Sexual Reproduction

Sexual Reproduction
It is a type of reproduction in which formation & fusion of sex cells called gametes occurs.
  • It generally involves two parents so also called bi-parental reproduction, except Tinia or Fasciola.
  • It involves formation & fusion of gametes.
  • It involves meiosis during gametogenesis & mitoic division during development of zygote.
  • It is also called Germinal Reproduction.
  • It involves low multiplication.
  • Unit of reproduction are haploid gametes.
  • Daughter cells are genetically different from the parents as variation appear due to new combination of genes during crossing over so it plays an important role in evolution.
  • It is found in higher plants & animals.

1.SYNGAMY: it is also called fertilization. Eg : Human
2.CONJUGATION: it involves temporary pairing of  two parents called Conjugants, to exchange their male pro-nuclei. It is found in ciliate protozoans. For eg: paramecium.
 Pre-fertilization events:
b.Gamete transfer
Post- fertilization events:
a.Zygote development
1.Gametogensis:- It involves the formation of haploid gametes from diploid Gametogonia by meiosis in gonads.
Examples: spermatogenesis in Testis
Oogenesis in ovary in animals.
2.Cells undergoing meiosis to form haploid gametes are called as Meiocytes. Due to meiosis every gamete gets only one set of chromosome.
3.In haploid organism, gametes formed by mitosis. For eg: Like in several organisms in Monera, in Algae & Fungi plant body is haploid.
a.No. of chromosome in elephant is 56.
b.No. of chromosome in Butterfly is 380.
4.Gamete Transfer: In many algae & fungi both the gametes can be non-motile/motile. Water is required for the movement of male gamete in simple plants of algae. Examples: Bryophytes & Pteridophytes.
5.Fertilization : it involves the fusion of haploid gametes to form diploid zygote.
a.Tapeworm & Liverfluke are hermaphrodites & show uniparental sexual reproduction & involve self fertilization
b.Embryogenesis: It refers to the process of development of embryo from the zygote. During embryogenesis zygote undergoes cell division (mitosis) and cell differentiation.
c.Von Bear is the father of modern embryology.
d.Hermaphrodites/ Bisexual/ Monoecious : these are those organisms which have both male & female reproductive organs in the same individual. For example earthworm, leech etc.
Oviparous Animals:
a. There are the animals which lay fertilized or unfertilized eggs.
b.The fertilized eggs have a calcareous to protect from the harsh environment.
Viviparous Animals:
a.These are the animals which give birth to young individuals.
b.The eggs have no calcareous shell as they are protected inside the mothers body.
External Fertilization:
a.When fusion of gametes (syngamy) occurs outside the body of the organism, it is called external fertilization.
b.A large no. of gametes are released in the surrounding medium by such animals. Eg: bony fish, amphibian etc.
Internal fertilization:
a.When fusion of gametes (syngamy) occurs inside the body of the organism, it is called internal fertilization.
b.The no. of ova produced is less, but a large no. of male gametes are formed, as they have to travel towards the ovum. Eg: birds, mammals, earthworm etc.
  • The later part of the development process, which leads to maturity to the ultimate complete loss of organization & function is termed as senescence.
  • It includes only degenerative & deteriorative changes in a plant or its parts
  • Senescence occur as a result of ageing & leads to DEATH. Eg : Auxin have been found to state senescence in certain trees, although they  cannot always be shown to have this effect in all plants.
  • Ethylene strongly promote scnescence in many tissues.
  • Senescence is often of great advantage to the plants. Eg. Loss of leaves in a deciduous tree is an essential part of winter cold avoidance.

It is the development of an unfertilized ovum into a fully formed haploid organism, so it is monoparental.
For example: It is found in no. of invertebrates (honey bee, wasps etc) & a few vertebrates (domestic turkey among birds)
On the basis of life cycle, the parthenogenesis is of two types:-
Natural Parthenogenesis: In this the parthenogenesis is a regular & characteristic feature of life cycle.
Artificial or Induced Parthenogenesis: In this the egg of some normally sexually reproducing animals can be induced by artificial means to develop parthenogeniticallly.
a.This avoid the wastage of germplasm as sperm & ova.
b.There are no chance of separation of useful combination of genes by crossing over & are transmitted as such.
c.The offspring are exactly similar to parents.
a.It stops the chances of new combination of genes & thus avoid selection in population.
b.It decreases the chances of adaptability followed by extinction.
When two sexes of the species are morphologically & physiologically different from each other then this phenomena is called sexual dimorphism. For example: human.
Life cycles:


Life span


15 yrs.


6 months


100 yrs.


100-150 yrs.


200 yrs.

Vegetative propogules:

Vegetative propogules





Ginger & Banana




Water hyacinth






Onion , Garlic



Important questions and answers:
Differentiate between oestrous and menstrual cycle:

S. No.

Oestrous cycle

Menstrual cycle

It refers to the cyclic changes in the female reproductive system of non-primates.
It refers to the cyclic changes in the reproductive system of primate females.
There is no menstruation in the end of oestrous cycle.
There is menstruation at the end of menstrual cycle.
At the time of ovulation, animals show ‘estrus’ or ‘heat’ and copulation occurs only at that period.
There is no heat period during ovulation and copulation occurs during any part of cycle.

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