All organisms create offspring, or reproduce, either through asexual or sexual reproduction. Asexual reproduction does not involve the fusion of gametes. Offspring are genetically identical to the parent and are usually formed when one organism replicates its genetic material and then divides itself in half. In sexual reproduction, male and female cells are formed and combine to form a new offspring. Each of these cells is called a gamete (egg or sperm) and is produced from cells called germ cells.
Sexual reproduction involves the fusion of two haploid (n) gametes. Each haploid cell has half the total chromosomes of the parent. When these haploid gametes join, they form a diploid (2n) zygote, having twice as many chromosomes as each gamete. The zygote has the entire complement of chromosomes the organism needs. In humans each gamete has 23 chromosomes. When these two cells come together, the resulting zygote has 46 chromosomes. The zygote will divide, copying each of those chromosomes every time, so that the full-grown person has 46 chromosomes in every one of his or her somatic, or body, cells. The germ cells, however, will again give rise to haploid gametes.
Sperm and egg production happen throughout the reproductive lives of the organism. In humans, both sexes reach reproductive age around 12 years old. Males then continue producing sperm for the rest of their lives. While viability and mobility of the sperm does decrease with age (in all mammals), most males can have children up until death. Females, on the other hand, have a finite reproductive life. In humans, reproductive capability of females usually lasts until about 50 years old. Human females are born with all the eggs they will have. The human female will release one egg per month until she reaches a stage of life called menopause, when her body stops the cycle that releases viable eggs. This indicates the end of her reproductive life. Other mammals, such as chimpanzees and whales, go through similar changes.