All creatures that reproduce sexually produce sex cells to create their offspring, from fish to human beings.
In humans, the sex cells are very different. The male sex cells, or sperm cells, are very small and able to move on their own. They contain either an X or Y chromosome. The female ova are much larger and not motile. They always contain an X chromosome.
Both types of cells are produced by a process called meiosis. In the first stage, two daughter cells, connected together at what’s called a centromere, are created. In the second stage the two daughter cells divide and separate, resulting in four separate sex cells.
Male meiosis is called spermatogenesis. It occurs constantly within the male testes, as it takes hundreds of millions of sperm for fertilization to occur. It takes fewer female sex cells, so female meiosis, or oogenesis, starts during an embyro’s development and ends shortly before or after a female infant’s birth. These cells contain some of the parent’s genetic material in the form of chromosomes.
Each human sex cell has 23 chromosomes.When a sperm cell and an ovum meet, they fuse together in a process called fertilization. The new cell, called a zygote, becomes an embryo and then a baby. The zygote cell has 46 chromosomes, all of the chromosomes of the sex cells that created it, so the resulting offspring is a mix of the parents’ genes. If an ovum is fertilized by a sperm cell with an X chromosome, the resulting baby will be a boy. If the ovum is fertilized by a sperm cell with a Y chromosome, the baby will be a girl.
Fertilization usually occurs after sexual intercourse. However, scientific advancements now see fertilization occurs through medical procedures like in-vitro fertilization and intrauterine insemination.