Genes are the smallest units of heredity. The information from all the genes, taken together, makes up the blueprint or plan for the human body and its functions. A gene is a short segment of DNA which is interpreted by the body as a plan or template for building a specific protein.


Some diseases, such as sickle cell anemia, can be caused by a change in a single gene (one of approximately 30,000 genes that make up the plan for the entire human body).

Genes are arranged along DNA strands similar to beads on a string. These DNA strands make up the chromosomes.

Chromosomes exist in matched pairs, each containing one copy of a given gene. The gene occurs in the same position on each chromosome. One chromosome has the gene from the mother, the other (matching) chromosome has the gene from the father.

The exception is in males, where 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 controllable by one gene in the pair.  
  • Recessive traits require both genes in the gene pair to work together to control the trait.

Related topics:

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

Johns Hopkins patient information

Last revised: December 3, 2012
by Levon Ter-Markosyan, D.M.D.

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