During sexual intercourse sperm are transferred from the penis of the male into the vagina of the female. They then swim through the uterus to the oviduct and can fertilise an egg cell if one is present.
Step one: Once every 28 days an egg cell is released from the ovary and moves into the oviduct (ovulation).
Step two: During sexual intercourse sperm is ejaculated into the vagina. Sperm moves through the vagina, cervix, uterus and along the oviduct.
Step three: It is in the oviduct that fertilisation takes place. The nucleus of one sperm fuses with the nucleus of an egg forming a zygote that contains genetic material from both parents. This random combination of parental gametes leads to variation within the species.