Geneticists are so interested in aneuploidy because its relationship to certain inherited disorders in humans. Even though most people are born with a normal number of chromosomes, alterations in chromosome number occur fairly frequently during gamete formation. About 5-10% of all fertilized human eggs result in an embryo with an abnormality in chromosome number. In most cases, these abnormal embryos do not develop properly and result in a spontaneous abortion very early pregnancy. Approximately 50% of all spontaneous abortions are due to alterations in chromosome number.

In some cases, an abnormality in chromosome number produces an offspring that can survive. Several human disorders involve abnormalities in chromosome number. The most common are trisomies of chromosomes 21, 18 or 13, or abnormalities in the number of the sex chromosomes. Most of the known trisomies involve chromosomes that are relatively small. Trisomies of the other human autosomes and monosomies of the autosomes usually produce a lethal phenotype that causes early spontaneous abortion.

Some human abnormalities in chromosome number are influenced by the age of the parents. Older parents are more likely to produce children with abnormalities in chromosome number. For example, the incidence of Down Syndrome rises with the age of the parents, particularly that of the mother. This syndrome was first described by the English physician John Landdon Down in 1866. This association between maternal age and Down Syndrome was later discovered by L.S. Penrose in 1933, even before the chromosomal basis for the disorder was identified by the French scientist Jerome Lejeune in 1959. Down Syndrome is most commonly caused by nondisjunction at meiosis I in the oocyte.

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