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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    20 / GENETICS IN OBSTETRICS AND GYNECOLOGY a reciprocal and presumably balanced translocation. Such an individual, known as a translocation heterozygote, is phenotypically normal. During meiosis I there are several ways in which the two translocation chromosomes and their two structurally normal homologues may segregate. Gametes are genetically balanced if both translocation chromosomes pass to one gamete, the two normal chromosomes passing to the other gamete. However, if either of the translo¬ cation chromosomes and one of the normal chromosomes pass to the same gamete, the gamete will be genetically imbalanced. Translocations are an im¬ portant cause of repetitive spontaneous abortions and developmental abnor¬ malities. In an inversion the order of genes on a chromosome is altered by two or more break points, but no duplication or deficiency occurs. If all breaks occur on the same side of the centromere (paracentric), the inversion can be recog¬ nized only by banding techniques or by meiotic or genetic studies. An indi¬ vidual carrying a paracentric inversion would be phenotypically normal unless a position effect occurred. A paracentric inversion may, however, lead to un¬ balanced gametes if crossing-over occurs within a paracentric loop. If an in¬ version arises following breaks on both sides of the centromere, the relative position of the centromere will be altered (pericentric inversion], assuming the two breaks are not equidistant from the centromere. Inversions may also lead to repetitive spontaneous abortions, because crossing-over within an inversion may lead to unbalanced gametes. isochromosomes have identical arms. They arise following horizontal, rather than longitudinal, division of the centromere (Fig. 1-13). The acentric portion, composed of one entire arm, is lost; the remaining telocentric portion is unstable, replicating at the next cell division to form a metacentric chro¬ mosome. The two arms of an isochromosome are mirror images, both struc¬ turally and genetically. An isochromosome thus consists of complete dupli¬ cation of one arm and complete deficiency of the other arm. An isochromosome can be composed of identical long arms (e.g., 46,X,i(Xq)) or identical short arms (e.g., 46,X,i(Xp)), depending upon which arm remains with the centromere. An isochromosome for the X long arm is the most common X-structural abnormality in patients with gonadal dysgenesis. И a lost Replication С d e e d с d e Figure 1-13. Diagrammatic representation of the origin of an isochromosome. (From Simpson JL: Disorders of Sexual Differentiation. Etiology and Clinical Delineation. New York: Academic Press, 1976, p 25. Reprinted with permission.)