CBSE Important Questions Class 10 Science Chapter 8 – Heredity and Evolution

Class 10 Science Chapter 8 Heredity and Evolution is part of the CBSE Class 10 Science syllabus. It explains how traits pass from parents to children, Mendel's experiments on pea plants, monohybrid and dihybrid crosses, dominant and recessive traits, how the sex of a child is decided, and how variation and natural selection lead to evolution over long periods of time.

These CBSE important questions for Class 10 Science Chapter 8 cover the meaning of heredity and genes, the phenotypic and genotypic ratios of a monohybrid cross, sickle cell anaemia, Darwin's theory of natural selection, sex determination in humans, germinal variation, errors in DNA copying, the dihybrid cross and Mendel's contribution. Every question has a clear step-by-step answer in simple English with figures, so students can practise on their own, parents can check the answers, and teachers can use them for class tests.

Q1. (a) Define heredity. What is the physical basis of heredity?
(b) What are the phenotypic and genotypic ratios in the F2 generation in a monohybrid cross?
(c) A boy has parents with two different hair colours: one with red hair and the other with black hair. The boy has black hair. If black is dominant over red, what could be the possible genotype of the boy?

Answer: (a) Heredity is the passing of characters and traits from one generation to the next. Genes are the physical basis of heredity.
(b) In the F2 generation of a monohybrid cross, the phenotypic ratio is 3 : 1 and the genotypic ratio is 1 : 2 : 1.
(c) Let B stand for black hair (dominant) and b for red hair (recessive). The red-haired parent must be bb. The black-haired parent gives a B allele. So the boy gets B from one parent and b from the other. His genotype must be Bb, and because B is dominant, his hair is black.

Q2. Sickle cell anaemia is a genetic disorder caused by mutation of genes. Choose the option which correctly describes it.

Defective allele Affects Provides protection against
A HbA HbS White blood cells Malaria
B HbA HbA Red blood cells Dengue
C HbS HbS Lymphocytes Dengue
D HbS HbS Red blood cells Malaria

(a) A
(b) B
(c) C
(d) D

Answer: (d) D
Sickle cell anaemia appears when a person has two copies of the defective allele, HbS HbS. The disease affects the red blood cells, which become sickle (crescent) shaped instead of round. People carrying the HbS allele also get some protection against malaria, which is why the allele is common in malaria-prone regions.

Q3. Darwin's theory of evolution describes that
(a) within any population, there is natural variation.
(b) asexual reproduction brings about evolution.
(c) individuals of various species coexist without competition for food, space and mate.
(d) environment has no role in variation.

Answer: (a) within any population, there is natural variation.
Darwin observed that the individuals of any population are not all alike; there are small natural differences between them. Nature then selects those variations that help the organism survive. Options (c) and (d) are wrong, because there is always competition for food, space and mates, and the environment plays a major role in deciding which variations survive.

Q4. The theory of natural selection, proposed by Darwin to explain organic evolution, was based on
(a) modification of organs through use and disuse.
(b) struggle for existence and survival of the fittest.
(c) inheritance of acquired characters.
(d) appearance and inheritance of sudden large variations.

Answer: (b) struggle for existence and survival of the fittest.
Darwin said that organisms produce more offspring than can survive, so there is a struggle for existence. Those with favourable variations survive and reproduce, and this is called survival of the fittest. Options (a) and (c) belong to Lamarck's theory, and option (d) is the mutation theory of de Vries.

Q5. Explain the method of sex determination in humans.

Sex determination in humans through X and Y chromosomes

Answer: Human beings have 23 pairs of chromosomes. The 23rd pair decides the sex. A female has two X chromosomes (XX) and a male has one X and one Y chromosome (XY).
The mother can give only an X chromosome to the child, so every child gets one X from the mother.
The father can give either an X or a Y chromosome. If the child gets an X from the father, it becomes XX, a girl. If the child gets a Y from the father, it becomes XY, a boy.
So the sex of the child is decided by the chromosome received from the father, not the mother.

Q6. Which type of variation is inherited?

Answer: Germinal variation is inherited. These are variations that happen in the germ cells (the cells that make eggs and sperms), so they are passed on to the next generation. Variations in body cells, called somatic variations, are not inherited.

Q7. What is the effect of inaccuracy in DNA copying during the process of reproduction?

Answer: When DNA is copied during reproduction, the copying is not always perfectly accurate. These small errors create differences in the DNA, which are called variations. Such variations in the offspring are the raw material for evolution, because natural selection acts on them.

Q8. Observe the flowchart depicting a dihybrid cross and answer the following:
1. Fill the empty boxes with appropriate answers.
2. What were Mendel's findings with respect to inheritance of traits in the F1 and F2 generations?

Dihybrid cross between round-green and wrinkled-yellow pea plants

Answer: 1. The gametes of the round, green parent (RRyy) are Ry and those of the wrinkled, yellow parent (rrYY) are rY. The F1 generation is RrYy, which is round and yellow. On selfing, the F2 generation gives the ratio 9 : 3 : 3 : 1, that is 9 round yellow, 3 round green, 3 wrinkled yellow and 1 wrinkled green.
2. Mendel's findings were:
• The F1 generation showed both the dominant traits (round and yellow).
• The F2 generation showed the parental combinations as well as two new combinations, called recombinants (round green and wrinkled yellow). This happens because the two pairs of characters are inherited independently of each other.

