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 / 17 only repulse one another and move toward opposite poles (anaphase). Telo¬ phase may or may not occur following meiosis I, its occurrence being species- specific. Meiosis II Between meiosis I and meiosis II, an interphase Gl may or may not occur. Little or no DNA synthesis (S) is required, and the occurrence of G2 varies from species to species. The stages of meiosis II—metaphase, anaphase, and telo¬ phase—are similar to the corresponding stages of mitosis. However, meiosis II lacks a prophase stage, chromosomes passing directly into metaphase. After completion of meiosis II, a single original diploid cell will have divided into four haploid cells. If recombination occurs, as it usually does, no two cells will be genetically identical (Fig. 1-9]. CHROMOSOME ERRORS Chromosomal errors may be divided into those characterized by numerical changes (deviations from the normal number) or by structural changes (abnor¬ malities in chromosome morphology). Numerical Errors If a haploid gamete or a diploid cell lacks the expected number of chro¬ mosomes (n or 2n, respectively), aneuploidy exists. If the complement contains one additional whole chromosome (2n -I- 1), trisomy exists. This term can be applied to both autosomal or sex chromosomal anomalies. If the number of sex chromosomes is increased, the term polysomy is sometimes used. If one entire chromosome is lacking (2n - 1), monosomy exists. Polyploidy refers to the presence of more than two haploid complements within a single cell; triploidy (3n = 69) and tetraploidy (4n = 92) are the most common types of polyploidy in humans. Trisomy and monosomy may arise by several mechanisms. First, aneuploi¬ dy may arise de novo, presumably following a meiotic or mitotic error. De novo aneuploidy on this basis is called primary nondisjunction. During mitosis, sister chromatids may not disjoin properly, one daughter cell receiving both sister chromatids. Nondisjunction during mitosis can lead to more than one cell line (mosaicism) (Fig. 1-10); nondisjunction during meiosis causes aneu- ploid gametes, but the embryo will contain only one cell line (Fig. 1-11). Tri¬ somie or monosomic parents would be expected to produce equal numbers of normal (n) gametes and either n + 1 or n - 1 gametes, depending upon the parental complement. If either of these gametes had been fertilized by a normal gamete, the zygote would be chromosomally abnormal as a result of secondary nondisjunction. This is a rare cause of aneuploidy in mammals, as aneuploid mammals are usually either sterile or fail to produce aneuploid offspring in the expected proportion. However, offspring of mothers with Down syndrome may show trisomy 21, and 47,XXX may produce 47,XXX or 47,XXY offspring (Simpson, 1981a). A third source of monosomic cells is anaphase lag, a situ-