An autosomal recessive gene is one which is required in two copies on an autosome to be active in an organism. Blue eye color is a common example of this gene type in humans.

For another example, a person who carries two copies of the same abnormal recessive gene (in other words, this person is homozygous for the gene) will experience effects from that gene (for instance, he or she might then suffer from a disease coded by the gene). The gene can be inherited from both parents who carry the gene; sons and daughters have an equal chance of inheriting the gene.

If a person has a genetic disease like cystic fibrosis or sickle cell anemia but his or her parents seem normal, then this probably means that:

  1. The parents of this person are heterozygous, unaffected carriers of the disease, or
  2. The person inherited one copy of the disease gene from one parent, and the other "good" copy underwent a mutation early in the person's embryonic development

If both parents are heterozygous carriers of a recessive gene, there is a 25% chance that any child they produce will inherit both copies of the gene and express it. The child has a 50% chance of inheriting one copy of the recessive gene and thus also being a carrier. If one parent suffers from the disease and the other parent is a carrier, the child will definitely at least be a carrier and has a 50% chance of having the disease.

Part of the information in this writeup was taken from the science dictionary at; I oversaw the development of the dictionary (the website was mothballed in 1998) and I believe I wrote the entry this is based on.

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