Is Parthenogenesis known as Sexual reproduction or Asexual reproduction?

Is Parthenogenesis known as Sexual reproduction or Asexual reproduction?

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I have seen that in some books refer to Parthenogenesis as sexual and in others as asexual. The ones that say it's a form of sexual reproduction,they focus on the fact that meios has happened in the process and also,we are using a sexual reproduction system in that animal to produce the offspring. The ones saying it is a from of asexual reproduction focus on the fact that although meios is involved,we only have one parent in the reproduction process and we cannot have any new alleles in the offspring than the ones the parent has.(unless we have mutation)

so how can we conclude in the end?which is it ?

and also,what is the evolutionary explanation for this phenomenon(parthenogenesis)?

Only a matter of definition

It is only a matter of definition. When it comes to sex, there is such a diversity of mating system that it is very hard to make a good definition that won't be challenged by limit cases.

Sexual reproduction can typically be defined based on the presence of meiosis (but again this will lead to limit-cases), on the need for two distinct parents + meiosis, or on the need for two distinct parents which have different gender (anisogamy) + meiosis.

Diversity of reproductive systems

At the end of the day, what one wish to call sexual reproduction does not matter too much as long as the definition is being made clear especially when dealing with potentially limit-cases.

If I remember correctly the intro book to evolutionary biolgy called Evolution by Futuyma makes a good overview of the diversity of reproductive system. Having a vague understanding of this diversity will help you undermine your hope of finding a clearly defined definition for sexual reproduction.

Applying the definition of wikipedia to parthenogenesis

Here is the definition from wikipedia

Sexual reproduction is a form of reproduction where two morphologically distinct types of specialized reproductive cells called gametes fuse together, involving a female's large ovum (or egg) and a male's smaller sperm

Using this definition and applying it to the specific case of parthenogenesis, one would conclude that parthenogenesis is not a form of sexual reproduction (despite the presence of meiosis). Note by the way the even within the term parthenogenesis, there is quite a bit of diversity (see the wikipedia entry for parthenogenesis).

what is the evolutionary explanation for this phenomenon(parthenogenesis)?

I will ignore this question because

  1. A post should always be limited to a single question
  2. This question is very broad
  3. There is not a single answer to this question given the diversity of types of parthenogenesis.

For a start on the subject, you might want to have a look at Jaenike and Selander, 1979 for example.

Is Parthenogenesis known as Sexual reproduction or Asexual reproduction? - Biology

By the end of this section, you will have completed the following objectives:

  • Describe advantages and disadvantages of asexual and sexual reproduction
  • Discuss asexual reproduction methods
  • Discuss sexual reproduction methods

Animals produce offspring through asexual and/or sexual reproduction. Both methods have advantages and disadvantages. Asexual reproduction produces offspring that are genetically identical to the parent because the offspring are all clones of the original parent. A single individual can produce offspring asexually and large numbers of offspring can be produced quickly. In a stable or predictable environment, asexual reproduction is an effective means of reproduction because all the offspring will be adapted to that environment. In an unstable or unpredictable environment asexually-reproducing species may be at a disadvantage because all the offspring are genetically identical and may not have the genetic variation to survive in new or different conditions. On the other hand, the rapid rates of asexual reproduction may allow for a speedy response to environmental changes if individuals have mutations. An additional advantage of asexual reproduction is that colonization of new habitats may be easier when an individual does not need to find a mate to reproduce.

During sexual reproduction the genetic material of two individuals is combined to produce genetically diverse offspring that differ from their parents. The genetic diversity of sexually produced offspring is thought to give species a better chance of surviving in an unpredictable or changing environment. Species that reproduce sexually must maintain two different types of individuals, males and females, which can limit the ability to colonize new habitats as both sexes must be present.

NEET DPP Biology Ch-23 Reproduction in Organism

Answer (b) Cloning is a technique by which genetically same individuals can be produced without including any sexual reproduction eg. Dolly sheep.

Q.3. Natural parthenogenesis occurs in:
(a) Protozoans
(b) Earthworm
(c) All insects
(d) Honeybee

Q.4. Retention of larval characters even after sexual maturity is called
(a) Parthenogenesis
(b) Ontogenesis
(c) Paedogenesis
(d) Neoteny

Q.5. Asexual reproduction is an effective strategy in stable environments because
(a) gametogenesis is most efficient under these conditions.
(b) the offspring, genetically identical to their parents, are preadapted to the environment.
(c) asexual parthenogenesis produces a large amount of genetic diversity.
(d) animal cells tend to be more totipotent under stable conditions.

Answer (b) The parents that have survived to reproduce asexually are able to survive in the current stable environment. Therefore, the offspring should be preadapted for this stable environment.

Q.6. If you compared the genetic makeup of an animal produced by parthenogenesis with that of its mother, which of the following would you expect?
(a) About 100 percent genetic similarity
(b) About 50 percent genetic similarity
(c) No genetic similarity
(d) Parthenogenetic animals have no mother

Answer (a) Species that exhibit parthenogenesis develop from unfertilized eggs produced by the mother. Therefore, the genetic make-up should be 100 percent the same as the mother. environment.

Q.7. Which of the following statements about animals that utilize external fertilization is false?
(a) They are divided equally between terrestrial and aquatic species.
(b) Many produce large numbers of gametes to ensure successful reproduction.
(c) The behaviours associated with mating are often highly synchronized.
(d) The probability of any one egg being fertilized and developing into an adult can be low.

Answer (a) Since external fertilization can only take place in an aquatic habitat, there are no terrestrial animals that use it.

Q.8. Which of the following statements about animal reproduction is false ?
(a) Species that reproduce sexually cannot also reproduce asexually.
(b) Viviparity, but not ovoviviparity, is common in mammals.
(c) Male insects can remove spermatophores deposited in a female by other males.
(d) Oogenesis and spermatogenesis both occur in simultaneous hermaphrodites.

Answer (a) Many animals reproduce both by asexual and sexual means.

Q.9. Which of the following animals qualifies as a sexually reproducing, oviparous species ?
(a) Human
(b) Chicken
(c) Kangaroo
(d) Sea star

Answer (b) All these animals can reproduce sexually. However, only the chicken lays an external egg.

