The purpose of sexual reproduction is to produce genetic variability in individuals, that is, babies that will develop to be different from their mother and father and from their siblings. This is achieved through the processes of gamete production (meiosis) and the fusion of a sperm with an ovum. Males result when fertilisation produces a zygote with an X and a Y chromosome, while a female results from a zygote with two X chromosomes. The organs of the reproductive tract that are present at birth are called the primary sex characteristics, and you should know them. The physical features that develop after puberty are known as the secondary sex characteristics.