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.

Fig. 5 - Non-disjunction in meiosis 1 during spermatogenesis

Syndrome de Klinefelter
(phénotypiquement masculin)
Syndrome de Turner
(phénotypiquement féminin)
Méiose I
Méiose II

Fig. 5

Origin of trisomy/monosomy of the sex chromosomes due to a non-disjunction in meiosis 1 (orange). Germ cells are formed thereby that lead, at the time of fertilization, to a number of chromosomes other than 46.

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Sex chromosome-
aberrations are relatively frequent. The two most frequent gonosomal aberrations of the numerical kind are: Klinefelter's syndrome and Turner's syndrome.

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.

Fig. 6 - Origin of trisomy/monosomy in sex chromosomes
Non-disjunction in meiosis step 2

Triple X syndrome (female phenotype)
Turner syndrome (female phenotype)
XYY syndrome
Meiosis step 1
Meiosis step 2

Fig. 6

If a disorder occurs in the meiosis 2 disjunction (orange), germ cells will be formed that again have too few or too many chromosomes.

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.

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Down syndrome: karyogram and some clinical data.

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 age of the mother. 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 (compare robertsonian translocation.

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).

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In trisomy 13 the following symptoms are present:

  • Microcephaly with receding forehead, holoprosencephaly, aplasia of the corpus callosum
  • Microphthalmus and coloboma
  • Cleft palate, lip and jaw
  • Polydactyly
  • Cardiac and renal abnormalities
  • 80-90% lethality in the first year of life

In trisomy 18 the following symptoms are present: