Porth's Essentials of Pathophysiology, 4e

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Genetic Control of Cell Function and Inheritance

C h a p t e r 5

Ocular albinism Hypophosphatemia, hereditary Duchenne/Becker muscular dystrophy Chronic granulomatous disease Retinitis pigmentosa Wiskott-Aldrich syndrome

22

2

p

21

1

11

SCID, X-linked

1

13

21

Agammaglobulinemia Fabry disease

q

2

25

Hemophilia B Fragile X syndrome Color blindness Diabetes insipidus, nephrogenic G6PD deficiency Hemophilia A

FIGURE 5-8. Crossing over of DNA at the time of meiosis.

If the centromere is in the center and the arms are of approximately the same length, the chromosome is said to be metacentric; if it is not centered and the arms are of clearly different lengths, it is submetacentric; and if it is near one end, it is acrocentric. The short arm of the chro- mosome is designated as “p” for “petite,” and the long arm is designated as “q” for no other reason than it is the next letter of the alphabet. The arms of the chromosome are indicated by the chromosome number followed by the p or q designation (e.g., 15p). Chromosomes 13, 14, 15, 21, and 22 have small masses of chromatin called satellites attached to their short arms by narrow stalks. The banding patterns of a chromosome are used in describing the geographic position of a gene on a chro- mosome, which can be useful, for example, in com- municating the location of genes involved in genetic diseases. Each arm of a chromosome is divided into regions, which are numbered from the centromere outward (e.g., 1, 2). The regions are further divided into bands, which are also numbered (Fig. 5-10). These

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numbers are combined to designate the position of a gene; for example, Xp22 refers to band 2, region 2 of the short arm (p) of the X chromosome. FIGURE 5-10. Localization of inherited diseases as represented on the banded karyotype of the X chromosome. Notice the nomenclature of arms (P, Q), regions (1, 2), and bands (e.g., 22 [region 2, band 2]). G6PD, glucose-6-phosphate dehydrogenase deficiency; SCID, severe combined immunodeficiency disease. (Modified from Peiper S, Strayer DS. In: Rubin R, Strayer DS, eds. Rubin’s Pathology: Clinicopathologic Foundations of Medicine. 6th ed. Philadelphia, PA: Wolters Kluwer Health | Lippincott Williams &Wilkins; 2012:251.) ■■ The genetic information in a cell is organized, stored, and retrieved as small structures called chromosomes.There are 46 chromosomes arranged in 23 pairs, 22 of which are alike for males and females, and 1 pair of sex chromosomes, with XX pairing in females and XY pairing in males. ■■ Cell division involves the duplication of the chromosomes. Duplication of chromosomes in somatic cell lines involves mitosis, in which each daughter cell receives 23 pairs of chromosomes. Meiosis is limited to replicating germ cells (ovum and sperm) and results in formation of a single set of 23 chromosomes. ■■ A karyotype is an image of a person’s chromosomes. It is prepared by special laboratory techniques in which body cells are cultured, fixed, and then stained to display identifiable banding patterns. A photomicrograph is then made. Often the individual chromosomes are cut out and regrouped according to chromosome number. SUMMARY CONCEPTS

Centromere

Chromatid

Metacentric

Submetacentric

Chromosome arm

Acrocentric

Satellite

FIGURE 5-9. Three basic shapes and the component parts of human metaphase chromosomes.The relative size of the satellite on the acrocentric is exaggerated for visibility. (Adapted from Cormack DH. Essential Histology. Philadelphia, PA: J.B. Lippincott; 1993.)