Every organism is mortal, It becomes old and wornout with advancing age (= ageing ) and ultimately dies ,Before death however ,it reproduces its own kind, so that its race species continues, Many protozoan and some lower metazoans reproduce asexually by mitosis .Inheritance in these is obviously uniparental some protozoans and most metazoans perpetuate their species by sexual reproduction . Inheritance in these is therefore biparental, because reproduction involves union of two sex cells or gametes ( male gametic cell or sperm and femal gametic cells or ovum), producing a single cells called zygote, from the zygote , then arises the entire multicellular body of a new progeny by its repeated mitotic division and different iation of resultant cells into the different types.
If the gametes were diploid like all body (= somatic) ,cell the zygote would have twice the number of diploid chromosomes (tetraploid -4 x or 4n and the number would go on multiplying in that order generation after generation .No species can survive if its karyotype multiplies in this way, Hence to prevent this situation ,sex cells or gametes are made to possess a single set of chromosomes i.e. only half the number of chromosomes present in the karyotpye ,This single set is called haploid or monoploid (x or n) gametic set in contrast to the diploid ( 2x or 2n) karyotypic or somatic set of chromosomes .
The haploid sex cells are formed insexual organs i.e. gonads (testes in males and ovaries in females ) from diploid germinal cells by a specialized process called gametogenesis (spermatogenesis in males and oogenesis in females) This process necessarily involves a single duplication of chromosomes but two consecutive cell division. The second division is mitotic but the first one is a special type of reduction cell division called meiosis (Gr meioum =to diminish ) which bring about the aforesaid reduction in chromosomal number to half in the resultant gametes . Conventionally , the two division are designated as first and second meiosis.