Heredity And Evolution Chapter 9 Class 10 Science NCERT Textbook PDF

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NCERT Class 10 Science Textbook Chapter 9 With Answer PDF Free Download

Heredity and Evolution

Chapter 9: Heredity and Evolution

We have seen that reproductive processes give rise to new individuals that are similar, but subtly different.

We have discussed how some amount of variation is produced even during asexual reproduction. And the number of successful variations is maximized by the process of sexual reproduction.

If we observe a field of sugarcane we find very few variations among the individual plants.

But in a number of animals including human beings, which reproduce sexually, quite distinct variations are visible among different individuals.

In this chapter, we shall be studying the mechanism by which variations are created and inherited.

The long-term consequences of the accumulation of variations are also an interesting point to be considered. We shall be studying this under evolution.

9.1 Accumulation Of Variation During Reproduction

Inheritance from the previous generation provides both a common basic body design and subtle changes in it, for the next generation.

Now think about what would happen when this new generation, in its turn, reproduces. The second generation will have differences that they inherit from the first generation, as well as newly created differences (Fig. 9.1).

Figure 9.1 would represent the situation if a single individual reproduces, as happens in asexual

If one bacterium divides, and then the resultant two bacteria divide again, the four individual bacteria generated would be very similar.

There would be only very minor differences between them, generated due to small inaccuracies in DNA copying.

However, if sexual reproduction is involved, even greater diversity will be generated, as we will see when we discuss the rules of inheritance.

Do all these variations in a species have equal chances of surviving in the environment in which they find themselves? Obviously not.

Depending on the nature of variations, different individuals would have different kinds of advantages.

Bacteria that can withstand heat will survive better in a heatwave, as we have discussed earlier.

The selection of variants by environmental factors forms the basis for evolutionary processes, as
we will discuss in later sections.

Language English
No. of Pages18
PDF Size2.9 MB

NCERT Solutions Class 10 Science 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 a trait B exists in 60% of the same population, which trait is likely to have arisen earlier?


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?


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?


Mendel showed that the traits can 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 fertilization 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?


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 color.

The color 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?


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.

Heredity and Evolution NCERT Textbook With Solutions PDF Free Download

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