Final answer:
A Robertsonian translocation is a chromosome abnormality that combines most of two acrocentric chromosomes into a long chromosome. This is different from pericentric and paracentric inversions, or reciprocal translocations, each of which affects chromosomal structure in unique ways.
Step-by-step explanation:
The type of chromosome abnormality that yields a long chromosome consisting of most of two acrocentric chromosomes is a Robertsonian translocation. This occurs when the long arms of two acrocentric chromosomes fuse at the centromere and the short arms are lost. The resulting translocated chromosome carries the genetic material from two chromosomes, which can lead to clinical consequences such as genetic disorders or can be asymptomatic, depending on the genes involved and whether the person has a balanced or unbalanced form of the translocation.
In contrast to Robertsonian translocation, a pericentric inversion includes the centromere and can change the relative lengths of the chromosome arms, making it easily identifiable, while a paracentric inversion occurs outside of the centromere and does not. A reciprocal translocation involves two nonhomologous chromosomes where segments interchange without loss or duplication of genetic material.