Final answer:
A mutation resulting in the repetition of a chromosome segment is known as a duplication, which can lead to multiple gene copies and potentially significant developmental effects.
Step-by-step explanation:
A mutation in which a segment of a chromosome is repeated is called a duplication. Such chromosomal mutations involve changes at the chromosome level and can lead to multiple copies of genes within the chromosome. This is different from a deletion, where a segment of DNA is lost from the chromosome.
Duplications can affect an organism in numerous ways, potentially leading to developmental issues and disorders, such as is seen in trisomy 21 or Down Syndrome. Chromosomal alterations, including duplications, are significant because they can lead to aneuploidy, an error in chromosome number.