SEGREGATION DISTORTION IN MYOTONIC-DYSTROPHY

Myotonic dystrophy (DM) is an autosomal dominant disease which, in the typical pedigree, shows a three generation anticipation cascade. This results in infertility and congenital myotonic dystrophy (CDM) with t he disappearance of DM in that pedigree. The concept of segregation di stortion, where there is preferential transmission of the larger allel e at the DM locus, has been put forward to explain partially the maint enance of DIM in the population. In a survey of DM in Northern Ireland , 59 pedigrees were ascertained. Sibships where the status of all the members had been identified were examined to determine the transmissio n of the DM expansion from affected parents to their offspring. Where the transmitting parent was male, 58.3% of the offspring were affected , and in the case of a female transmitting parent, 68.7% were affected . Studies on meiotic drive in DIM have shown increased transmission of the larger allele at the DM locus in non-DIM heterozygotes for CTGn. This study provides further evidence that the DM expansion tends to be transmitted preferentially.