NCERT Class X Science Solutions: Chapter 9 – Heredity and Evolution Part 3 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Question 7:

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


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) .

Inheritance in Dogs

In this case, all the offspring՚s will be heterozygous (Bb) .

Since black (B) is dominant, all the offspring՚s 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) offspring՚s.

Heterozygous Pups Are Crossed

Question 8:

Explain the importance of fossils in deciding evolutionary relationships.


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.

Sometime later, due to soil erosion or floods in that area, the rocks containing horse like 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 9:

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


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 () , carbon dioxide () , methane () , ammonia () , 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 () , methane () , ammonia () , and hydrogen gas () in a chamber and sparks were passed through this mixture using two electrodes. After one week, 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.

Result for Miller and Urey Experiment