What defines a ring chromosome?

A ring chromosome is characterized by a single chromosome that has formed a ring structure due to the loss of genetic material from both ends, followed by the ends of the chromosome fusing together. This process often results in a chromosome that is structurally altered, which can lead to various genetic disorders depending on the genes located in the deleted regions.

In contrast, the other options describe different genetic scenarios that do not accurately represent the definition of a ring chromosome. For instance, a fusion of two different chromosomes refers to a type of structural rearrangement but does not involve a single chromosome forming a ring. A chromosome with a centromere at each end does not fit the description of a ring chromosome either, as ring chromosomes typically have one functional centromere. Lastly, an extra small chromosome describes a situation where an additional chromosome is present rather than the specific structural characteristic of a ring chromosome.