haplotype in each family the founder dup15q24 25 haplotypes were easily identified among the other haplotypes with the duplication carried by new members of the family this high number of haplotypes 42 of the chromosomes with the duplication have different haplotypes indicates independent origins and suggests that this mutation is easily generated in at least 7 couples of these families both members carried the duplication this represents 44 of the couples in which both members were analysed for the duplication and is a clear evidence of assortative matting for the phenotype s associated with the 15q24 25 mutation there were several examples of abnormal segregation of the 15q24 25 duplication in the families indicating the generation of a de novo case the conversions from duplicated to non duplicated chromosomes and the conversion of one form of the duplication to the other mosaicism for the 15q24 25 duplication was first observed by fish in lymphocyte metaphase chromosomes when hybridised with probes t216 1 and c251 3 mosaicism for dup15q24 25 was scored in 100 interphase nuclei of each individual detecting a mediane of 55 duplicated cells 40 4 70 6 the hybridisation efficiency of the probe in interphase nuclei in non duplicated control subjects was 85 we have studied the 15q24 25 duplication in sperm from a non duplicated control subject and in a patient homozygous for the duplication 15 homozygous for the duplication and 85 heterozygous as.