Disease Example (cis configuration/trans configuration):


The four genes (HBA) that encode the alpha globin chains of normal adult hemoglobin are located on the short arm of chromosome 16, two genes situated in tandem on each chromosome.

Deletions of one or more of these four genes are most often associated with alpha-thalassemia trait, hemoglobin H disease, or hydrops fetalis.


O = HBA gene

Chromosome 16


Some Clinical Implications

  • A carrier of two HBA gene deletions in trans configuration is not at risk of having a child with hydrops fetalis, even if his/her partner carries two HBA deletions in cis configuration. Such a couple has a 50% risk of having a child with Hemoglobin H disease.
  • Hydrops fetalis can only occur in offspring of parents who both carry two HBA gene deletions in cis configuration.

Revised: 2-26-04

Related Terms: double heterozygote; pseudogene; variable expressivity

Image LearnMore.jpg Image Term.jpg Image FullGlossary.jpg Image Help.jpg

From: Illustrated Glossary

Cover of GeneReviews®
GeneReviews® [Internet].
Pagon RA, Adam MP, Ardinger HH, et al., editors.
Seattle (WA): University of Washington, Seattle; 1993-2015.
Copyright © 1993-2015, University of Washington, Seattle. All rights reserved.

For more information, see the GeneReviews Copyright Notice and Usage Disclaimer.

For questions regarding permissions: ude.wu@tssamda.

NCBI Bookshelf. A service of the National Library of Medicine, National Institutes of Health.