Q.10. Benefits of asexual reproduction include all of the following except
(a) it often allows for the production of many more offspring at the same time
(b) it is advantageous in changing environments in which population variety is the key to successful propagation of a species
(c) it is easier in certain environments to have offspring without searching for a mate
(d) allowing the conservation of resources otherwise allocated to finding mates and performing ritualized courtship.

Answer (b) All of these statements concerning asexual reproduction are correct, except that asexual reproduction is best in favorable, stable environments, ones that don’t change rapidly. The reason for this is that asexual reproduction, in contrast to its sexual counterpart, results in the formation of identical offspring. Although asexual organisms can often produce many more offspring in a single reproductive event than sexual organisms, these asexually produced young do not usually have the genetic variation caused by meiosis and crossingover to be able to survive a rapidly changing environment or times of environmental stress.

Q.11. All the ‘eyes’ of a potato tuber are taken out and it is sown in the ground normally. New plants will
(a) Not emerge
(b) Emerge normally
(c) Be weaker
(d) Be healthier

Answer (a) Buds in ‘eyes’ form new plants.

Q.12. Basal half of an onion bulb is removed and upper half is sown in the ground. New plant will
(a) Emerge normally
(b) Not emerge
(c) Be without leaves
(d) Be without flowers

Answer (b) Bud giving rise to new plant is present towards base.

Q.13. A small portion of cane-sugar stem between the two successive nodes is cut off and sown in the soil normally. New plants will
(a) Be formed normally
(b) Not be formed
(c) Be without juice
(d) Without nodes

Answer (b) New plants in cane-sugar are formed from nodes which are absent.

Q.14. A cutting of rose plant is thoroughly waxed and planted in the soil normally, It will form
(a) New rose plant
(b) A dead piece of rose stem after some time
(c) A rose plant of improved variety
(d) None of these

Answer (b) Water absorption & gaseous exchange stop due to presence of wax

Q.15. When an ovary develops into a fruit, without fertilization, it is called
(a) apospory
(b) apogamy
(c) parthenocarpy
(d) porogamy

Answer (c) Parthenocarpy is the development of a fruit without the formation of seeds as a result of lack of pollination, lack of fertilization and lack of development. This condition can be artificially induced by application of hormones.

Q.16. Asexual reproduction is the best strategy for plants
(a) that are well adapted to their stable environment.
(b) as winter approaches
(c) when new genes must be introduced
(d) that have underground stems.

Q.17. Bamboo reproduces by
(a) rhizomes
(b) tubers
(c) corms
(d) stolons

Q.18. Grafting is an example of asexual reproduction. Which of the following choices is an example of asexual reproduction involving nonvegetative parts of a plant ?
(a) Apomixis
(b) Production of corms
(c) Production of bulbs
(d) Production of rhizomes

Q.19. What is necessary for successful grafting to occur ?
(a) Each section must be able to form roots.
(b) The grafted section must be able to form seeds.
(c) Fusion of the two vascular tissues must occur.
(d) Fusion of the two cambial tissues must occur.

Q.20. Banana fruits are seedless, because
(a) auxins are sprayed for rapid development of fruit.
(b) vegetative propagation of plant.
(c) they are triploid plants.
(d) fruits are artificially ripened.

Answer (c) Most of banana varieties are triploid and triploidy is associated with seedlessness.

Q.21. Consider the following statements and choose the correct option.
(i) The genetic constitution of a plant is unaffected in vegetative propagation.
(ii) Rhizome in ginger serves as an organ of vegetative reproduction.
(iii) Totipotency of cells enables us to micropropagate plants.
(a) Statements (i) and (ii) alone are true
(a) Statements (ii) and (iii) alone are true
(c) Statement (ii) alone is true
(d) All the three statements (i), (ii) and (iii) are true

Q.22. Plants identical to mother plants can be obtained from
(a) seeds
(b) stem cutting
(c) Both (a) and (b)
(d) None of these

Q.23. Ploidy of ovary, anther, egg, pollen, male gamete and zygote are respectively-
(a) 2n, 2n, n, 2n, n, 2n
(b) 2n, 2n, n, n, n, 2n
(c) 2n, n, n, n, n, n
(d) 2n, 2n, n, 2n, 2n, 2n

Q.24. Offsprings of oviparous animals are at greater risk as compared to offsprings of viviparous animals because-
(a) Proper embryonic care and protection is lesser
(b) Embryo is not developed
(c) Progenies are with more variation
(d) Progenies are larger

Q.25. The parameter(s) of senescence or old age is/are-
(a) End of the reproductive phase
(b) Concomitant change in body (like slowing metabolism)
(c) Failure of metabolism decreases
(d) Both (a) and (b)

Q.26. The terms homothallic and monoecious are used to denote
(a) bisexual condition
(b) unisexual condition
(c) staminate flowers
(d) pistillate flowers

Answer (a) Homothallic and monoecious are terms used to denote bisexual condition. The example indudes fungi and plants. Heterothallic and dioecious are terms used to denote unisexual condition.

Q.27. During regeneration, modification of an organ to other organ is known as
(a) Morphogenesis
(b) Epimorphosis
(c) Morphallaxis
(d) Accretionary growth

Answer (b) Morphallaxis is a mechanism of regeneration involving reorganization of body cells. In epimorphosis, new cells proliferate from the surface of the wound to form the missing structure. In accretionary growth some specialized cells retain the ability to divide and produce new cells to replace the worn-out.

Q.28. Cells become variable in morphology and function in different regions of the embryo. The process is
(a) differentiation
(b) metamorphosis
(c) organisation
(d) rearrangement

Answer (a) Cells become variable in shape, size & getting their specialization for the formation of particular tissue or organ in future foetus. They place themselves at some specific regions in embryo for further organogeny.

Q.29. Earthworms, sponges, tapeworms are
(a) bisexual animals
(b) unisexual animals
(c) hermaphrodites
(d) Both (a) and (c)

Answer (d) Earthworm, sponges, tapeworms are bisexual animals and hermaphrodites as they prossess both male and female reproductive organs.

