Spinocerebellar ataxia type 1 and type 3 (SCA1, SCA3) are autosomal dominant neurodegenerative disorders caused by expanded CAG trinucleotide repeats in novel genes. In our collective of SCA1 and SCA3 families, we observed distortion of the Mendelian 1:1 segregation of the disease. The mutated alleles were preferentially transmitted by female carriers in SCA3, whereas a gender effect on clinical features such as age of onset was not obvious. The mechanism underlying segregation distortion remains to be established.