Final answer:
An isochromosome is formed when the centromere splits in the wrong plane during meiosis, resulting in a chromosome with identical arms.
Step-by-step explanation:
A chromosome that results when the centromere splits in the wrong plane during meiosis, forming identical arms, is known as a isochromosome. During anaphase of cell division, sister chromatids are normally separated into two distinct chromosomes and pulled to opposite poles of the cell. When the centromere splits incorrectly, it results in two identical arms, yielding an isochromosome. This is a type of chromosomal aberration where chromatids fail to separate properly, either during anaphase I when homologous chromosomes should separate, or anaphase II when sister chromatids should divide.