Q.30. The site of origin of the new plantlets in potato, dahlia, ginger and banana is-
(a) Floral buds present on stem
(b) Internodes of modified stem
(c) Nodes of modified stem
(d) Adventitious buds present on root

Q.31. Among the following which one is not a method of vegetative propagation?
(a) Budding
(b) Layering
(c) Sowing
(d) Tissue culture

Answer (c) Sowing is related with sexual reproduction.

Q.32. Vegetative propagation in mint occurs by:
(a) offset
(b) rhizome
(c) sucker
(d) runner

Answer (c) Vegetative propagation in mint occurs through sucker. Vegetative reproduction is a type of asexual reproduction for plants, and is also called vegetative propagation, vegetative multiplication, or vegetative cloning. It is a process by which new plant “individuals” arise or are obtained without production of seeds or spores. It is a natural process in many plant species (as well as non-plant organisms such as bacteria and fungi) and used or encouraged by horticulturists to obtain quantities of economically valuable plants. A related technique used in cultivation is tissue culture, which involves vegetative reproduction under sterile conditions.

Q.33. What is common between vegetative reproduction and apomixis?
(a) Both are applicable to only dicot plants
(b) Both bypass the flowering phase
(c) Both occur round the year
(d) Both produces progeny identical to the parent

Answer (d) Vegetative reproduction and apomixis, both are asexual methods of reproduction, which gives the progeny genetically similar to parent.

Q.34. Individuals of a clone-
(a) Are genetically similar but morphologically different
(b) Are morphologically similar but genetically different
(c) Are morphologically and genetically similar
(d) Are genetically and phenotypically different

Q.35. Some organisms are capable of asexual or sexual reproduction. Under favourable conditions, reproduction proceeds asexually. When conditions become more stressful reproduction switchess to a sexual mode. Why?
(a) Sexual reproduction is simple and more rapid allowing larger numbers of offspring to be produced
(b) Sexual reproduction requires two separate individuals, who can mutually provide nutrient support during stress
(c) Sexual reproduction produces individuals with new combinations of recombined chromosomes increasing diversity
(d) Asexual reproduction requires more energy

Q.36. Apomixis in plant means development of a plant
(a) from root cuttings
(b) without fusion of gametes
(c) from fusion of gametes
(d) from cuttings of stem

Q.37. Which of the following is not vegetative propagule ?
(a) Rhizome and sucker
(b) Tuber and offset
(c) Bulbil (e.g. in Agave), leaf buds and bulb
(d) Antherozoid

Q.38. Which of the following is false about external fertilization?
(a) Organisms showing external fertilization exhibit great synchrony between the sexes and release gametes.
(b) Gametes are produced in large number in water to enhance the chances of fertilization.
(c) A large number of gametes are wasted.
(d) A major advantage is that the offspring are protected from predators and there is a great chance of their survival upto adulthood.

Q.39. Modified stem present in Gladiolus is:
(a) bulb
(b) rhizome
(c) corm
(d) bulbil

Q.40. Which of the following are seasonal breeders?
(a) Frogs
(b) Birds
(c) Lizards
(d) All of these

Q.41. Select the wrong statement:
(a) Anisogametes differ either in structure, function or behaviour.
(b) In oomycetes female gamete is smaller and motile, while male gamete is larger and non-motile.
(c) Chalmydomonas exhibits both isogamy and anisogamy and Fucus shows oogamy.
(d) Isogametes are similar in structure, function and behaviour.

Answer (b) In oomycetes female gamete is large and non motile while male gamete is small & motile.

Q.42. Monoecious plant of Chara shows occurrence of :
(a) stamen and carpel of the same plant
(b) upper antheridium and lower oogonium on the same plant
(c) upper oogonium and lower antheridium on the same plant
(d) antheridiophore and archegoniophore on the same plant

Answer (c) Male sex organ is called antheridium or globule while female sex organ is called oogonium. They develop on the same branchlet in the same plant in chara.

Q.43. Which of the following statement(s) is/are false about internal fertilization?
(i) Male gametes are motile.
(ii) Male gametes are non-motile.
(iii) Male gametes are produced in large number.
(iv) Male gametes are produced in small number.
(v) There is a significant reduction in the number of eggs produced.
(a) (i), (iii) and (v)
(b) (iii) and (iv)
(c) (ii) and (iv)
(d) Only (v)

Q.44. Syngamy can occur outside the body of the organism in
(a) Fungi
(b) Mosses
(c) Algae
(d) Ferns

Answer (c) In most aquatic organisms, such as a majority of algae and fishes as well as amphibians, syngamy occurs in the external medium (water), i.e., outside the body of the organism. This type of gametic fusion is called external fertilisation.

Q.45. Select the correct sequence from the following.
(i) Juvenile phase → Senescent phase → Reproductive phase
(ii) Juvenile phase → Reproductive phase → Senescent phase
(iii) Reproductive phase → Juvenile phase → Senescent phase
(iv) Vegetative phase → Reproductive phase → Senescent phase
(a) (i) and (ii)
(b) (i) and (iv)
(c) (iii) and (iv)
(d) (ii) and (iv)

Sexual reproduction involves the fusion of male and female gametes to form a diploid zygote, which develops into a new organism. It leads to genetic variation. The types of sexual reproduction seen in animals are syngamy (fertilization) and conjugation. In syngamy, the fusion of two haploid gametes takes place to produce a diploid zygote. Depending upon the place where the fertilization takes place, it is of two types.

In external fertilization, the fusion of male and female gametes takes place outside the body of female organisms in the water medium. e.g. sponges, fishes and amphibians. In internal fertilization, the fusion of male and female gametes takes place within the body of female organisms. e.g. reptiles, aves and mammals.

Different kinds of syngamy (fertilization) are prevalent among living organisms.

The male and female gametes are produced by the same cell or same organism and both the gametes fuse together to form a zygote. e.g. Actinosphaerium and Paramecium.

The male and female gametes are produced by different parents and they fuse to form a zygote. So it is biparental. e.g. Human – dioecious or unisexual animal.

