NCERT Solutions for Class 12 Biology Chapter 1: Reproduction in Organisms
1. Why is reproduction essential for organisms?
Ans. Reproduction is essential for an organism because it is a process through which living organisms produce offsprings similar to them. Reproduction ensures the continuity of species. In the absence of reproduction the species will not able to exist for a long time and may extinct soon.
2. Which is a better mode of reproduction: sexual or asexual? Why?
Ans.Sexual reproduction is a better mode of reproduction because of the following reasons:
(i) Sexual reproduction creates diversity in genetic makeup.
(ii) In sexual reproduction both parents are involved hence offsprings get attribute of both parents.
(iii) Organisms produced by sexual reproduction have a greater survival rate as compared to asexual reproduction as organisms have variation which is adapted to various environments.
3. Why is the offspring formed by asexual reproduction referred to as clones?
Ans. The offspring formed by asexual reproduction is morphologically and genetically identical to the parent and to each other because they are uniparental. Hence, it is called a clone.
4. Offspring formed due to sexual reproduction have better chances of survival. Why? Is this statement always true?
Ans. Yes, it is true that the offsprings formed due to sexual reproduction because during sexual reproduction the male and female gametes fuse together. They got characters from both the parents and variations is also introduced in offsprings. As a result, they are better adapted to environment as these variations help them to survive better.
5. How does the progeny formed from asexual reproduction differ from those formed by sexual reproduction?
Ans. The progeny formed from asexual reproduction differ from those formed by sexual reproduction in following ways:
(i) Progeny formed from asexual reproduction is morphologically and genetically identical as they derived from single parent whereas the progeny formed from sexual reproduction is not exactly indentical to parents as they are derived from two parents i.e., male and female.
(ii) Progeny produced from asexual reproduction does not show any variation and hence called alone whereas progeny produced from sexual reproduction shows variation from each other and their parents.
6. Distinguish between asexual and sexual reproduction. Why is vegetative reproduction also considered as a type of asexual reproduction?
Ans.The differences between asexual and sexual reproduction are given in the following table:
|Asexual reproduction||Sexual reproduction|
||Sexual reproduction involves participation of separate parents.|
||It usually involves the sex organs.|
||It involves meiosis which occurs at th|
||The sexual reproduction requires fertilisation take place between opposite gametes leading to the production of a zygote.|
||The individuals are produced as a result of meiosis and gametic fusion, so exhibit genetic variation and differ from either of the two parents.|
||It is a very slow method of multiplication individuals.|
||Since there are variations, so it contributes evolution of the species.|
As vegetative reproduction does not involve two parents and any meiosis and syngamy so it is considered as a type of asexual reproduction. This term is used in the case of plants.
7. What is vegetative propagation? Give two suitable examples.
Ans. Vegetative propagation: Vegetative propagation is a process known to be the formation and growth of plants by an asexual means or by a fragment of a plant.
Example: Rhizome of ginger
Tuber of the potato
(i) Juvenile phase
(ii) Reproductive phase
(iii) Senescent phase
Ans. (i) Juvenile phase: The juvenile phase is the period of growth between the birth of an individual till reaches reproductive maturity. In plant, this is also called the vegetative phase.
(ii) Reproductive phase: The reproductive phase starts after the vegetative phase (juvenile phase) and the organisms produce offspring during this phase. A few plants show unusual flowering phenomena. Some of them such as bamboo species, flower only once in their lifetime, usually after 30-100 years. They produce large number of fruits and die. Among animals such as birds living in nature lay eggs seasonally while birds in captivity (as in poultry farm) can be made to lay eggs throughout the year.
(iii) Senescent phase: Senescent phase or old age begins at the end of reproductive phase. It is last phase of life span during which there is progressive deterioration in the body and loss in the capability of reproduction. Old age ultimately leads to death of the organism. In plants, it is characterized by the yellowing of leaves and leaf fall.
9. Higher organisms have resorted to sexual reproduction in spite of its complexity. Why?
Ans. Higher organisms have resorted to sexual reproduction to:
(i) Get over the unfavourable condition
(ii) Restore high gene pool in a population
(iii) Restore vigour and vitality of the race and get proper parental care
(iv) Introduce variation to enable better adaptive capacity.
10. Explain why meiosis and gametogenesis are always interlinked?
Ans. Gametogenesis is a process of gamete formation and meiosis is a process of cell division which reduces the number of chromosomes.
To maintain the constant number chromosome numbers over generation, gametogenesis occurs through meiosis to produce haploid gametes. Thus, the meiosis and gametogenesis are always interlinked.
11. Identify each part in a flowering plant and write whether it is haploid (n) or diploid (2n).
(v) Male gamete…..
Ans. (i) Ovary – Diploid(2n)
(ii) Anther – Diploid(2n)
(iii) Egg – Haploid(n)
(iv) Pollen – Haploid(n)
(v) Male gamete – Haploid(n)
(vi) Zygote – Diploid(2n)
12. Define external fertilization. Mention its disadvantages.
Ans. External fertilization: External fertilization is process in which fusion of male and female gametes takes place outside the female body in an external medium, generally in water.
Disadvantages of external fertilization:
1. In external fertilization, eggs have fewer chance of fertilization thus wastage of eggs.
2. Due to absence of parental care their is very low survival rate of progeny.
13. Differentiate between a zoospore and a zygote.
Ans. Difference between Zoospore and Zygote:
||It is sexually reproduced cell formed when male and female gametes units.|
||It is non-motile and does not contain flagella.|
||It is always diploid.|
14. Differentiate between gametogenesis from embryogenesis.
Ans.The difference between gametogenesis and embryogenesis:
||It is the formation and development of a multicellular embryo from unicellular zygote.|
||It involves cell division to increase the number of cells, cell growth and cell differentiation (formation of different kinds of tissues).|
||Mitosis occurs during embryogenesis.|
||It leads to organogenesis (organ formation)|
15. Describe the post-fertilization changes in a flower.
Ans. The post-fertilization changes that take place in a flower are as follows:
(i) The formation of zygote which later develops into an embryo and a primary endosperm cell which develops into an endosperm takes place.
(ii) While the sepals, petals and stamens are shed, the pistil remains intact.
(iii) The fertilized ovule develops into seeds.
(iv) The ovary matures into a fruit that later develops a thick protective wall, called the pericarp.
(v) Seeds after dispersal germinate under favourable conditions which later develop into a new plant.
16. What is a bisexual flower? Collect five bisexual flowers from your neighbourhood and with the help of your teacher find out their common and scientific names.
Ans. Bisexual flower: A flower which contains both male and female parts on same flower is called Bisexual flower.
|Common Name||Scientific Name|
17. Examine a few flowers of any cucurbit plant and try to identify the staminate and pistillate flowers. Do you know any other plant that bears unisexual flowers?
Ans. Cucurbit plant bears unisexual flowers as these flowers have either the stamen or the pistil. The staminate flowers bear bright, yellow-coloured petals along with stamens that represent the male reproductive structure. On the other hand, the pistillate flowers bear only the pistil that represents the female reproductive structure. Other examples of plants that bear unisexual flowers are corn, papaya, cucumber, etc.
18. Why are offspring of oviparous animals at a greater risk as compared to offspring of viviparous animals?
Ans. COffspring of oviparous animals at a greater risk as compared to offspring of viviparous animals because of absence of proper parental care and protection leading to law survival rate.