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Trisomy is the most common chromosome abnormality in humans and occurs in approximately 4% of all clinically recognised pregnancies.1 This numerical chromosome abnormality may lead to spontaneous abortions, polymalformation syndromes, mental retardation, and impairment of gametogenesis. Although the great majority of trisomies are the result of segregation errors during paternal or maternal meiosis, some trisomies are the result of postzygotic mitotic error.2
Parental age has been known to be the most important aetiological factor implicated in human trisomy formation. Advanced maternal age is a predisposing factor for most autosomal trisomies but maternal age effect shows considerable variation among chromosomes.3 Data on human trisomy from spontaneous miscarriages and offspring suggested that non-disjunctions at maternal meiosis I was the most common cause of trisomies for acrocentric chromosomes. Two possible mechanisms may be responsible for non-disjunction of acrocentric chromosomes: premature division of sister chromatids and non-disjunction of bivalent chromosomes. The second mechanism increases with maternal age.4,5 The majority of trisomy 18 are the result of an abnormal distribution of this chromosome during the second maternal meiotic division.6 Maternal age has variable effects on maternally derived constitutional aneuploidy for different human chromosomes. The maternal age effect is very strong for chromosome 21 and demonstrable for chromosomes 15, 16, and 21.3,7
The paternal contribution to aneuploidy is low and the possible relationship between paternal age and trisomy has been the subject of controversy. Most of the studies evaluating this parameter concerned aneuploidy discovered after miscarriages, prenatal diagnosis, or at birth. However, cases with proven paternal origin have been rarely reported.3,7 Using molecular analysis of trisomic patients, 10-30% of autosomal trisomies originate at paternal second meiotic division and chromosomes 13, 14, 15, 21, and 22 have been implicated.4 In trisomy 21, the extra chromosome 21 …