Lower organisms, sometimes the entire mature organisms do not form gametes but they themselves behave as gametes and the fusion of such mature individuals is known as hologamy e.g. Trichonympha.

(d) Paedogamy:

It is the sexual union of young individuals produced immediately after the division of the adult parent cell by mitosis.

The fusion of small sized and morphologically different gametes (merogametes) takes place.

The fusion of morphological and physiological identical gametes (isogametes) is called isogamy. e.g. Monocystis.

(g) Anisogamy:

The fusion of dissimilar gametes is called anisogamy (Gr. An-without iso-equal gam-marriage). Anisogamy occurs in higher animals but it is customary to use the term fertilization instead of anisogamy or syngamy. e.g. higher invertebrates and all vertebrates.

Conjugation is the temporary union of the two individuals of the same species. During their union both individuals, called the conjugants exchange certain amount of nuclear material (DNA) and then get separated. Conjugation is common among ciliates, e.g. Paramecium, Vorticella and bacteria (Prokaryotes).

Phases of life cycle:

Organisms have three phases – Juvenile phase, reproductive phase and senescent phase. Juvenile phase/vegetative phase is the period of growth between the birth of the individual upto reproductive maturity. During reproductive phase/ maturity phase the organisms reproduce and their offsprings
reach maturity period. On the basis of time, breeding animals are of two types: seasonal breeders and continuous breeders.

Seasonal breeders reproduce at particular period of the year such as frogs, lizards, most birds, deers etc., Continuous breeders continue to breed throughout their sexual maturity e.g. honey bees, poultry, rabbit etc., Senescent phase begins at the end of reproductive phase when degeneration sets in the structure and functioning of the body.

Parthenogenesis (Gr. Parthenos – virgin, Genesisproduce)

Development of an egg into a complete individual without fertilization is known as parthenogenesis. It was first discovered by Charles Bonnet in 1745. Parthenogenesis is of two main types namely, Natural Parthenogenesis and Artificial Parthenogenesis. In certain animals, parthenogenesis occurs regularly, constantly and naturally in their life cycle and is known as natural parthenogenesis.

Natural parthenogenesis may be of two types, viz., complete and incomplete. Complete parthenogenesis is the only form of reproduction in certain animals and there is no biparental sexual reproduction. There are no male organisms and so, such individuals are represented by females only.


A number of parthenogenic variations have been observed. Some aphids and water fleas undergo a type of parthenogenesis called heterogony or cyclic parthenogenesis. In these species, generations of offspring produced from fertilized eggs may alternate with those produced from unfertilized ones. Such an alternation of generations in both groups of insects is thought to result partly from seasonal temperature changes, with eggs produced through sexual reproduction having a greater ability to withstand the winter cold. They lie dormant until temperatures rise.

Pseudogamy (gynogenesis, or sperm-dependent parthenogenesis) is another variation, which appears in the life cycle of a few insects, mites, and salamanders as well as the flatworm Schmidtea polychroa. S. polychroa is hermaphroditic and may be diploid (which can reproduce sexually) or polyploid (that is, with one or more additional sets of chromosomes). Whereas sexual reproduction requires sperm for fertilization, parthenogenic reproduction in this species involves sperm only to stimulate the initial development of the egg the sperm’s genetic material is not used.

The Editors of Encyclopaedia Britannica This article was most recently revised and updated by John P. Rafferty, Editor.

CBSE Class 12 Biology –Chapter 1 Reproduction in Organism- Study Materials

Find CBSE Class 12th Biology notes for the chapter Reproduction in Organisms. Every concept is followed by the solved and unsolved questions. You can also find the various categories questions like short, very short and long answer type

Find CBSE Class 12th Biology notes for the chapter Reproduction in Organisms. Every concept is followed by the solved and unsolved questions. You can also find the various categories questions like short, very short and long answer type.

Some important terminologies and concepts are given below:

Reproduction-As a biological process in which an organisms gives rise to young ones similar to itself.

Clone-Morphologically and genetically similar individuals is called clone.

Binary Fission-Many single celled organisms reproduce by binary fission, where a cell divides in two halves and each rapidly -grows into adult.

e.g. Amoeba, Paramecium and yeast.

Vegetative Propagation-In plant the formation of vegetative propagation does not evolve two parent, the process Involved in a sexual so is called vegetative propagation.

Monoceious – Homothelics like cucurbits, Bisexual Coconut, Chara.

Dioecious – Heterothelic and Unisexual Papaya, date Palm, marchantia.

Parthenogenesis – When the female gamete undergoes development to form new organisms without fertilization, This Phenomenon is called parthenogenesis e.g. Rotifers, honeybees, lizards and birds (turkey).

Gametogenesis – The process of formation of the two type of gametes male and female. The male gamete is called the aniherozoid or sperm and female gamete is called egg or ovum.

Spermatogenesis – lt is the process of formation of haploid spermatozoa from diploid male germ cells of the testes.

A spermatogonium produces four spermatozoa (sperms).

Oogenesis – It is the process of formation of haploid ova from the gamete’ mother cells (oogania) in the ovany. An oogonium form only one ovum.

Lets discuss some unanswered questions from this chapter

Q1.Explain – in asexual reproduction a part of organism separates and forms a new organism?

Q2.Name the parasite which causes black fever

Q3 .How does reproduction take place by multiple fission?

Q4. What do you understand by the term daughter nuclei?

Q5.Colonies of yeast do not multiply in water but they do multiply in sugar solution. explain

Q6.How does bread mould formation takes place

Q7.Regeneration and its function. Why complex organisms cannot reproduce though regeneration?

Lets discuss 2 marks long answer questions from this chapter

Q. 1. Draw a labelled diagram of conidia of penicillium.

Q. 2. Diagrammatically represent the asexual reproduction in yeast.

Q. 3. What is fission ? Name the type of fission.

Ans. Fission : It is a type of sexual reproduction in which a fully grown parental organism divides into two or more than two daughters.

Q. 4. Diagrammatically represent the asexual reproduction in Amoeba ?

Q. 5. Write the two significance of vegetative propagation ?

Ans. (i) Characters of the parents plants are preserved. A good variety produced can be propagated by vegetative propagation?

