Ann Lindgren, age 15, of Casper, Wyo., for her question:
HOW DOES HEREDITY WORK?
Heredity is the passing on of certain characteristics from the parents to the children. But children are not usually exact copies of their parents nor are brothers and sisters exactly alike.
The difference between children and their parents and between siblings is known as variation. The study of heredity and variation is known as the science of genetics. This science discovers how genes, through reproduction, carry the information of heredity and variation from one generation to the next.
Back more than 100 years ago an Austrian monk named Gregor Mandel was the first to find the facts from which the modern science of genetics developed. In working with sweat peas in his garden, Mendel discovered in the seeds of the parent plants that there were many different factors, called genes. These always followed definite rules that controlled what a plant growing from the seeds would look like.
In 1900 other scientists saw how very important Mendel's work had been and they called his discoveries the Mendelian Laws.
A human starts life as an egg inside the mother. A human egg isn't much larger than the dot you make with your pen when you write the small latter "i." The egg is fertilized by receiving a sperm from the father. The father's sperm, looking like a tadpole, is even smaller than the mother's egg.
The human egg and sperm each contain 23 string like beads called chromosomes. The mother's and father's hereditary factors, called genes, are within these beads. There are literally thousands of genes.
The mother's egg and the father's sperm unite in what is called conception, and then both sets of genes start working together to produce the new baby's inherited traits.
Each gene has a special job: to color the baby's eyes and hair, build the facial features, establish the size of the baby's skeleton, and so on.
During the nine months that it takes for the new baby to develop, the genes are multiplying themselves as they go along, so that every one of the billions of cells in the new baby's body has duplicates of all the original genes.
The genes continue to play a part in the way a body develops and they work all through life. The genes aren't finished with their job when the baby is born.
How can brothers and sisters inherit different traits from their parents? It's easy. Each parent carries two genes for every trait but gives each child only one gene of every pair. One parent may carry a brown hair gene and a blond hair as well as a straight hair gene and a curly hair gene. One child could inherit brown, straight hair while his brother or sister could inherit curly, blond hair.
Brothers and sisters may differ greatly is various hereditary traits.