Case Study Based Questions: Reproduction in Animals | Science Class 8 PDF Download

Case Study 1

Do you recall the processes of digestion, circulation and respiration which you have studied in your previous classes? These processes are essential for the survival of every individual. You have also learnt about the process of reproduction in plants. Reproduction is essential for the continuation of a species. Imagine what would have happened if organisms had not reproduced. You will realise that reproduction is very important as it ensures the continuation of similar kinds of individuals, generation after generation. Modes of Reproduction: Have you seen the young ones of different animals? Try to name some of the young ones by completing shown in examples at S. No.1 and 5.You must have seen the young ones of various animals being born. Can you tell how chicks and caterpillars are born? How are kittens and puppies born? Do you think that these young ones looked the same before they were born as they do now? Let us find out. Just as in plants, there are two modes by which animals reproduce. These are: (i) Sexual reproduction, and(ii) Asexual reproduction. Sexual Reproduction Try to recall reproduction in plants which you studied in Class VII. You will remember that plants that reproduce sexually have male and female reproductive parts. Can you name these parts? In animals also, males and females have different reproductive parts or organs. Like plants, the reproductive parts in animals also produce gametes that fuse to form a zygote. It is the zygote which develops into a new individual. This type of reproduction beginning from the fusion of male and female gametes is called sexual reproduction. Let us find out the reproductive parts in humans and study the process of reproduction in them.

Q1: Reproduction is essential for the continuation of a …………………...
(a) Genus
(b) Species
(c) Breed
(d) Group
Ans: (b)

Q2: Which among the following is the correct option stating the young one of a butterfly?
(a) Caterpillar
(b) Calf
(c) Moth
(d) Chick
Ans: (a)

Q3: Calf is the young one of which of the following animals given below?
(a) Human
(b) Frog
(c) Hen
(d) Cow
Ans: (d)

Q4: Explain in detail the term “sexual reproduction” in animals.
Ans: In sexual reproduction, the reproductive parts of animals produce gametes (sperm and egg) that fuse to form a zygote. This fusion process is called fertilization. The zygote then develops into a new individual, inheriting genetic material from both parents, which leads to genetic diversity in the offspring.

Q5: Name the two modes of reproduction in animals.
Ans: The two modes of reproduction in animals are:
(i) Sexual reproduction
(ii) Asexual reproduction

Case Study 2

Male Reproductive Organs: The male reproductive organs include a pair of testes (singular, testis), two sperm ducts and a penis. The testes produce the male gametes called sperms. Millions of sperms are produced by the testes. Though sperms are very small in size, each has a head, a middle piece and a tail. Does it appear to be a single cell? Indeed, each sperm is a single cell with all the usual cell components. Female Reproductive Organs: The female reproductive organs are a pair of ovaries, oviducts (fallopian tubes) and the uterus. The ovary produces female gametes called ova (eggs). In human beings, a single matured egg is released into the oviduct by one of the ovaries every month. Uterus is the part where development of the baby takes place. Like the sperm, an egg is also a single cell. Fertilisation: The first step in the process of reproduction is the fusion of a sperm and an ovum. When sperms come in contact with an egg, one of the sperms may fuse with the egg. Such fusion of the egg and the sperm is called fertilisation. During fertilisation, the nuclei of the sperm and the egg fuse to form a single nucleus. This results in the formation of a fertilised egg or zygote. Did you know that the zygote is the beginning of a new individual? The process of fertilisation is the meeting of an egg cell from the mother and a sperm cell from the father. So, the new individual inherits some characteristics from the mother and some from the father. Look at your brother or sister. See if you can recognise some characters in them similar to those of your mother or your father. Fertilisation which takes place inside the female body is called internal fertilisation. Internal fertilisation occurs in many animals including humans, cows, dogs and hens. Boojho and Paheli’s teacher once told them in the class that in some women oviducts are blocked. These women are unable to bear babies because sperms cannot reach the egg for fertilisation. In such cases, doctors collect freshly released egg ands perms and keep them together for a few hours for IVF or in vitro fertilisation (fertilisation outside the body). In case fertilisation occurs, the zygote is allowed to develop for about a week and then it is placed in the mother’s uterus. Complete development takes place in the uterus and the baby is born like any other baby. Babies born through this technique are called test-tube babies. This term is actually misleading because babies cannot grow in test tubes. You will be surprised to know that in many animals’ fertilisation takes place outside the body of the female. In these animals, fertilisation takes place in water. Let us find out how this happens. During spring or rainy season, frogs and toads move to ponds and slow-flowing streams. When the male and female come together in water, the female lays hundreds of eggs. Unlike hen’s egg, frog’s egg is not covered by a shell and it is comparatively very delicate. A layer of jelly holds the eggs together and provides protection to the eggs. As the eggs are laid, the male deposits sperms over them. Each sperm swims randomly in water with the help of its long tail. The sperms come in contact with the eggs. This results in fertilisation. This type of fertilisation in which the fusion of a male and a female gamete takes place outside the body of the female is called external fertilisation. It is very common in aquatic animals such as fish, starfish, etc.