(ii) It is easier and cheaper method of propagation.

Q. 6. When artificial methods of vegetative propagation are utilised ?

Ans (i) For quick production of new plants.

(ii) For combining good qualities of two different varieties.

Q. 7. Write the Zoological name of given animals.

Rhesus monkey, Goat, Guinea, Pig, Ascaris.

Q. 8. Identify each part and write whether it is haploid (n) or diploid (2n) Ovary, Anther, Egg, Pollen, male gamete and Zygote.

Q. 9. If the Chromosome numbers in meiocytes of human beings, rat, elephant, rice, butterfly and onion. 46, 42, 56, 24, 380, and 32 respectively. What will be the chromosome number in gametes of these species.

Ans. Human beings = 23

Lets discuss 3 marks answer questions from this chapter

Q. 1. In extreme summer and winter, certain animals like frogs and hazards abandon active life.

This is popularly called summer sleep and winter sleep respectively.

(i) What are the technical terms for summer sleep and winter sleep?

(ii) State any two changes in the body that occur during the above mentioned dormant stale.

Ans. Technical terms are as follows :

Winter sleep – Hibernation

Summer sleep — Aestivation

(i) Rate of metabolism declines

(ii) Respiration takes place through skin only (Cutaneous respiration).

Q. 2 Show by a series of diagrams the manner of regeneration in a hydra if it is cut into two pieces transversely at the, middle.

Show by a series of diagrams the manner of transverse binary fission in Planaria.

Q. 3. What is vegetative propagation? Give two suitable examples.

Ans. Vegetative propagation in plants is a, type of asexual reproduction in which new individual appears from any vegetative parts of parents plant.

In plants, The units of vegetative propagation such as runner, rhizome, sucker, tuber, offset, bulb are all capable of giving rise to new offsprings.

Such structures are called vegetative propagules.

Examples of vegetative propagation are:

(i) Vegetative propagation by leaves, in Bryophyllum.

(ii) Vegetative propagation by Stem (rhizome) in Ginger, Turmeric etc.

Q. 4. Define the following terms:

(i) Juvenile Phase,

(ii) Reproductive Phrase

(iii) Senescent Phase

Ans. (i) Juvenile Phase- The period of growth and maturity in life when then organisms reproduce sexually. This period of growth is called Juvenil Phase, or Vegetative phase.

(ii) Reproductive Phase-The end of Juvenile Phase is indicated by many of them showing morphological and physiological changes prior to active reproductive behaviour,

(iii) Senscent Phase-The end of reproductive phase is one of the parameters of senescene or old stage. At this stage body metabolism is shown down and ultimately this stage leads to death.

Q. 5. Explain why meiosis and gamete genesis are always Inter linked ?

Ans. Gametogenesis refers to the process of formation of two types of gametes male and female. Gametes are haploid cells. Gametes in all heterogametic species are of two types i.e., male and female. A haploid parent produced gametes by mitosis.

But in majority of organisms, Parent body is diploid. Thus meiosis is required by such diploid. Organisms, to produce haploid gametes. In haploid organisms meiocytes undergo meiosis. At the end of meiosis only one set of chromosome is incorporated into each gametes.

Let’s discuss 5 marks long answer questions from this chapter

Also Read: CBSE Class 12th Solved Question Papers PCB – eBook

Q. 1. Define fission, what are two types of fusion?
Describe multiple .fission irt Amoeba and Binary fission.

Ans. Fission is a type of sexual reproduction in which a fully grown parental organism divides into two or more than two daughters. It is of two type.

(i) Binary fission-A type of asexual reproduction in which the Parental Organisms divides in two daughters cells during favourable conditions.

(ii) Multiple fission-A type of sexual reproduction in which the Parental Organisms Produces many daughter cells simultaneously during unfavourable conditions.

Q. 2. The- unicellular organisms which reproduce by binary fission are causidered immortal Instify.

Ans. Asexual reproduction is common among single-celled organisms. Thus in these organisms cell division is itself a mode of reproduction. In binary fission. The parent body divides into two halves and each rapidly growing into an adult. There is no remains of parents body cell and parent cannot be said to have dead. In fact after binary fission, parent continues living as two daughter- individuals. Hence, the unicellular organisms which undergo binary fission are considered immortal. Example-Amoeba and Paramecium.

There are some small and I marks questions based on this chapter

Q. 1. What is life Span ?

Ans. The period between birth to the natural death of an organisms is known as life span.

Q. 2. Define reproduction.

Ans. Reproduction is a biological process by which an organisms produces another organisms of its own kind.

Q. 3. Name two monoecious plants.

Ans. Cucurbita and Coconuts.

Q.4. Name two dioecious plants.

Ans. Papaya and date palm.

Q. 5. Name one bisexual and one unisexual animal.

Ans. Bisexual – Earthworm. Unisexual – Cockroach.

Q. 6. What are post-fertilisation events?

Ans. The post-fertilisation events included:
1. Formation of diploid zygote.
2. Development of embryo from zygote by the process called embryogenesis.

Q. 7. “Amoeba is immortal”. Explain.

Ans. Amoeba is considered immortal because it does not undergo natural death.

Q. 8. Which is the most critical event in Sexual Reproduction.

Ans. Fertilisation of gametes is the most critical event in Sexual Reproduction.

Q. 9. Give two example which reproduce a sexually by binary fission.

Ans. Bacteria and Amoeba reproduce by binary fission.

Q. 10. What are the Vegetative Propagules.

Ans. In plants, the units of vegetative propagation, such as tuber, bulb, rhizome etc. are called vegetive propagules.

Q. 11. Why offsprings of Oviperous animals are at a greater risk as compared to offsprings of viviparous animals.

Ans. Oviparous animals lay eggs in a safe place in the environment. In an open area, the eggs are not always safe and the offsprings are always at a risk.

Q. 12. How many type of natural vegetative reproduction take place in flowering plants?

Ans. The natural vegetative reproduction takes place by modified tuberous roots having adventitious buds, under ground modified stems, creeping stems, leaves, balbs and turions.

