Duplications typically arise from an event termed unequal crossing-over (recombination) that occurs between misaligned homologous chromosomes during meiosis (germ cell formation). The chance of this event happening is a function of the degree of sharing of repetitive elements between two chromosomes.

What does it mean to have a duplicated chromosome?

What are duplications? The term "duplication" simply means that a part of a chromosome is duplicated, or present in 2 copies. This results in having extra genetic material, even though the total number of chromosomes is usually normal.

When does chromosome replication occur?

At the heart of the eukaryotic cell-division cycle are the precise duplication and segregation of chromosomal DNA. These two events take place at distinct points of the cell cycle: DNA is replicated during S phase, and then the replicated DNA strands are accurately partitioned to the two daughter cells during M phase.