Class 4 Science Chapter 6 Question Answers - Reproduction in Animals

The hen reproduces through internal fertilization, in which the egg is already fertilized before coming out in the form of an egg.
In the case of the frog, there is external fertilization; hence, the eggs are fertilized outside the female’s body.
The female lays coverless eggs in the water, and later the male deposits his sperm over them. With the help of their tails, the sperms swim and fuse with the eggs.
If these eggs had a hard covering, it would be impossible for the sperm to fuse with them, and thus reproduction would not be possible.

Ans: 

Reproduction is a process that is essential to maintain life.
Organisms reproduce for two main reasons:

1. To continue their progeny.

2. To provide group immortality by replacing dead individuals with new ones, ensuring the survival of the species on Earth.

Ans: Cloning is defined as the production of an exact copy of a cell, any other living part, or a complete organism.
Cloning of an animal was first performed successfully by Ian Wilmut and his colleagues. For the first time, a sheep named Dolly was cloned successfully.
Dolly was born on 5 July 1996 and was the first mammal to be cloned.

The process of cloning involved collecting a cell from the mammary gland of a female Finn Dorsett sheep, and an egg (whose nucleus was removed) from a Scottish Blackface ewe. The nucleus from the mammary gland cell of the Finn Dorsett sheep was inserted into the egg of the Scottish Blackface ewe. The egg thus produced was implanted into the Scottish Blackface ewe. Development of this egg took place normally, and finally Dolly was produced.

Ans: The process of producing young ones of the same species is known as reproduction. It is a very important process as it helps in the continuation of a species on Earth.

Modes of Reproduction:
There are two modes of reproduction:
(i) Sexual reproduction
(ii) Asexual reproduction

(i) Sexual reproduction: The process of reproduction in which two parents (male and female) are involved is called sexual reproduction. In this process, the male produces male gametes and the female produces female gametes. Both gametes fuse to form a zygote, which develops into a baby.
Example: Cow, cat, dog, and human beings.

(ii) Asexual reproduction: The mode of reproduction in which only one parent is involved to produce new young ones is called asexual reproduction.
Budding, fragmentation, spore formation, and binary fission are the methods of asexual reproduction, as seen in Hydra, Amoeba, and bacteria.

Ans: When sperms come in contact with an egg, one of the sperms may fuse with the egg. This fusion of the sperm and egg is called fertilization. In this process, the formation of a zygote takes place.

There are two types of fertilization:
(i) Internal fertilization
(ii) External fertilization

(i) Internal fertilization: The fertilization in which the fusion of the egg and sperm takes place inside the female’s body is called internal fertilization. It occurs in animals like cows, dogs, hens, and human beings.

(ii) External fertilization: The fertilization in which the fusion of the egg and sperm takes place outside the female’s body is called external fertilization. It is very common in aquatic animals like fish, frogs, and starfish.

Ans: In frogs, external fertilization takes place. During the rainy season, frogs move to ponds and slow-flowing streams. When the male and female frogs come together in water, the female lays hundreds of eggs. The eggs of a frog are not covered by a shell and are comparatively delicate.

A layer of jelly holds the eggs together and provides protection. As the eggs are laid, the male deposits sperms over them. Each sperm swims randomly in the water with the help of its tail. When sperms come in contact with the eggs, they fuse together. This results in fertilization.

Ans: Fertilization results in the formation of a zygote, which begins to develop into an embryo. The zygote divides repeatedly to form a ball of cells. These cells then begin to form groups that develop into different tissues and organs of the body. This developing structure is termed an embryo.

The embryo gets embedded in the wall of the uterus for further development. It continues to grow in the uterus and gradually develops body parts such as hands, legs, head, eyes, and ears. This stage is called a foetus. The mother gives birth to the baby when the foetus is fully developed.

Fig. 9.15 (a) Zygote formation and development of an embryo from the zygote; (b) Ball of cells (enlarged); (c) Embedding of the embryo in the uterus (enlarged).

Ans: 

Cloning is the production of an exact copy of a cell, part, or complete body. Dolly was the first clone, born on 5th July 1996, and was the first mammal to be cloned. It was cloned by Ian Wilmut and his colleagues.

During the process of cloning Dolly, a cell was collected from the mammary gland of a female Finn Dorsett sheep. Simultaneously, an egg was obtained from a Scottish Blackface ewe, and its nucleus was removed. The nucleus from the mammary gland cell of the Finn Dorsett sheep was inserted into the egg of the Scottish Blackface ewe. The development of this egg followed normally, and finally Dolly was born.

Fig. 9.16 (a) Finn Dorsett sheep (b) Scottish blackface ewe (c) Dolly (July 1996 to Feb. 2003).

Dolly was found to be identical to the Finn Dorsett sheep and produced several offspring of her own through normal sexual means. Dolly died on 14th February 2003 due to a lung disease.

Simultaneously, an egg was obtained from a Scottish blackface ewe. The nucleus was removed from the egg, then the nucleus of mammary gland cell from the Finn Dorsett sheep was inserted into egg of Scottish blackface ewe. Development of this egg followed normally and finally Dolly was born. It was found to be absolutely identical to the Finn Dorsett sheep and produced several offspring of her own through normal sexual means. Dolly died on 14^th February 2003 due to a certain lung disease. 

Ans: The diseases spread only by sexual contact are called Sexually Transmitted Diseases (STDs). When a healthy person comes in sexual contact with an infected person, the microorganisms are transmitted from the infected person to the healthy one, causing infection.

Syphilis, gonorrhoea, herpes, and AIDS are some examples of STDs. Herpes and AIDS are incurable and fatal diseases that destroy the immune system of the patient. Maintaining sexual hygiene and having safe sexual relationships are the only ways to prevent sexually transmitted diseases.