Q. 13. Name the artificial means of vegetative reproduction.

Ans. The artificial means of vegetative reproduction are cutting, layering, grafting and micropropagation.

Reproduction Methods

Animals produce offspring through asexual and/or sexual reproduction. Both methods have advantages and disadvantages. Asexual reproduction produces offspring that are genetically identical to the parent because the offspring are all clones of the original parent. A single individual can produce offspring asexually and large numbers of offspring can be produced quickly. In a stable or predictable environment, asexual reproduction is an effective means of reproduction because all the offspring will be adapted to that environment. In an unstable or unpredictable environment asexually-reproducing species may be at a disadvantage because all the offspring are genetically identical and may not have the genetic variation to survive in new or different conditions. On the other hand, the rapid rates of asexual reproduction may allow for a speedy response to environmental changes if individuals have mutations. An additional advantage of asexual reproduction is that colonization of new habitats may be easier when an individual does not need to find a mate to reproduce.

During sexual reproduction the genetic material of two individuals is combined to produce genetically diverse offspring that differ from their parents. The genetic diversity of sexually produced offspring is thought to give species a better chance of surviving in an unpredictable or changing environment. Species that reproduce sexually must maintain two different types of individuals, males and females, which can limit the ability to colonize new habitats as both sexes must be present.

Asexual Reproduction

Asexual reproduction occurs in prokaryotic microorganisms (bacteria) and in some eukaryotic single-celled and multi-celled organisms. There are a number of ways that animals reproduce asexually.


Fission, also called binary fission, occurs in prokaryotic microorganisms and in some invertebrate, multi-celled organisms. After a period of growth, an organism splits into two separate organisms. Some unicellular eukaryotic organisms undergo binary fission by mitosis. In other organisms, part of the individual separates and forms a second individual. This process occurs, for example, in many asteroid echinoderms through splitting of the central disk. Some sea anemones and some coral polyps ([link]) also reproduce through fission.


Budding is a form of asexual reproduction that results from the outgrowth of a part of a cell or body region leading to a separation from the original organism into two individuals. Budding occurs commonly in some invertebrate animals such as corals and hydras. In hydras, a bud forms that develops into an adult and breaks away from the main body, as illustrated in [link], whereas in coral budding, the bud does not detach and multiplies as part of a new colony.

Watch a video of a hydra budding.


Fragmentation is the breaking of the body into two parts with subsequent regeneration. If the animal is capable of fragmentation, and the part is big enough, a separate individual will regrow.

For example, in many sea stars, asexual reproduction is accomplished by fragmentation. [link] illustrates a sea star for which an arm of the individual is broken off and regenerates a new sea star. Fisheries workers have been known to try to kill the sea stars eating their clam or oyster beds by cutting them in half and throwing them back into the ocean. Unfortunately for the workers, the two parts can each regenerate a new half, resulting in twice as many sea stars to prey upon the oysters and clams. Fragmentation also occurs in annelid worms, turbellarians, and poriferans.

Note that in fragmentation, there is generally a noticeable difference in the size of the individuals, whereas in fission, two individuals of approximate size are formed.


Parthenogenesis is a form of asexual reproduction where an egg develops into a complete individual without being fertilized. The resulting offspring can be either haploid or diploid, depending on the process and the species. Parthenogenesis occurs in invertebrates such as water flees, rotifers, aphids, stick insects, some ants, wasps, and bees. Bees use parthenogenesis to produce haploid males (drones). If eggs are fertilized, diploid females develop, and if the fertilized eggs are fed special diet (so called royal jelly), a queen is produced.

Some vertebrate animals—such as certain reptiles, amphibians, and fish—also reproduce through parthenogenesis. Although more common in plants, parthenogenesis has been observed in animal species that were segregated by sex in terrestrial or marine zoos. Two female Komodo dragons, a hammerhead shark, and a blacktop shark have produced parthenogenic young when the females have been isolated from males.

Sexual Reproduction

Sexual reproduction is the combination of (usually haploid) reproductive cells from two individuals to form a third (usually diploid) unique offspring. Sexual reproduction produces offspring with novel combinations of genes. This can be an adaptive advantage in unstable or unpredictable environments. As humans, we are used to thinking of animals as having two separate sexes—male and female—determined at conception. However, in the animal kingdom, there are many variations on this theme.


Hermaphroditism occurs in animals where one individual has both male and female reproductive parts. Invertebrates such as earthworms, slugs, tapeworms and snails, shown in [link], are often hermaphroditic. Hermaphrodites may self-fertilize or may mate with another of their species, fertilizing each other and both producing offspring. Self fertilization is common in animals that have limited mobility or are not motile, such as barnacles and clams.

Sex Determination

Mammalian sex determination is determined genetically by the presence of X and Y chromosomes. Individuals homozygous for X (XX) are female and heterozygous individuals (XY) are male. The presence of a Y chromosome causes the development of male characteristics and its absence results in female characteristics. The XY system is also found in some insects and plants.

Avian sex determination is dependent on the presence of Z and W chromosomes. Homozygous for Z (ZZ) results in a male and heterozygous (ZW) results in a female. The W appears to be essential in determining the sex of the individual, similar to the Y chromosome in mammals. Some fish, crustaceans, insects (such as butterflies and moths), and reptiles use this system.

The sex of some species is not determined by genetics but by some aspect of the environment. Sex determination in some crocodiles and turtles, for example, is often dependent on the temperature during critical periods of egg development. This is referred to as environmental sex determination, or more specifically as temperature-dependent sex determination. In many turtles, cooler temperatures during egg incubation produce males and warm temperatures produce females. In some crocodiles, moderate temperatures produce males and both warm and cool temperatures produce females. In some species, sex is both genetic- and temperature-dependent.

Individuals of some species change their sex during their lives, alternating between male and female. If the individual is female first, it is termed protogyny or “first female,” if it is male first, its termed protandry or “first male.” Oysters, for example, are born male, grow, and become female and lay eggs some oyster species change sex multiple times.

