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A single gene from a parent transmits a dominant or recessive trait to the child. This method goes back to Mendel and his peas. The child inherits all of the many genes involved in the inheritance of a disease. An entire chromosome is abnormal. For example, if a female ends up with only one X chromosome, that lack can produce a condition known as Turner’s syndrome, which often includes a thyroid disorder.

Inheriting a disease through a single gene People inherit many diseases through a single gene, often as a result of a gene mutation, in one of three ways: As a recessive gene: Both parents must supply the same gene in order for the disease to appear. As a dominant gene: Only one parent supplies the gene necessary to cause the disease. With the X chromosome in a recessive form: A male gets the disease (because he has only one X chromosome), but a female is spared unless both her X chromosomes have the gene. You can find the entire list of diseases transmitted by single-gene inheritance at www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=OMIM, the homepage of Online Mendelian Inheritance in Man (OMIM), a huge database compiled by Dr. Victor McKusick at Johns Hopkins University and others. If you search the term “thyroid” from the homepage, 554 different thyroid diseases appear (at the time of this writing). A full description accompanies each one, with citations of all the research that has been done to define the defect and a complete bibliography at the end of each description. Inherited thyroid diseases can affect every step in thyroid hormone production, transportation, and action. The ones I list in the following sections are only 14 of the 554 currently listed in the OMIM database. Scientists are discovering new conditions all the time. These 14 are the best understood.

Recessive inheritance Many conditions where the body doesn’t make thyroid hormone properly fall into the category of recessive inheritance. Two “bad” genes are necessary to develop one of these conditions. If you have just one bad gene, you’re a carrier of the disease, but you don’t experience it yourself. The phenotype (the way this gene makes itself known) is usually a large thyroid that doesn’t produce sufficient thyroid hormone. Note that several of the conditions appear to be the same because the final result of the condition is absence of thyroid hormone, but each condition involves a defect in

Thyroid for dummies  
Thyroid for dummies  
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