Chromosome number can evolve by smaller-scale changes than duplication of entire chromosom...

Chromosome number can evolve by smaller-scale changes than duplication of entire chromosome sets. For example, domestic horses have 64 chromosomes per diploid set while Przewalski’s horse, an Asian subspecies, has 66. Przewalski’s horse is thought to have evolved from an ancestor with 2n = 64 chromosomes. The question is: Where did its extra chromosome pair originate? It seems unlikely that an entirely new chromosome pair was created from scratch in Przewalski’s horse. To generate a hypothesis explaining the origin of the new chromosome in Przewalski’s horse, examine the adjacent figure. The drawing at right shows how certain chromosomes synapse in the hybrid offspring of a domestic horse–Przewalski’s horse mating (Short et al. 1974). The remaining chromosomes show a normal 1:1 pairing. Do you think this sort of gradual change in chromosome number involves a change in the actual number of genes present, or just rearrangement of the same number of genes?

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