Laverne Barretto, age 12, of Agincourt, Ont., Canada, for her question:
WHAT ARE GENES?
An Austrian monk by the name of Gregor Mendel discovered the principles of genetics during the 1860s. He found that garden peas inherited various traits in a predictable manner. Mendel concluded that many characteristics are passed on from generation to generation by factors that are now called genes. Organisms inherit two forms of the gene for each trait one from each parent. Genes are the hereditary material of cells, and they control many characteristics of an organism. Tiny bodies in a cell are called chromosomes. The chromosomes carry the genes.
Every living thing has a certain number of chromosomes inside the cell nucleus. In man, body cells contain 46 chromosomes. Body cells are all cells except egg and sperm cells. Each body cell in mice has 40 chromosomes and each one in corn has 20.
Every human body cell contains two sets of 23 chromosomes. Each can be matched with a particular chromosome in the other set. Egg and sperm cells, however, have only one set of 23 chromosomes. These are formed in a special way, and end up with only half the number of chromosomes found in body cells. As a result, when an egg and a sperm come together, the fertilized egg cell will contain the 46 chromosomes of a normal body cell.
The egg cell starts to grow as soon as the sperm cell fertilizes it. First the egg cell divides into two cells. The two then divide and produce four cells. The four divide to become eight, and so on. The division eventually produces the trillions of cells that make a man's body.
Genes are tiny particles that determine all hereditary traits, and they are carried on the chromosomes. Each chromosome except the Y chromosome bears hundreds of genes, arranged in a line along its length. The Y chromosome is much smaller than the other chromosomes and carries many fewer genes.
Every person inherits two genes for a particular trait ¬one from his mother and one from his father. But with some traits the action of one gene completely overpowers the action of the other. The powerful gene is called dominant and the other recessive.
All the genes on the same chromosome are usually inherited together. For this reason, they are called linked genes. Every cell in the body has 10,000 or more genes.
The key chemical compound of the gene is a nucleic acid called DNA. DNA is short for deoxyribonucleic acid. DNA is a threadlike molecule with a diameter of only about one ten millionth of an inch.