Section Summary

Reproduction may be asexual when one individual produces genetically identical offspring, or sexual when the genetic material from two individuals is combined to produce genetically diverse offspring. Asexual reproduction occurs through fission, budding, and fragmentation. Sexual reproduction may mean the joining of sperm and eggs within animals’ bodies or it may mean the release of sperm and eggs into the environment. An individual may be one sex, or both it may start out as one sex and switch during its life, or it may stay male or female.

Review Questions

Which form of reproduction is thought to be best in a stable environment?

Sexuality in Organisms

Plants having male and female structures within the same plant are known as bisexual plants. In fungi, the word homothallic is used whereas for plants monoecious word is used for unisexual plants. Example of monoecious plant is cucurbits. Plants having male and female structures on different plants, are known as bisexual plants. The heterothallic and dioecious are used for fungi and plants respectively. Example of dioecious plant is papaya. Unisexual flowers with stamen are known as staminate whereas unisexual female flowers with pistils are known as pistillate.

Bisexual organisms having both male and female reproductive structures are known as hermaphrodite. For example, Earthworm. In earthworm, testis and ovaries are present in the same organism.

Fig. 7. Sexuality in organisms

Organisms belonging to monera, fungi, algae etc., have haploid parent body. But complex organisms such as plants belonging to gymnosperms, angiosperms and animals including humans are diploid in nature. For reproduction, these organisms produce haploid gametes via process of meiosis. Cells that form gametes are known as meiocytes. These meiocytes undergo meiosis to form gametes.

For the formation of new organism, gametes need to come together for fusion. Exceptions are there in case of asexual reproduction. In some plants, both the gametes are motile. But in case where one gamete is motile and other gamete is non-motile there is a need of a medium for mating of the gametes. For example, in algae, bryophytes, pteridophytes, water act as a medium for gamete transfer.

Water serves as a poor medium of gamete transfer, as most of the sperm produced do not reach the target female gamete. To compensation this, many male gametes are produced as compared to female gamete.

Other higher organisms use pollination as the mode of gamete transfer. It is the process of transfer of pollen from the anther to the stigma of the flower. Pollen grain lands on the stigma of the flower, then it reaches the egg and discharge the male gametes. One of the male gamete fertilizes the egg to form zygote and another one is involved in the formation of endosperm. This is known as double fertilization. This is the characteristic mechanism of Angiosperms.

Virgin births seen in wild vipers

Snake broods are first examples of wild parthenogenesis in sexually reproducing species.

It usually takes two snakes — a female and a male — to make a litter of baby copperheads. But research now shows that copperheads (Agkistrodon contortrix) and their venomous cousins cottonmouths (Agkistrodon piscivorus) don’t always require a partner to establish the next generation. These vipers are capable of virgin births.

For some vertebrates, parthenogenesis — asexual reproduction in which embryos develop without fertilization — is the norm. The New Mexico whiptail lizard (Cnemidophorus neomexicanus), for example, is an all-female species that reproduces without any genetic contribution from a male.

But in zoos and aquariums, zoologists have begun to document the strange phenomenon of facultative parthenogenesis: females of species that usually reproduce sexually, delivering offspring without mating. Surprise pregnancies have been documented among birds, sharks, snakes and Komodo dragons (Varanus komodoensis), but until now, only in captivity.

Warren Booth, a molecular ecologist at the University of Tulsa in Oklahoma, now reports the first known case of wild facultative parthenogenesis, publishing the study today in Biology Letters 1 .

In work conducted while he was at North Carolina State University in Raleigh, Booth and his colleagues captured pregnant wild copperheads and cottonmouths, which gave birth in the lab. The researchers suspected that some of the snakes had reproduced without male input: in comparison with those born from sexual unions, says Booth, asexually reproduced snake litters typically have a large number of failures in development such as stillborn babies, and few viable males. When he saw that some of the snakes had delivered broods with these characteristics, “these litters were at the top of my agenda to genotype”, says Booth.

Booth examined genetic markers in the mothers and offspring to check whether the young snakes had really been born as a result of facultative parthenogenesis, or were unusual broods sired by males that were genetically similar to the mother.

“When I got the results of the DNA sequencer, I was floored,” he says. The genotyping compared the genetic make-up of the offspring with the populations from which the snakes were collected the results indicated that the chance of a male contribution was “infinitesimally small”. Researchers had always believed that facultative parthenogenesis took place in the wild, Booth notes, but he and his colleagues were “stunned” at finally finding the evidence.

It is not clear how asexual reproduction evolved in normally sexual species, but the absence of a process called genomic imprinting may have had a role. In mammals, genomic imprinting causes a set of genes from one parent to dominate over the other, and this interaction requires genes from both parents to create viable offspring. Reptiles don't undergo genomic imprinting, so mating isn't required for mothers to develop their young — but it is not known why.

Nor is it known what spurred the female snakes to reproduce asexually. Booth points out that isolation from males is not the key: the snakes were collected from habitats with males, which undoubtedly were on the lookout for females. For whatever reason, the females forsook their potential mates, or rejected sperm from pairings, to deliver parthenogenic litters. Booth says that the finding removes the “prevailing dogma” that facultative parthenogenesis occurs only when females are isolated.

Phill Watts, an ecological geneticist at the University of Liverpool, UK, agrees that the study calculated “a convincingly low probability for male parentage”. The discovery may motivate other researchers to search for further examples. “We still lack data to understand when and why facultative parthenogenesis happens in the wild,” says Watts. There are almost certainly other asexually reproducing snakes, sharks and lizards out there, but the biological impetus for their wonderful births is, as yet, a mystery.

Is Parthenogenesis known as Sexual reproduction or Asexual reproduction? - Biology

Unit Six. Animal Life

Few subjects pervade our everyday thinking more than sex few urges are more insistent. They are no accident, these strong feelings. They are a natural part of being human. All animals share them. The cry of a cat in heat, insects chirping outside the windows, frogs croaking in swamps, wolves howling in a frozen northern scene—all these are the sounds of the living world’s essential act, an urgent desire to reproduce that has been patterned by a long history of evolution. It is a pattern that all of us share. The reproduction of our families spontaneously elicits in us a sense of rightness and fulfillment. It is difficult not to return the smile of a new infant, not to feel warmed by it and by the look of wonder and delight to be seen on the faces of parents like this nursing mother. This chapter deals with sex and reproduction among the vertebrates, of which we human beings are one kind. Few subjects are of more direct concern to students than sex. Because many students must make important decisions about sex, the subject is of far more than academic interest, and is one about which all students need to be well informed.

