Explain how, offspring and parents of organisms reproducing sexually have the same number of chromosomes?

Explanation

In sexual reproduction. though the genetic material from a male and female gamete combines together to form a zygote, the amount of DNA in zygote does not get doubled. This is because the gametes are reproductive cell which contain only half the amount of DNA as compared to the parent cell. So, when a male gamete combines with a female gamete during sexual reproduction, the zygote formed will have the normal amount of DNA (i.e., same number of chromosomes as parent).
e.g., if the male gamete has 'n' chromosomes and female gamete also has 'n' chromosome. So, during fertilisation, the zygote formed will haven + n = 2n chromosomes, which is the normal number of chromosomes in parents.