CBSE Class X

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 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 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 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 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 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 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).

An example of dihybrid crosses

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.

Independent inheritance of two different traits

In the above cross, more than two factors are involved, and these are independently

inherited.

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 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.

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:

In asexual reproduction, the reproducing cells produce a copy of their DNA through

some chemical reactions. However, this copying of DNA is not accurate and therefore,

the newly formed DNA has some variations.

It can be easily observed in the above figure that in asexual reproduction, very few

variations are allowed. Therefore, if a trait is present in only 10% of the population,

it is more likely that the trait has arisen recently. Hence, it can be concluded that

trait B that exists in 60% of the same population has arisen earlier than trait A.

Question 11:

What could be the reasons for adopting contraceptive methods?

Answer:

Contraceptive methods are mainly adopted because of the following reasons:

(i) To prevent unwanted pregnancies.

(ii) To control population rise or birth rate.

(iii) To prevent the transfer of sexually transmitted diseases.

Question 10:

How does reproduction help in providing stability to populations of species?

Answer:

Living organisms reproduce for the continuation of a particular species. It helps in

providing stability to the population of species by producing a new individual that

resembles the parents. This is the reason why cats give birth to only cats or dogs

give birth to only dogs. Therefore, reproduction provides stability to populations of

dogs or cats or any other species.

Question 9:

How are the modes for reproduction different in unicellular and multicellular

organisms?

Answer:

In unicellular organisms, reproduction occurs by the division of the entire cell. The

modes of reproduction in unicellular organisms can be fission, budding, etc. whereas

in multicellular organisms, specialised reproductive organs are present. Therefore,

they can reproduce by complex reproductive methods such as vegetative propagation,

spore formation, etc. In more complex multicellular organisms such as human beings

and plants, the mode of reproduction is sexual reproduction.

Question 8:

What are the different methods of contraception?

Answer:
The contraceptive methods can be broadly divided into the following types:

• Natural method→ It involves avoiding the chances of meeting of sperms and

   ovum. In this method, the sexual act is avoided from day 10th to 17th of the

   menstrual cycle because during this period, ovulation is expected and therefore,

   the chances of fertilization are very high.
• Barrier method → In this method, the fertilization of ovum and sperm is

   prevented with the help of barriers. Barriers are available for both males and

   females. Condoms are barriers made of thin rubber that are used to cover penis

   in males and vagina in females.

• Oral contraceptives →In this method, tablets or drugs are taken orally. These

   contain small doses of hormones that prevent the release of eggs and thus

   fertilization cannot occur.

• Implants and surgical methods →Contraceptive devices such as the loop or

   Copper-T are placed in uterus to prevent pregnancy. Some surgical methods can

   also be used to block the gamete transfer. It includes the blocking of vas deferens

   to prevent the transfer of sperms known as vasectomy. Similarly, fallopian tubes

   of the female can be blocked so that the egg will not reach the uterus known as

   tubectomy.

Question 7:

Draw a labelled diagram of the longitudinal section of a flower.

Answer:

Question 6:

Why does menstruation occur?

Answer:

Menstruation is a process in which blood and mucous flows out every month through

the vagina. This process occurs every month because one egg is released from the

ovary every month and at the same time, the uterus (womb) prepares itself to

receive the fertilized egg. Thus, the inner lining of the uterus gets thickened and

is supplied with blood to nourish the embryo. If the egg does not get fertilised,

then the lining of the uterus breaks down slowly and gets released in the form of

blood and mucous from the vagina.

Question 5:

What are the functions performed by the testis in human beings?

Answer:

The testes are the male reproductive organs that are located outside the abdominal

cavity within a pouch called scrotum.

Functions of testes:

• Produce sperms

• Produce a hormone called testosterone, which brings about secondary sexual

   characters in boys.

Question 4:

What are the advantages of sexual reproduction over asexual reproduction?

Answer:

Advantages of sexual reproduction:

• In sexual reproduction, more variations are produced. Thus, it ensures survival of

   species in a population.

• The new formed individual has characteristics of both the parents.

• Variations are more viable in sexual mode than in asexual one. This is because in

   asexual reproduction, DNA has to function inside the inherited cellular apparatus.

Question 3:

The anther contains

(a) sepals.

(b) ovules.

(c) carpel.

(d) pollen grains.

Answer:

(d) The anther contains pollen grains.

Question 2:

Which of the following is not a part of the female reproductive system in human

beings?

(a) Ovary

(b) Uterus

(c) Vas deferens

(d) Fallopian tube

Answer:

(c) Vas deferens is not a part of the female reproductive system in human beings.