Origin of the deviating number of chromosomes
The loss of a whole chromosome leads to a monosomy and the presence of an additional chromosome to a trisomy. Both of these forms have a number of chromosomes other than 46, a condition that is also termed aneuploidy (Greek: "not good duplication"). Responsible for the occurrence of mono-trisomy is a non-disjunction in the gametogenetic meiotic process.
During spermato/oogenesis gametes with additional but also with missing chromosomes can be formed. Since meiosis takes place in two steps, the disorders can occur in the disjunctions of the first or of the second meiotic divisions.
In Figs. 5 and 6 disorders of the maturation divisions of the germ cells are shown. Thereby monosomies and trisomies can form. Trisomies of the sex chromosomes (XXX, XXY or XYY) occur relatively frequently and can be lived with, whereas monosomies are generally lethal, those with Turner's syndrome forming the single exception.
Among the trisomies of the autosomes, trisomy 21 (Down syndrome) is the most well known. The occurrence of trisomy 21, as well as many other chromosomal aberrations caused by non-disjunctions, is highly correlated with the compare robertsonian translocation.The probability of bearing a child with trisomy 21 increases exponentially with age. If a young mother apparently gives birth to a child bearing a trisomy 21, however, a differential diagnosis of an inherited chromosomal aberration must by all means be considered (
The two further, most frequent types of trisomy after trisomy 21 encountered in living newborns are trisomy 13 (Patau's syndrome) and trisomy 18 (Edwards's syndrome).