How Heredity Works

In this topic you will learn about how heredity is determined by a small, complex molecule.
Every cell is surrounded by a membrane. Inside the cell membrane is a fluid called cytoplasm. In the center of the cytoplasm is the cell nucleus, which contains chromosomes. Chromosomes contain the factors for hereditary traits.
Living organisms are made of two types of cells, body cells and sex cells. Most of the cells in any organism are body cells. Body cells contain a full set of chromosomes. A human body cell contains 46 chromosomes. Sex cells are produced inside the sex organs of an adult organism. Each sex cell contains only one-half the number of chromosomes of body cells. A human sex cell contains 23 chromosomes.
Cells grow. When they reach a certain size, they divide into two new cells. While a body cell is dividing into two identical cells, the nucleus divides by a process called mitosis. Mitosis is a gradual, continuous process. During mitosis a second set of chromosomes is formed inside the cell. When the cell splits and produces two new cells, each new cell has a full set of chromosomes and is identical to the original cell.
A sex cell has only half the number of chromosomes as a body cell. A sex cell is formed as a result of a process called meiosis. Meiosis is the division of a nucleus resulting in sex cells with half as many chromosomes as in other cells.
When an organism reproduces, a sperm cell and an egg cell unite to create a single, new cell. This new cell receives half of the needed chromosomes from the female's egg cell and half from the male's sperm. The new cell represents a new individual with a new combination of genetic material.
A portion of a chromosome that controls a particular inherited trait is called a gene. Genes contain the information needed for cells to function. Genes are arranged along the length of a chromosome. Organisms have two copies of each gene for a given trait. They get one copy from each parent. Both genes are located in the same place on a chromosome.
Genes are portions of a long, complex molecule called deoxyribonucleic acid, or DNA. DNA contains the codes that tell each cell how to operate. Each gene on a chromosome is a short section of the long DNA molecule that makes up the chromosome. The genetic characteristics of every living organism are contained in that organism's DNA. The DNA in each cell of a given organism is the same. The DNA of every organism is different from that of every other organism. The shape of the DNA molecule looks something like a spiral staircase or twisted ladder. This shape is called a double helix.