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.creating a large metacentric chromosome.

Fragment a fragment that often —- | fails to segregate and is lost.

9.16 In a Robertsonian translocation, the short arm of one acrocentric chromosome is exchanged with the long arm of another.

sess one normal copy of each chromosome and one translocated copy ( FIGURE 9.17a). Each of these chromosomes contains segments that are homologous to two other chromosomes. When the homologous sequences pair in prophase I of meiosis, crosslike configurations consisting of all four chromosomes ( FIGURE 9.17b) form.

Notice that N1 and T1 have homologous centromeres (in both chromosomes the centromere is between segments B and C); similarly, N2 and T2 have homologous centromeres (between segments N and O). Normally, homologous centromeres separate and move toward opposite poles in anaphase I of meiosis. With a reciprocal translocation, the chromosomes may segregate in three different ways. In alternate segregation ( FIGURE 9.17c), N1 and N2 move toward one pole and T1 and T2 move toward the opposite pole. In adjacent-1 segregation, N1 and T2 move toward one pole and T1 and N2 move toward the other pole. In both alternate and adjacent-1 segregation, homologous centromeres segregate toward opposite poles. Adjacent-2 segregation, in which N1 and T1 move toward one pole and T2 and N2 move toward the other, is rare.

The products of the three segregation patterns are illustrated in FIGURE 9.17d. As you can see, the gametes produced by alternate segregation possess one complete set of the chromosome segments. These gametes are therefore functional and can produce viable progeny. In contrast, gametes produced by adjacent-1 and adjacent-2 segregation are not viable, because some chromosome segments are present in two copies, whereas others are missing. Adjacent-2 segregation is rare, and so most gametes are produced by alternate and adjacent segregation. Therefore, approximately half of the gametes from an individual heterozygous for a reciprocal translocation are expected to be functional.

(a) ly An individual heterozygous for this translocation possesses one normal copy of each chromosome (N-| and N2)...

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