Nondisjunction

In order to understand why some aneuploid conditions are fatal and others (such as those mentioned above) are not, one must understand the concept of gene dosage and its importance in development. A normal human possesses twenty-two pairs of autosomes and two sex chromosomes (XY in the case of males and XX in the case of females). Such an individual develops normally because there is a situation of genetic balance: Each gene is present in the correct amount (or dose), such that its contribution towards development is appropriate and ideal. However, if a chromosome is either removed from or added to the normal set, a situation of imbalance is immediately established: The contribution (or gene dosage) of each gene contained within that chromosome is altered and as a result development is compromised. While the duplication or silencing of an individual gene is not usually fatal, the wholesale addition or loss of a chromosome, which contains a thousand or more genes, almost always is.

It is obvious from this reasoning that a small change is more likely to be tolerated (albeit at a cost) than a large one. Down syndrome is caused by trisomy of chromosome 21, which is one of the smallest human chromosomes (containing a relatively small number of genes). This provides an explanation as to why this condition is not fatal, while a trisomy involving another, larger autosome would most likely be fatal.

With the sex chromosomes, a lot more flexibility is allowed: Although the X chromosome is very large, only one is used in development (in every femalecell one of the two X chromosomes is inactivated at random). The Y chromosome, on the other hand, contains very few genes and is not necessary for normal female development. It is only required for male development. With a knowledge of these facts it is relatively easy to understand why aneuploidies involving the sex chromosomes tend not to be fatal. Note, however, that the YO condition is fatal due to the lack of the essential X chromosome.