Figure 5.3 Chromosome segregation from a reciprocal translocation heterozygote. The box at the top illustrates the process by which unequal crossing over between sequences on different chromosomes can lead to the production of reciprocal translocation heterozygotes. The unequal crossover event will have actually occurred in a meiotic cell that gave rise to a balanced transloaction gamete (as shown in the lower part of the Figure) that participated in the formation of the zygote from which the animal derived. The box in the center illustrates the process of chromosome pairing and synapsis that will occur for just the two chromosome pairs involved in the translocation. Through the process of normal disjunction, each gamete will receive just one homolog of each chromosome pair. At the bottom right is an illustration of the two gametes (sperm in this case) produced by "adjacent-1 segregation" in which the adjacent homologs of chromosomes 2 and 8 assort together. At the bottom left is an illustration of the two gametes produced by "alternate segregation" in which non-adjacent homologs of the two chromosomes assort together. A third much less frequent possibilty, not illustrated here, is "adjacent-2 segregation"; in this case of non-disjunction, both chromosome 2 centromeres would assort to one gamete, and both chromosome 8 centromeres would assort to the brother gamete.