Info

Down N1 + N2

Translocation genotype

Normal

Both complete and viable

Figure 15-24 The two most commonly encountered chromosome segregation patterns in a reciprocal translocation heterozygote. N1 and N2, normal nonhomologous chromosomes; T1 and T2, translocated chromosomes. Up and Down designate the opposite poles that homologs migrate to during anaphase I.

Translocation Heterozygote Corn Plant

Figure 15-25 Photomicrograph of normal and aborted pollen of a semisterile corn plant. The clear pollen grains contain chromosomally unbalanced meiotic products of a reciprocal translocation heterozygote. The opaque pollen grains, which contain either the complete translocation genotype or normal chromosomes, are functional in fertilization and development. [William Sheridan.]

Figure 15-25 Photomicrograph of normal and aborted pollen of a semisterile corn plant. The clear pollen grains contain chromosomally unbalanced meiotic products of a reciprocal translocation heterozygote. The opaque pollen grains, which contain either the complete translocation genotype or normal chromosomes, are functional in fertilization and development. [William Sheridan.]

translocated chromosomes. The segregation of each of the structurally normal chromosomes with one of the translocated ones (Tj + N2 and T2 + Nj) is called adjacent-1 segregation. Each of the two meiotic products is deficient for a different arm of the cross and has a duplicate of the other. These products are inviable. On the other hand, the two normal chromosomes may segregate together, as will the reciprocal parts of the translocated ones, to produce Nj + N2 and Tj + T2 products. This segregation pattern is called alternate segregation. These products are both balanced and viable.

Adjacent-J and alternate segregations are equal in number, so half the overall population of gametes will be nonfunctional, a condition known as semisterility or "half sterility." Semisterility is an important diagnostic tool for identifying translocation heterozygotes. However, semisterility is defined differently for plants and animals. In plants, the 50 percent of meiotic products that are from the adjacent-1 segregation generally abort at the gametic stage (Figure 15-25). In animals, these products are viable as gametes but lethal to the zygotes they produce upon fertilization.

Remember that heterozygotes for inversions may also show some reduction in fertility, but by an amount dependent on the size of the affected region. The precise 50 percent reduction in viable gametes or zygotes is usually a reliable diagnostic clue for a translocation.

Genetically, genes on translocated chromosomes act as though they are linked if their loci are close to the translocation breakpoint. Figure 15-26 shows a translocation heterozygote that has been established by crossing an a/a ; b/b individual with a translocation homozygote bearing the wild-type alleles. On testcrossing the heterozygote, the only viable progeny are those bearing the parental genotypes, so linkage is seen between loci

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