Aneuploidy refers to a disruption in an organism's normal number of chromosomes. Many serious birth defects and congenital conditions are linked to aneuploidy, as the chromosomes are the repository of DNA, a crucial component of life. In many cases, a fetus with aneuploidy will be spontaneously aborted before it is fully developed, and in others the organism may have a difficult time surviving after birth. In other instances, aneuploidy represents a handicap, but the organism is able to live a happy and healthy life despite the challenge.
Humans typically include testing for aneuploidy in their routine pre-natal testing. If an aneuploidy is identified, the parents are forewarned about the issues which may face their child. In some cases, parents choose to terminate the pregnancy if the aneuploidy is associated with severe, crippling birth defects which 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 which normally has a haploid set of chromosomes, meaning that it contains 50% of the genetic information of the parent organism. When a gamete joins with a gamete from another organism, a complete set of chromosomes is created, and it develops into an entirely new organism.
The first step in meiosis involves the duplication of a cell into two complete copies. Each copy then splits in two, creating four cells from the original parent cell. When the chromosomes in a cell fail to completely divide, the daughter cell may end up with an extra chromosome within an otherwise haploid set. When the gamete joins with another gamete, the resulting condition is trisomy, meaning that one of the chromosome pairs has turned into a triad.
If, on the other hand, a chromosome becomes deleted or damaged, the condition is known as monosomy. Monosomy refers to the fact that one of the chromosomes in an otherwise 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 badly damaged.
These chromosome disorders can be readily recognized during prenatal testing, since chromosomes can be readily dyed and counted. Genetic conditions are linked to specific chromosomes; trisomy 21, for example, refers to an extra copy of the 21st chromosome, resulting in Down's Syndrome.