A gene is a short piece of DNA, which tells the body how to build a specific protein. There are approximately 30,000 genes in each cell of the human body. The combination of all genes makes up the blueprint for the human body and its functions.

A person's genetic makeup is called a genotype.

Genes are located on strands of DNA, just like beads on a string. The DNA strands make up the chromosomes.

Chromosomes contain matching pairs of one copy of a specific gene. The gene occurs in the same position on each chromosome.

In females, one sex chromosome gets its gene from the mother, the other matching sex chromosome has the gene from the father.

In males, a single X chromosome comes from the mother and a non-matching Y chromosome comes from the father.

Genetic traits, such as eye color, are described as dominant or recessive:

• Dominant traits are controlled by one gene in the pair.
• Recessive traits require both genes in the gene pair to work together to control the trait.

Many personal characteristics, such as height, are determined by more than one gene. However, some diseases, such as sickle cell anemia, can be caused by a change in a single gene. For detailed information, see heredity and disease.

See also:

• Autosomal dominant
• Autosomal recessive
• Genetic counseling and prenatal diagnosis
• Sex-linked dominant
• Sex-linked recessive