Q1: Which of the following is NOT a part of the female reproductive system?
(a) Fallopian tube
(b) Ovaries
(c) Testes
(d) Uterus
Ans: (c)

Q2: The type of fertilization in which the fusion of male and female gametes takes place outside the body is called ………………………...
(a) Internal fertilization
(b) External fertilization
(c) Systemic fertilization
(d) Directional fertilization
Ans: (b)

Q3: Which among the following is the correct full form of IVF?
(a) In voluntary fertilization
(b) Inter vesicular fistula
(c) Intra venous fertilization
(d) In vitro fertilization
Ans: (d)

Q4: Define the process of fertilization in detail.
Ans: When sperm comes in contact with the egg, one of the sperm fuses with the egg to form a zygote; this process is called fertilization. The nuclei of the sperm from the male and the nuclei of the ovum from the female fuse to form a single fertilized egg or zygote.

Q5: What are test tube babies?
Ans: Test tube babies are those conceived through in vitro fertilization (IVF). In this process, eggs and sperm are collected from the parents and are kept together in a laboratory setting for fertilization. Once fertilization occurs, the resulting zygote is allowed to develop for a few days and then is placed in the mother's uterus, where it can grow into a baby.

Case Study 3

How could a single cell become such a big individual? Development of Embryo: Fertilisation results in the formation of zygote which begins to develop into an embryo. The zygote divides repeatedly to give rise to a ball of cells. The 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. The embryo continues to develop in the uterus. It gradually develops body parts such as hands, legs, head, eyes, ears etc. The stage of the embryo in which all the body parts can be identified is called a foetus. When the development of the foetus is complete, the mother gives birth to the baby. Internal fertilisation takes place in hens also. But, do hens give birth to babies like human beings and cows? You know that they do not. Then, how are chicks born? Let us find out. Soon after fertilisation, the zygote divides repeatedly and travels down the oviduct. As it travels down, many protective layers are formed around it. The hard shell that you see in a hen’s egg is one such protective layer. After the hard shell is formed around the developing embryo, the hen finallylays the egg. The embryo takes about 3weeks to develop into a chick. You must have seen the hen sitting on the eggs to provide sufficient warmth. Did you know that development of the chick takes place inside the egg shell during this period? After the chick is completely developed it bursts open the egg shell. In animals which undergo external fertilisation, development of the embryo takes place outside the female body. The embryos continue to grow within their egg coverings. After the embryos develop, the eggs hatch. You must have seen numerous tadpoles swimming in ponds and streams. Viviparous and Oviparous Animals: We have learnt that some animals give birth to young ones while some animals lay eggs which later develop into young ones. The animals which give birth to young ones are called viviparous animals. Those animals which lay eggs are called oviparous animals. The following activity will help you understand better and differentiate between viviparous and oviparous animals. The eggs of a few animals are easy to observe because their mothers lay them outside their bodies. These are examples of oviparous animals. But you would not be able to collect the eggs of a dog, cow or cat. This is because they do not lay eggs. The mother gives birth to the young ones. These are examples of viviparous animals. Young Ones to Adults: The new individuals which are born or hatched from the eggs continue to grow till they become adults. In some animals, the young ones may look very different from the adults. Recall the lifecycle of the silkworm (egg → larva or caterpillar → pupa → adult) you studied in Class VII. Frog is another such example. Observe the different stages of frog starting from the egg to the adult stage. We find that there are three distinct stages, that is, egg → tadpole (larva) →adult. Don’t the tadpoles look so different from the adults? Can you imagine that these tadpoles would someday become frogs? Similarly, the caterpillar or the pupa of silkworm looks very different from the adult moth. The features that are present in the adult are not found in these young ones. Then what happens to the tadpoles or caterpillars there after? You must have seen a beautiful mothemerging out of the cocoon. In the case of tadpoles, they transform into adults capable of jumping and swimming. The transformation of the larva into an adult through drastic changes is called metamorphosis. What about the changes that we observe in our body as we grow? Do you think we too under gometamorphosis? In human beings, body parts similar to those present in the adults are present from the time of birth.

