Chapter

Genetic factors relating to the thyroid with emphasis on complex diseases

Francesca Menconi, Terry F. Davies and Yaron Tomer

in Oxford Textbook of Endocrinology and Diabetes

Second edition

Published on behalf of Oxford University Press

ISBN: 9780199235292
Published online July 2011 | e-ISBN: 9780199608232 | DOI: http://dx.doi.org/10.1093/med/9780199235292.003.3084

Series: Oxford Textbooks

Genetic factors relating to the thyroid with emphasis on complex diseases

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The nucleus of each human cell encodes approximately 30 000 genes. A large fraction of the genes in each individual exist in a form that can vary between individuals. These variable genetic forms are termed polymorphisms, and they account for much of the normal variation in body traits, such as height and hair colour. The genetic information encoded in the DNA is stored on the chromosomes and each somatic cell contains 46 chromosomes (22 autosomes and two sex chromosomes), arranged in 23 pairs, one of each derived from each parent.

Since each individual inherits two copies of each chromosome (for autosomes), one from each parent, there are also two copies of each gene. The chromosomal location of a gene is termed the locus of the gene. When the gene in a certain locus exists in two or more forms, these variants of the gene are termed alleles. When an individual’s two alleles at a locus are identical, that individual is said to be homozygous at that locus, and when the two alleles are different, the individual is a heterozygote.

Female somatic cells contain two X chromosomes, whereas male somatic cells contain only one X chromosome. Nevertheless, the activity of genes coded for by the X chromosome is no higher in females than in males. This is due to inactivation of most of the genes on one of the two X chromosomes. Thus, in female somatic cells only one X chromosome gene is expressed, and this process of suppression is called X-chromosome inactivation. X-chromosome inactivation occurs early in embryonic life and, thereafter, in each cell either the maternal or paternal chromosome is inactivated. This results in a tissue mosaic of paternally and maternally expressed X-chromosomal alleles, with an average of 1:1 distribution. As a result, a female who is heterozygous for an X-linked gene will show a mosaic-like distribution of cells expressing either one of the two alleles. Recently X-inactivation has been postulated to play a role in autoimmune diseases and may help explain the female preponderance of autoimmune diseases (see below).

Chapter.  11942 words.  Illustrated.

Subjects: Endocrinology and Diabetes

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