Using FISH, different percentages of disomy for the different chromosomes have been observed. The median value for all chromosomes reported is about 0.3% [29]. The large variations observed between studies can be at­tributed to technical factors such as decondensation and denaturation of the DNA in the specimen and distortion of the morphology after pretreatment. If 0.3% is truly repre­sentative for the disomy rate among all the 23 chromo­somes in sperm, an overall rate of 7% would be found using the FISH technique. This figure agrees with that found using the zona free hamster technique [30,31]. Be­sides technical factors to explain a proportion of the varia­tion observed, there may also be differences between pa­tients. Recent studies have shown that the percentage of numerical abnormalities is significantly increased in pa­tients with abnormal semen characteristics [32,33]. When FISH was performed on sperm samples from 20 couples who had experienced three or more recurrent first trimes­ter abortions, a high rate of sperm disomy (14.5-15.5%) was scored in two cases [34].

A higher incidence of numerical chromosome abnor­malities in sperm has also been detected in intracyto­plasmatic sperm injection (ICSI) zygotes [35]. Finally, sperm of non obstructive azoospermic men had a higher incidence of chromosome abnormalities, which was pre­dominantly in the sex chromosomes [36].