Spinocerebellar ataxia type 3/Machado-Joseph disease (SCA3/MJD) is a dominant 
neurodegenerative disease caused by the expansion of a CAG repeat tract in 
ATXN3. Anticipation and worsening of clinical picture in subsequent generations 
were repeatedly reported, but there is no indication that SCA3/MJD frequency is 
changing. Thus, we performed a systematic review and meta-analysis on phenomena 
with potential effect on SCA3/MJD recurrency in populations: instability of CAG 
repeat transmissions, anticipation, fitness, and segregation of alleles. 
Transmission of the mutant allele was associated with an increase of 1.23 CAG 
repeats in the next generation, and the average change in age at onset showed an 
anticipation of 7.75 years per generation; but biased recruitments cannot be 
ruled out. Affected SCA3/MJD individuals had 45% more children than related 
controls. Transmissions from SCA3/MJD carriers showed that the expanded allele 
was segregated in 64% of their children. In contrast, transmissions from normal 
subjects showed that the minor allele was segregated in 54%. The present 
meta-analysis concluded that there is a segregation distortion favoring the 
expanded allele, among children of carriers. Therefore, further studies on 
transmissions and anticipation phenomena as well as more observations about 
fertility are required to clarify these selective forces over SCA3/MJD.