In sexually reproducing organisms, why are there exactly two chromosomes in each homologous pair?

Ans.

Sexually reproducting organisms include those organisms, in which both male and female gametes or cells take part in reproduction to form zygote. The zygote develops into an organism that has characteristics of both the parents. Cells of these individuals have two sets of chromosome, one from each parent as they receive genetic material from each parent.

Thus, in these organisms, there are two copies of each chromosomes (or two chromosomes in a homologous pair), since one copy of a chromosome derives from male parent and one from female parent.

meiosis is the first to occur after the nuclei of opposite of opposite mating types fuse