New life is created by reproduction. The mother and father produce sex cells, which join to form an embryo. These sex cells also determine the child’s HEREDITY.
Male cells are called sperm. They are produced by the million in the testes—a pair of ball-shaped glands that hang outside the body in the scrotum. A female sex cell is called an ovum (plural ova). A woman’s ova form before she is born, in two organs called ovaries, in her abdomen.
Sperm and ova come together as a result of sexual intercourse (sex). During sex, a man places his penis inside a woman’s body. Sperm leave the penis and swim into the woman’s reproductive organs. If the ovaries have released an ovum, a sperm cell may fuse with it and form an embryo. This is called fertilization.
The embryo develops inside an organ called the uterus (womb). It sinks into the soft lining of the uterus and absorbs food from the mother through an organ called the placenta, which develops from the embryo. The uterus has a very stretchy wall so that it can expand as the baby grows.
The process of inheriting characteristics from parents is called heredity. Many of our characteristics are passed on through genes—instructions made up of DNA molecules in our chromosomes.
People inherit half their genes from their mother and half from their father. The genes are passed on in chromosomes carried by sperm and ova. The chromosomes interact with each other in different ways, so children from the same parents may share characteristics, but are also very different. Apart from identical twins, everyone has a unique set of genes.
Genes determine most of your physical characteristics, such as eye color. Eye color is controlled by only a handful of genes, but other characteristics, such as height, involve lots of genes. Genes can also affect mental characteristics such as personality and intelligence, but these are heavily influenced by your experiences as well. The study of genes and how they affect you is called genetics.