Q9. Describe Mendel's experiment on the pea plant. What were his observations? What did he conclude from the results?

Monohybrid cross between a tall and a short pea plant with the F2 Punnett square

Answer: Mendel chose pea plants with clearly contrasting characters, such as tall and short plants, violet and white flowers, and round and wrinkled seeds.
He crossed a pure tall plant (TT) with a pure short plant (tt) and obtained the F1 generation. He then self-pollinated the F1 plants to get the F2 generation.
Parents: TT (tall) × tt (short)
F1: all Tt (tall)
F1 selfed: Tt × Tt → F2: TT, Tt, Tt, tt
Observations:
(a) All the plants of the F1 generation were tall.
(b) The F2 generation had both tall and short plants, with about one quarter short and three quarters tall.
Conclusions:
(a) Only one trait is seen in the F1 generation; this is called the dominant trait, and the hidden one is the recessive trait.
(b) Both traits were still present in the F1 plants, so each organism carries two copies of a trait and the characters do not blend.
(c) The two factors stay together in the F1 generation but separate during the formation of gametes and appear again in the F2 generation.

Q10. What is the effect on the reproduction process of DNA copying which is not perfectly accurate?

Answer: Imperfect DNA copying produces new variations in the offspring. These variations are passed on during reproduction. Over many generations, such variations lead to new traits and may even lead to the formation of new species, so this is a basic reason for the diversity of life.

Q11. What is natural selection? How can it help an organism to be 'selected'?

Answer: Natural selection is the process by which nature itself decides which organisms will survive in a particular environment.
In any area there are certain environmental conditions, such as heat, cold or drought. The organisms living there must face these conditions. Only those individuals whose variations suit the conditions are able to survive; the others die out. The survivors reproduce and their numbers increase generation after generation.
In this way an organism is 'selected' by nature for that environment. Over a long time, this process can change the whole population and lead to evolution.

Q12. What is Mendel's contribution to heredity and evolution?

Answer: Gregor Johann Mendel worked with garden peas and crossed plants with contrasting characters. From his experiments he found that:
• The F1 generation showed only one of the two traits, while the other stayed hidden; both traits appeared again in the F2 generation.
• Therefore each sexually reproducing organism carries two copies of every trait, one from each parent, and these do not blend.
• Traits are controlled by units he called 'factors', which we now call genes.
• Different pairs of traits are inherited independently of each other.
These results became the Laws of Inheritance. By showing how variation is passed on and recombined, Mendel laid the foundation of modern genetics, which explains how evolution works.

Related Study Material on Extramarks

Q.1 The process by which a seed produces a new plant under favourable condition is called __________________.

Marks:1

Ans

A seed contains the embryo that gives rise to a new plant on germination. Certain conditions such as availability of water, air and light are required for seed germination.

Q.2 A seed contains _______  that gives rise to a new plant on germination.

(a) zygote

(b) testa

(c) plumule

(d) embryo

Marks:1

Ans

A seed contains the embryo that gives rise to a new plant on germination.

Q.3 The process of transfer of male gametes to the female sex organ in plants is known as

(a) Pollination

(b) Budding

(c) Gametogenesis

(d) Fertilisation

Marks:1
Ans

Pollination

Q.4 a) Mention three ways of preventing unwanted pregnancy.
b) Which method would a couple follow to permanently avoid unwanted pregnancies? Why?

Marks:5

Ans

a) Many ways have been devised to avoid pregnancy. These contraceptive methods fall in a number of categories.

i. Mechanical Barrier: Condoms on the penis or similar coverings worn in the vagina can prevent the sperm to reach the egg.
ii. Contraceptives: They function by changing the hormonal balance of the body so that eggs are not released and fertilisation does not occur. These drugs commonly need to be taken orally as pills. However, since they change hormonal balances, they can cause side-effects too.
iii. Surgery: The transfer of sperm is prevented by blocking the vas deferens (tubes carrying the sperms) in the males. This method is called vasectomy. Similarly, the fallopian tube in the females is blocked to prevent the egg from reaching the uterus. This method is called tubectomy. Both of these surgical methods prevent the fertilisation, that is, fusion of sperm and ovum (egg), thus prevent unwanted pregnancy.

b) The best option for a couple who has completed its family is the surgical method, since this is a permanent method available.
Either the male can undertake vasectomy or the female can take tubectomy to prevent fertilisation. These are very simple surgical operations that can be carried out in any government run centre, free of cost. The patient is discharged within a few hours of the completion of surgery. The surgical methods are safe in the long run.

Q.5 Give any one reason of the following questions with appropriate answers:

  1. Why sexual reproduction is more advantageous over asexual reproduction?
  2. Why fertilisation in flowering plants is called double fertilisation?
  3. Why is menstruation important for a woman?

Marks:3
Ans

  1. Sexual reproduction is more advantageous over asexual reproduction because it brings variation in organisms, which can make them resistant to environmental stress.
  2. Fertilisation in flowering plants is called double fertilisation because it takes place twice, which includes syngamy and triple fusion.
  3. The menstrual cycle in the women is the body’ way of secreting tissue that it no longer needs. It is a normal biological process.

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