Christine Armstrong, 13, of Brooklyn, N.Y., for her question:
HOW DO WE INHERIT TRAITS?
Heredity is the passing on of characteristics from parents to offspring. These characteristics, called traits, are determined by certain bodies in the cells called chromosomes and genes. The chromosomes carry the genes.
Chromosomes are tiny objects inside the cell nucleus. Every living thing has a certain number of chromosomes.
In humans, all body cells except those of egg and sperm cells contain 46 chromosomes. In mice, each body cell contains 40 chromosomes while each cell in corn has 20 chromosomes.
Every human body cell actually has two sets of 23 chromosomes. These two sets look very much alike. Each chromosome in one set can be matched with a particular chromosome in the other set. Egg and sperm cells have only one set of 23 chromosomes.
When an egg cell and a sperm cell come together, the fertilized egg cell will contain 46 chromosomes. Half of the chromosomes come from the father while the other half come from the mother.
Genes are tiny particles that actually determine all hereditary traits. They are carried on the chromosomes. The chromosomes bear hundreds of genes arranged in lines.
Each person inherits two genes for a particular trait: one from his mother and the other from his father. But in many traits, the action of one gene completely overpowers the action of the other. The powerful gene is called dominant and the other recessive.
Suppose a dark haired mother and a light haired father have a dark haired child. The child actually has cells that contain genes for both light and dark hair, but in this case it is apparent that the gene for dark hair was dominant over the gene for light hair.
All genes on the same chromosome are usually inherited together. For this reason, they are called linked genes. Sometimes, however, genes on the same chromosome are not inherited together.
For example, when a sex cell divides to form egg cells or sperm cells, each chromosome pairs off with its partner. As they lie side by side, groups of genes from one chromosome may change places with groups of genes form its partner.
This process of exchange, called crossing over, makes it possible for members in a family to inherit different combinations of linked traits.
Genes that are close together on the same chromosome are rarely separated from each other by the process of crossing over. Genes that are farther apart separate more often.
Sometimes children inherit diseases from their parents. Included are hemophilia, a disease in which the blood does not clot normally, and muscular dystrophy.