Q.Discuss in detail the significance of meiosis, covering both the halving of the chromosome number and the generation of genetic variation through crossing over and independent assortment, and explain what would happen to the chromosome number of a sexually reproducing species across generations if meiosis did not occur before gamete formation.

[!TLDR]

Meiosis halves chromosome number (so fertilisation restores, not doubles, it) AND generates variation (crossing over + independent assortment). Without meiosis, chromosome number would double every generation. …

Meiosis is indispensable to organisms that reproduce sexually because of two distinct consequences of the process.

The first significance concerns chromosome number. Sexual reproduction works by the fusion of two gametes, one from each parent, at fertilisation, to form a single zygote. If gametes carried the full diploid (2n) chromosome number of the parent's ordinary body cells, every act of fertilisation would double the chromosome number in the resulting zygote compared with either parent, and this doubling would compound with each successive generation. Meiosis prevents this by halving the chromosome number during gamete formation, from diploid to haploid (n), specifically at anaphase I when whole homologous chromosomes -- not sister chromatids -- separate, so that fusion of two haploid gametes at fertilisation exactly restores the diploid number rather than doubling it, keeping the chromosome number characteristic of the species constant, generation after generation.

The second significance is the generation of genetic variation among the gametes an individual produces, and so among its potential offspring. This arises through two distinct mechanisms built into meiosis. Crossing over, taking place during pachytene of prophase I, is the reciprocal exchange of segments of genetic material between non-sister chromatids of a homologous pair, producing chromosomes with new combinations of alleles not present together on either original parental chromosome. Independent assortment, operating at metaphase I, arises because each bivalent orients itself at the equatorial plate independently of every other bivalent, so the particular mix of maternal and paternal chromosomes ending up in any one gamete is essentially a matter of chance -- and with many chromosome pairs assorting independently, the number of possible combinations is enormous. …

Address both halves of the question separately -- first the two genuine significances (halving + variation), then a distinct 'what if not' par …

Discussing only genetic variation and omitting the chromosome-number-halving significance (or vice ver …