that the fossils found closer to the surface of the

earth are more recent ones than the fossils present in deeper layers.

Question 7:

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

Answer:

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 6:

Explain the terms analogous and homologous organs with examples.

Answer:

Homologous organs are similar in origin (or are embryologically similar) but

perform different functions. For example, the forelimbs of humans and the wings

of birds look different externally but their skeletal structure is similar. It means

that their origin is similar (as wings in birds are modifications of forearm) but

functions are different – the wings help in flight whereas human forearm helps in

various activities.

Analogous organs, on the other hand, have different origin but perform similar

functions. For example, the wings of a bird and a bat are similar in function but

this similarity does not mean that these animals are more closely related. If we

carefully look at these structures, then we will find that the wings of a bat are

just the folds of skin that are stretched between its fingers whereas the wings

of birds are present all along the arm. Therefore, these organs are analogous

organs.

Question 5:

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

Answer: 

Classification involves grouping of organism into a formal system based on similar-

ities 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 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 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 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 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:

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:

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 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 2:

Find out, from Table, the medium having highest optical density. Also find the

medium with lowest optical density.

 

Answer:

Highest optical density = Diamond

Lowest optical density = Air

Optical density of a medium is directly related with the refractive index of that

medium.

A medium which has the highest refractive index will have the highest optical density

and vice-versa.

It can be observed from table 10.3 that diamond and air respectively have the

highest and lowest refractive index. Therefore, diamond has the highest optical

density and air has the lowest optical density.

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 evolution-

arily 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:

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 where

as 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 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.