Genetics in obstetrics and gynecology / Joe Leigh Simpson [and others].

Date:
[1982], ©1982
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Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Genetics in obstetrics and gynecology / Joe Leigh Simpson [and others]. Source: Wellcome Collection.

    13/344 (page VII)
    1 Principles of Human Genetics Both normal phenotypic variation, such as blood pressure, and abnormal phen- otypic variation, such as Down syndrome or cystic fibrosis, may be considered in terms of several etiologic categories. These categories include (1) cytogenetic disorders, which result from changes in the number or structure of the chro¬ mosomes; (2) Mendelian disorders, which result from mutations at a single genetic locus; (3) polygenic (multifactorial) disorders, which result in part from the cumulative effects of more than one gene; and (4) teratogenic disorders, which depend primarily upon environmental factors. These categories are use¬ ful in classifying anomalies and other disorders (Lancet, 1974a). Other genetic factors, such as cytoplasmic inheritance, are of less immediate relevance to physicians. MOLECULAR ASPECTS OF THE GENE The principles of inheritance were discovered in 1865 by Mendel and were subsequently rediscovered around 1900 by others. In 1902, Sutton and Boveri first suggested that chromosomes served as the physical basis of inheritance. The chemical basis of inheritance was not elucidated until several decades later. In experiments involving genetic transformation of pneumococci (Griffith, 1928; Avery et al., 1944), deoxyribose nucleic acid (DNA) was shown to be the hereditary material of all living organism except certain viruses. Subsequently, several types of ribonucleic acid (RNA) were shown to be required to translate the information inherent in the genes. Structure and Replication of DNA In 1953, Watson and Crick proposed that DNA existed in the form of a double helix; this structural configuration helped explain how DNA could replicate. The double helix may be envisioned as a twisted ladder (Fig. 1-1). Each vertical column consists of alternating deoxyribose sugar residues and 1