Q1: The developing zygote called the embryo gets embedded in which part of the female reproductive system?
(a) Oviduct
(b) Wall of uterus
(c) Cervix
(d) Ovary
Ans: (b)

Q2: Complete the missing stage in the life cycle of silkworm: egg→ larva or caterpillar→ ……………. →Adult
(a) Pupa
(b) Moth
(c) Tadpole
(d) Nymph
Ans: (a)

Q3: Which among the following is NOT an example of viviparous animals?
(a) Monkey
(b) Cow
(c) Dog
(d) Frog
Ans: (d)

Q4: Define the term “metamorphosis”.
Ans: Metamorphosis is the transformation of a larva into an adult through a series of drastic changes in form and structure. This process is common in insects and amphibians.

Q5: What do you mean by oviparous animals? Give one example.
Ans: Oviparous animals are those that lay eggs, with the embryo developing and hatching outside the mother's body. An example of an oviparous animal is a chicken.

Case Study 4

Asexual Reproduction: So far, we have learnt about reproduction in some familiar animals. But what about very small animals like hydra and microscopic organisms like amoeba? Do you know how they reproduce? Let us find out. In each hydra, there may be one or more bulges. These bulges are the developing new individuals and they are called buds. Recall the presence of buds in yeast. In hydra too the new individuals develop as outgrowths from a single parent. This type of reproduction in which only a single parent is involved is called asexual reproduction. Since new individuals develop from the buds in hydra, this type of asexual reproduction is called budding. Another method of a sexual reproduction is observed in the microscopic organism, amoeba. Let us see how this happens. You have already learnt about the structure of amoeba. You will recall that amoeba is a single-celled organism. It begins the process of reproduction by the division of its nucleus into two nuclei. This is followed by division of its body into two, each part receiving a nucleus. Finally, two amoebae are produced from one parent amoeba. This type of asexual reproduction in which an animal reproduces by dividing into two individuals is called binary fission. Apart from budding and binary fission, there are other methods by which a single parent reproduces young ones. You will study about these in your higher classes. Story of Dolly, the Clone: Cloning is the production of an exact copy of a cell, any other living part, or a complete organism. Cloning of an animal was successfully performed for the first time by Ian Wilmut and his colleagues at the Roslin Institute in Edinburgh, Scotland. They successfully cloned a sheep named Dolly. Dolly was born on 5th July 1996 and was the first mammal to be cloned. 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. The nucleus was removed from the egg. Then, the nucleus of the mammary gland cell from the Finn Dorsett sheep was inserted into the egg of the Scottish blackface ewe whose nucleus had been removed. The egg thus produced was implanted into the Scottish blackface ewe. Development of this egg followed normally and finally Dolly was born. Though Dolly was given birth by the Scottish blackface ewe, it was found to be absolutely identical to the Finn Dorsett sheep from which the nucleus was taken. Since the nucleus from the egg of the Scottish blackface ewe was removed, Dolly did not show any character of the Scottish blackface ewe. Dolly was a healthy clone of the Finn Dorsett sheep and produced several offspring of her own through normal sexual means. Unfortunately, Dolly died on 14th February 2003 due to a certain lung disease. Since Dolly, several attempts have been made to produce cloned mammals. However, many die before birth or die soon after birth. The cloned animals are many-a-times found to be born with severe abnormalities.

Q1: The process in which the bulges in the body of Hydra called the buds develop into a new individual is called ……………………………….
(a) Binary fission
(b) Bulbs
(c) Budding
(d) Budging
Ans: (c)

Q2: Dolly was a healthy clone of which of the following sheep?
(a) Finn Dorsett sheep
(b) Scottish blackface ewe
(c) Suffolk
(d) Cheviot
Ans: (a)

Q3: In which of the following year did Ian Wilmut successfully clone a sheep named Dolly?
(a) 1996
(b) 1997
(c) 2003
(d) 1995
Ans: (a)

Q4: Define the process of “asexual reproduction”.
Ans: The type of reproduction in which only a single parent is involved is known as asexual reproduction. Budding is one of the examples of asexual reproduction.

Q5: What is the process of cloning?
Ans: Cloning is the process of producing an exact genetic copy of a cell, living part, or a complete organism. The first successfully cloned animal was a sheep named Dolly, who was created by Ian Wilmut in 1996.