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Key Points

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  • Disease summary:

    • Cytogenetics is the study of chromosome structure, function, and disorders by combining cytology (the study of cells) with clinical genetics (the study of inherited variation). The normal diploid complement of chromosomes consists of 46 chromosomes: 22 pairs of numbered autosomal chromosomes and one pair of lettered sex chromosomes (X and Y) (Fig. 167-1). One haploid complement is composed of 23 chromosomes, inherited from the mother in the egg or the father in the sperm. A complete diploid set of 46 chromosomes is present in the zygote, and upon replication, they are present in every cell nucleus of the offspring. A normal chromosome complement (or karyotype) is designated as 46,XX for females and 46,XY for males. Chromosomes each have a short arm (p) and a long arm (q). Chromosome regions as well as gene locations are annotated as the chromosome number followed by “p” or “q” (for the short or long arm, respectively) followed by the band number (organized by the banding pattern chromosomes uniformly display on in vitro staining). During metaphase the region between the p and q arms is condensed and constricted and is designated the centromeric region where the spindle apparatus is attached. Some chromosomes such as chromosome 1, 6, 7, 11, 14, 15, 18, 19, and 20 contain regions that are differentially methylated, or “imprinted,” based on the parent of origin; imprinted portions of chromosomes are transciptionally silenced, meaning genes in those segments are not translated to protein. The term chromosome disorder refers to a clinical condition secondary to an abnormality in the quantity, content, and/or arrangement of the 23 pairs of chromosomes comprised of the nuclear genomic DNA.

  • Chromosomal abnormalities (see Table 167-1):

    • Aneuploidy, alteration in the total number of chromosomes, such as trisomy and monosomy

    • Euploidy, a multiple of the normal number of chromosomes, 46 (eg, 69,XXX or 92,XXXX, which are embryonic lethal)

    • Deletion, loss of a portion of a chromosome (eg, DiGeorge or velocardiofacial syndrome due to deletion of 22q11.2, or chromosome 22, q arm region, band 11.2 [designated “one one point two”])

    • Duplication, gain of a portion of a chromosome (eg, Charcot-Marie-Tooth type 1 due to duplication of the PMP22 gene in chromosome17p12)

    • Insertion, gain of a portion of a chromosome

    • Pericentric inversion, inversion of a portion of a chromosome including the centromeric region

    • Paracentric inversion, inversion of a portion of a chromosome arm not including the centromeric region

    • Robertsonian translocation, fusion of two acrocentric chromosomes 13, 14, 15, 21, or 22 (chromosomes with a very small p arm) with breakpoints at or near the centromere with loss of repeated DNA sequences from the short p arms that usually is without clinical consequence. A karyotype with a robertsonian translocation has a total of 45 chromosomes.

    • Reciprocal translocation, rearrangement of portions of two different chromosomes formed by the reciprocal exchange between portions from autosomal or sex chromosomes (Fig. 167-2)

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