31.1. Asexual and Sexual Reproduction

Not all reproduction involves two parents. Asexual reproduction, in which the offspring are genetically identical to one parent, is the primary means of reproduction among protists, cnidari- ans, and tunicates, and also occurs in some more complex animals.

Through mitosis, genetically identical cells are produced from a single parent cell. This permits asexual reproduction to occur in the Euglena in figure 31.1 by division of the organism, or fission. The DNA replicates and cell structures, such as the flagellum, duplicate. The nucleus divides with identical nuclei going to each daughter cell. Cnidaria commonly reproduce by budding, where a part of the parent’s body becomes separated from the rest and differentiates into a new individual. The new individual may become an independent animal or may remain attached to the parent, forming a colony.

Unlike asexual reproduction, sexual reproduction occurs when a new individual is formed by the union of two cells. These cells are called gametes, and the two kinds that combine are generally called sperm and eggs (or ova). The union of a sperm and an egg produces a fertilized egg, or zygote, that develops by mitotic division into a new multicellular organism. The zygote and the cells it forms by mitosis are diploid they contain both members of each homologous pair of chromosomes. The gametes, formed by meiosis in the sex organs, or gonads—the testes and ovaries—are haploid (see chapter 9). The processes of spermatogenesis (sperm formation) and oogenesis (egg formation) are described in later sections.

Figure 31.1. Asexual reproduction in protists.

The protist Euglena reproduces asexually: A mature individual divides by fission, and two complete individuals result.

Parthenogenesis, a type of reproduction in which offspring are produced from unfertilized eggs, is common in many species of arthropods. Some species are exclusively parthenogenic, whereas others switch between sexual reproduction and parthenogenesis in different generations. In honeybees, for example, a queen bee mates only once and stores the sperm. She then can control the release of sperm. If no sperm are released, the eggs develop parthenogenetically into drones, which are males if sperm are allowed to fertilize the eggs, the fertilized eggs develop into other queens or worker bees, which are female.

The Russian biologist Ilya Darevsky reported in 1958 one of the first cases of unusual modes of reproduction among vertebrates. He observed that some populations of small lizards of the genus Lacerta were exclusively female, and he suggested that these lizards could lay eggs that were viable even if they were not fertilized. In other words, they were capable of asexual reproduction in the absence of sperm, a type of parthenogenesis. Further work has shown that parthenogenesis occurs among populations of other lizard genera.

Hermaphroditism, another variation in reproductive strategy, is when one individual has both testes and ovaries and so can produce both sperm and eggs. The hamlet bass in figure 31.2a are hermaphroditic, producing both eggs and sperm. During mating each fish switches from producing eggs that are fertilized by its partner, to producing sperm that fertilizes its partner’s eggs. A tapeworm is hermaphroditic and can fertilize itself as well as cross fertilize, a useful strategy because it is unlikely to encounter another tapeworm living inside its host. Most hermaphroditic animals, however, require another individual to reproduce. Two earthworms, for example, are required for reproduction—like the hamlet bass, each functions as both male and female. Each leaves the encounter with fertilized eggs.

Sequential hermaphroditism, in which individuals can change their sex, occurs in numerous fish genera. Among coral reef fish, for example, both protogyny (“first female,” a change from female to male) and protandry (“first male,” a change from male to female) occur. In the protogynous blue-head wrasse in figure 31.2b, the sex change appears to be under social control. These fish commonly live in large groups, or schools, where successful reproduction is typically limited to one or a few large, dominant males. If those males are removed, the largest female rapidly changes sex and becomes a dominant male (the blue-headed fish in the photo).

Figure 31.2. Hermaphroditism and protogyny.

(a) The hamlet bass (genus Hypoplectrus) is a deep-sea fish that is a hermaphrodite. In the course of a single pair-mating, one fish may switch sexual roles as many as four times. Here, the fish acting as a male curves around its motionless partner, fertilizing the upward- floating eggs. (b) The bluehead wrasse Thalassoma bifasciatium is protogynous. Here a large male, or sex-changed female, is seen among females, which are typically much smaller.

Among the fish just described, and in some species of reptiles, environmental changes can cause changes in the sex of the animal. In mammals, the sex is determined early in embryonic development. The reproductive systems of human males and females appear similar for the first 40 days after conception. During this time, the cells that will give rise to ova or sperm migrate to the embryonic gonads, which have the potential to become either ovaries in females or testes in males. If the embryo is XY, it is a male and will carry a gene on the Y chromosome whose product converts the gonads into testes (as on the left in figure 31.3). In females, who are XX, this Y chromosome gene and the protein it encodes are absent, and the gonads become ovaries (as on the right). Recent evidence suggests that the sex-determining gene may be one known as SRY (for “sex-determining region of the Y chromosome”). The SRY gene appears to have been highly conserved during the evolution of different vertebrate groups.

Figure 31.3. Sex determination.

Sex determination in mammals is made by a gene on the Y chromosome designated SRY. Testes are formed when the Y chromosome and SRY are present ovaries are formed when they are absent.

Once testes form in the embryo, they secrete testosterone and other hormones that promote the development of the male external genitalia and accessory reproductive organs (indicated in the blue box). If testes do not form, the embryo develops female external genitalia and accessory reproductive organs. The ovaries do not promote this development of female organs because the ovaries are nonfunctional at this stage. In other words, all mammalian embryos will develop female sex accessory organs and external genitalia by default unless they are masculinized by the secretions of the testes.

Key Learning Outcome 31.1. Sexual reproduction is most common among animals, but many reproduce asexually by fission, budding, or parthenogenesis. Sexual reproduction generally involves the fusion of gametes derived from different individuals of a species, but some species are hermaphroditic.

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Watch the video: What Is Asexual Reproduction. Genetics. Biology. FuseSchool (August 2022).