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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    22 / GENETICS IN OBSTETRICS AND GYNECOLOGY could be formed by fusion after (1] fertilization of both an ovum and its polar body, (2] fertilization of both within a single binucleated follicle, or (3) fertil¬ ization of ova derived from different follicles. If two or more genotypes are present in nonhematogenous tissues (skin, gonads] or persist in hematogenous tissue for a long time, whole-body chimerism can be assumed. The frequency of whole-body chimerism is probably underestimated, be¬ cause chimeric individuals are usually discovered because of abnormal sexual development, specifically true hermaphroditism. Almost all whole-body chi¬ meras are heterosexual (unlike-sexed), whereas one might expect an equal number of isosexual (like-sexed) chimeras. Detection of Mosaicism and Chimerism Detection of mosaicism or chimerism depends upon many factors, but most importantly upon the number of cells analyzed per tissue and the number of tissues analyzed. The number of cells that needs to be analyzed depends upon the percentage of the minority cell line, which itself depends upon the time the mosaicism originates and whether any of the mosaic lines have a selective disadvantage or advantage. Several investigators have calculated the number of cells statis¬ tically necessary to exclude a minority cell population. For example, analysis of fifty cells without detection of at least one cell representing a minor line excludes any population comprising 10 percent or more of all cells in the tissue studied (p < 0.05) (Ford, 1969). If mitotic nondisjunction occurs relatively early in organogenesis, yet at a time when the embryo contains at least several hundred cells (1- to 2-week- old embryo), analysis of a single tissue might suffice, assuming absence of cell selection. If, on the other hand, monosomy X originates by nondisjunction or anaphase lag in the older fetus, mosaicism may pass undetected if only a single tissue were studied. Thus, ideally, multiple tissues should be studied; however, practicality will more often dictate analysis of a single tissue. CYTOLOGIC PROPERTIES OF SEX CHROMOSOMES X Chromosome The X chromosome is a submetacentric chromosome intermediate in length between Nos. 7 and 8. Its banding pattern distinguishes it from all other chro¬ mosomes. In addition, the late-replicating X (inactive) and the early-replicating X (active) can be distinguished from one another by autoradiography or by exposing cells to bromodeoxyuridine (BUdR), staining with the fluorochrome 33258 Hoechst, and analyzing by fluorescent microscopy. In females one of the two X chromosomes is the last of the complement to complete DNA synthesis, probably because during interphase it is more tightly coiled than other chromosomes. During interphase, this chromosome, said to represent the heterochromatic X, forms a planoconvex body called X chromatin (Fig. 1-5). In diploid lines the number of X-chromatin masses equals the number of X chromosomes minus one. All X chromosomes in excess of one are genetically inactive. The rela-