Chapter 063. Chromosome Disorders (Part 5) Chromosome segregation in meiosis. A. In meiosis I, each of the 23 pairs of chromosomes finds its "partner," or homologue, and exchanges genetic material (recombines) with it. At metaphase, each homologous pair aligns on the equatorial plate; at anaphase, each member of the homologous pair segregates from its partner. Thus, at the end of meiosis I, each daughter cell contains 23 chromosomes, with each chromosome consisting of two sister chromatids. B. In meiosis II, each chromosome aligns on the metaphase plate, and at anaphase, each of the two sister chromatids divides from the other. Thus, at the end of meiosis II, each daughter cell (e.g., the oocyte or spermatocyte) contains 23 chromosomes, with each chromosome consisting of one sister chromatid. In mitosis, the chromosomes behave exactly as they do in meiosis II, except that somatically dividing cells contain 46 chromosomes, not the 23 that are present in the meiosis II cell. Chromosome segregation is more complicated in germ cell division, since the number of chromosomes must be reduced from 46 to 23 in the mature sperm and eggs. This is accomplished by two rounds of division—meiosis I and meiosis II (Fig. 63-3). In meiosis I, homologous chromosomes pair and exchange genetic material, then align on the metaphase plate, and finally separate from one another. Thus, by the end of meiosis I, only 23 of the original 46 chromosomes are represented in each of the two daughter cells. Meiosis II quickly follows meiosis I and is essentially a "haploid mitosis," involving separation of the sister chromatids in each of the 23 chromosomes. Although the fundamentals of meiosis are the same in males and females, there are important distinctions, particularly in the timing of meiotic divisions. In males, meiosis begins with puberty and continues throughout the individual's lifetime. In females, meiosis begins prenatally, with oocytes proceeding through the first stages of meiosis I but arresting at mid-prophase. At the time of birth, the first meiotic division is suspended in oocytes. Only after ovulation many years later do oocytes complete meiosis I and proceed to the metaphase stage of meiosis II; if fertilized, the oocyte then completes the second meiotic division. Thus, in females, the first meiotic division takes at least 10–15 years and as many as 40–45 years to complete. Maternal age–related increases in the incidence of trisomy are likely the consequence of this protracted process of cell division. Incidence and Types of Chromosome Abnormalities Errors in meiosis, or in early cleavage divisions, occur with extraordinary frequency. At least 10–25% of all pregnancies, for example, involve chromosomally abnormal conceptions. A large proportion of these terminate in the earliest stages of pregnancy, many of which go unrecognized. Nevertheless, even among clinically recognized pregnancies, nearly 10% of fetuses are chromosomally unbalanced. For the three types of clinically recognized pregnancies—spontaneous abortions, stillbirths, and livebirths—the frequencies of different chromosomal abnormalities are summarized in Table 63-2. The most common abnormalities are numerical, involving fetuses with additional (trisomy) or missing (monosomy) chromosomes, or those with one (triploidy) or two (tetraploidy) additional sets of chromosomes. Structural chromosome abnormalities are much less common, although several of the most important clinical chromosomal disorders involve structural rearrangements (see below). Table 63-2 Frequency and Distribution of Chromosome Abnormalities in Different Types of Clinically Recognizable Pregnancies Frequency of Abnormality Chromoso me Abnormality Spontane ous Abortion Stillbir th Livebi rth Probabil ity of Surviving to Term, % Trisomy, all 25.1 4.0 0.3 5 +13, 18, 21 4.5 2.7 0.14 15 +16 7.5 — — 0 Sex chromosome monosomy (45,X) 8.7 0.1 0.01 1 Triploidy 6.4 0.2 — 0 Tetraploid y 2.4 — — 0 Structural 2.0 0.8 0.3 45 abnormality Total abnormalities 50.0 5.1 0.6 5 By far the most common abnormality is trisomy, which is identified in ~25% of spontaneous abortions and 0.3% of newborns. Trisomies for all chromosomes have now been identified in embryos or fetuses, but there is considerable variation in frequency for various chromosomes. For example, trisomy 16 is extraordinarily common, accounting for about one-third of all trisomies in spontaneous abortions, whereas trisomies 1, 5, 11, and 19 have been identified less often. Available evidence suggests two reasons for this variation: (1) some chromosomes (e.g., chromosome 16) are more likely to segregate abnormally or undergo nondisjunction during meiosis than are others; and (2) the potential for development varies widely among different trisomic conditions, with some being eliminated very early in gestation, others surviving to the time of clinical pregnancy recognition, and some (e.g., trisomies 13, 18, and 21 and sex chromosome trisomies) being compatible with survival to term.[newpage] . Chapter 063. Chromosome Disorders (Part 5) Chromosome segregation in meiosis. A. In meiosis I, each of the 23 pairs of chromosomes finds its "partner,". end of meiosis I, each daughter cell contains 23 chromosomes, with each chromosome consisting of two sister chromatids. B. In meiosis II, each chromosome aligns on the metaphase plate, and at. cell (e.g., the oocyte or spermatocyte) contains 23 chromosomes, with each chromosome consisting of one sister chromatid. In mitosis, the chromosomes behave exactly as they do in meiosis II,