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.

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    PRINCIPLES OF HUMAN GENETICS / 19 H- lost Terminal + lost interstitial Figure 1-12. Diagrammatic representation of the origin of terminal and interstitial deletions. (From Simpson JL: Disorders of Sexual Differentiation: Etiology and Clinical Delineation. New York, Academic Press, 1976, p 22. Reprinted with permission.) of a chromosome are involved. Nonetheless, if the change is large enough to be recognized by light microscopy, many genes are duplicated or deficient. A phenotypic abnormality may also result if the position of a gene with respect to its neighboring genes is altered (position effect). A simple deficiency involves loss of one portion of a chromosome. A deficiency can either be terminal or interstitial; only with banding can these be distinguished (Fig. 1-12). Deficiency in an autosome often results in death or malformation of the embryo, but deficiency in a sex chromosome is usually less deleterious. A ring chromosome arises following a break in both the long arm and the short arm. The centromeric ends fuse; the telomeric fragments are acentric, and thus are lost, causing a portion of each arm to be deleted. Ring chromosomes are unstable because their replication can lead to double-sized rings, dicentric rings, or loss of the entire chromosome. Duplication may originate by duplication of a portion of a single chro¬ mosome or by unequal crossing-over between nonsister chromatids. Duplica¬ tion may also arise if a portion of one chromosome is interchanged with a portion of another (translocation). Duplications are usually less lethal than deficiencies, especially in lower animals; however, in humans duplications are usually associated with phenotypic abnormalities. If, following chromosome breakage, material is exchanged between two or more chromosomes, a translocation is said to have occurred (Fig. 1-7). Both chromosomes may appear structurally abnormal, one showing a duplication and the other a deficiency. If no genetic material is lost, an individual carries