Most mammals , including humans , are genetically determined as such by the XY sex-determination system where males have an XY (as opposed to XX) sex chromosome . It is also possible in a variety of species, including humans, to be XXY or have other intersex / hermaphroditic qualities, though one would still be considered genotypically (if not necessarily phenotypically) male so long as one has a Y-chromosome. During reproduction , a male can give either an X sperm or a Y sperm, while a female can only give an X egg. A Y sperm and an X egg produce a male, while an X sperm and an X egg produce a female .
Meiosis is the type of cell division that produces both male and female gametes. Spermatogenesis is the production of male gametes, and oogenesis is the production of female gametes. For both types of gametes, meiosis serves to halve the number of chromosomes so that upon fertilization the offspring has the requisite number of chromosomes. However, each round of spermatogenesis produces four sperm cells while each round of oogenesis results in a single egg cell and three non-functional cells, or polar bodies. The polar bodies eventually degrade; their only purpose is to provide an outlet for the extra chromosome sets.
The sperm then move to the vas deferens, or sperm duct. The seminal vesicles and prostate gland produce a whitish fluid called seminal fluid , which mixes with sperm to form semen when a male is sexually stimulated. The penis, which usually hangs limp, becomes hard when a male is sexually excited. Tissues in the penis fill with blood and it becomes stiff and erect (an erection). The rigidity of the erect penis makes it easier to insert into the female's vagina during sexual intercourse. When the erect penis is stimulated, muscles around the reproductive organs contract and force the semen through the duct system and urethra. Semen is pushed out of the male's body through his urethra — this process is called ejaculation . Each time a guy ejaculates, it can contain up to 500 million sperm.