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end. A series of other steps that you don’t need to know to understand hereditary thyroid disease bind the amino acids into a protein that may be an enzyme or a muscle or whatever. Genes also contain large segments of bases that don’t code anything and, in fact, have to be cut out from their complementary RNA before the RNA can produce a protein. These sequences are called introns, and scientists don’t know what their function is.

Determining whether to be a liver or a heart A fertilized egg reproduces itself by the process of mitosis, creating two identical cells. But how can each identical cell transform itself into a thyroid cell or a liver cell or even a brain cell? The gene uses a number of techniques to turn gene action on and off. Short sequences of bases called promoters precede the active gene. When various factors bind to a promoter, it turns on the action of the gene to begin transcription. Other sequences of elements called enhancers increase the activity of the gene further, while silencers tend to shut down the activity of the gene. The process of transcription from DNA to messenger RNA comes to a halt when certain sequences of bases are reached. Because all cells contain the same genetic information, a thyroid cell differs from a brain cell as a result of the particular genes that are expressed (active) in each cell. Many different factors determine whether a gene is expressed. Promoters and enhancers increase their activity in response to various hormones or growth factors that may be present in one cell but not another. The expression of a gene may be controlled at the level of the gene itself, or it may be controlled after transcription has taken place so that the messenger RNA never makes the protein. In these and in many other ways scientists have yet to discover, cells use only the genes they need to function within their environment.

Probing the Origins of Genetic Thyroid Diseases Now that you have a basic understanding of genetics, you can apply your knowledge to thyroid diseases that are transmitted through inheritance. A child inherits a thyroid disease in one of three basic ways:

Thyroid for dummies  
Thyroid for dummies