In sexual reproduction, though the genetic material DNA (in the form of chromosomes) from two gametes, male and female gametes, combines together to form a new cell ‘zygote’ but the amount of DNA in zygote does not get doubled. This can be explained as follows:
The gametes are special type of cells called reproductive cells which contain only half the amount of DNA (or half the number of chromosomes) as compared to the normal body cells of an organism. So, when a male gamete combines with a female gamete during sexual reproduction, then the new cell ‘zygote’ will have the normal amount of DNA (or normal number of chromosomes in it).
For example, the human sperm has 23 chromosomes and the human egg (or ovum) has also 23 chromosomes. So, when a sperm and an egg fuse together during fertilisation, then the zygote formed will have 23 + 23 = 46 chromosomes, which is the normal number of chromosomes.