Final answer:
An autotriploid organism can arise due to nondisjunction during meiosis, leading to gametes with an extra set of chromosomes that result in offspring with triploid chromosome numbers upon fertilization.
Step-by-step explanation:
An autotriploid can arise in an organism through nondisjunction during meiosis. This error during cell division can result in a gamete with an extra set of chromosomes. For example, a plant with a diploid number of 2n = 6 that fails to undergo cytokinesis during mitosis might produce diploid gametes with 2n = 6 instead of the expected haploid number n = 3. Such a gamete, upon fertilization with another gamete having the same or a similar error, could lead to an offspring with a triploid number of chromosomes (3n). This autotriploid organism would have three complete sets of chromosomes from its own species and would be capable of self-pollination or breeding with other autotriploid individuals, but not with the ancestral diploid species.