Aneuploidy refers to a disruption in the normal number of chromosomes in an organism. Many serious birth defects and congenital conditions are related to aneuploidy, since chromosomes are the repository for DNA, a crucial component of life. In many cases, a fetus with aneuploidy will miscarry before it is fully developed, and in others the body may have difficulty surviving after birth. In other cases, aneuploidy represents a disability, but the body can live a happy and healthy life despite the challenge.
Humans generally include aneuploidy tests in their routine prenatal tests. If an aneuploidy is identified, parents are warned about the problems their children may face. In some cases, parents choose to terminate the pregnancy if the aneuploidy is associated with serious and disabling birth defects that would make the baby's life short and unpleasant.
The causes of aneuploidy are not fully understood, but the mechanics are. Aneuploidy is caused by an error in meiosis, the division of cells to create gametes. A gamete is a cell that normally has a haploid set of chromosomes, meaning that it contains 50% of the genetic information of the original organism. When a gamete unites with a gamete from another organism, it creates a complete set of chromosomes and becomes a completely new organism.
The first step in meiosis involves the duplication of a cell into two complete copies. Each copy is split in two, creating four cells from the original cell. When the chromosomes in a cell do not divide completely, the daughter cell may end up with an extra chromosome within a haploid set. When the gamete unites with another gamete, the resulting condition is trisomy, which means that one of the pairs of chromosomes has become a triad.
If, on the other hand, one chromosome is deleted or damaged, the condition is known as monosomy. Monosomy refers to the fact that one of the chromosomes in a diploid set is unpaired. In humans, this would mean that the baby only has 45 chromosomes, instead of the normal 46, or that one of the chromosomes in a pair is severely damaged.
These chromosomal disorders can be easily recognized during prenatal testing, as the chromosomes can be easily stained and counted. Genetic conditions are linked to specific chromosomes; Trisomy 21, for example, refers to an extra copy of chromosome 21, resulting in Down syndrome.