People's genetic information is contained within structures in the cell nuclei called chromosomes, which are constructed primarily of DNA molecules. DNA (short for deoxyribonucleic acid) is formed of various chemical substances arranged into long, chainlike molecules whose shape resembles ladders twisted into spirals. A person's genetic details are encoded in the patterns and combinations of the chemicals in these DNA strands, forming instructions that tell cells how to operate. All the cells in a person's body contain the same DNA because they all developed as replications of the first cell from which that person originated.

Every cell in a body contains an identical set of 46 chromosomes, grouped into 23 pairs. One of those pairs determines a person's gender--either an X and a Y chromosome for males, or two Xs for females. The remaining 22 pairs (called autosomal chromosomes) determine various non-sex related traits, mapped into segments of the chromosomes called genes.

Every chromosome is divided up into thousands of genes--there are more than 50,000 in a single cell. Because genes are a part of chromosomes, they also come in pairs, and each gene pair works together to control a specific function or activity within the cell. Some have relatively small significance, such as defining hair or eye color. Some genes control important activities, such as the production of vital enzymes needed for healthy functioning.

The following diagram shows the relationships between chromosomes, genes, and DNA.

People's chromosomes and genes come in pairs because during the process of reproduction, the egg and sperm each contribute half the set of genetic material for the new life. For example, with the sex chromosomes, the XX mother will pass one of her X chromosomes onto her child; the XY father will pass either an X or a Y chromosome. Thus the sex of the child will be determined by the final combination received from the parents: XX (female) or XY (male).

Inheritance of the autosomal chromosomes works similarly: the mother will pass on half the pair and the father the other half. The outcome in the child will depend on how the received gene pairs work together. If the two genes determine the same thing--for example, if they both code for blue eyes--the outcome is straightforward.

Sometimes a certain type of gene is stronger, or "dominant," and will override another "recessive" gene type.

Previous | Next