The GAA repeats in the FGF14 gene are unstable when they are passed from parent to child, which gives SCA27B a more complicated inheritance pattern. 

The instability of the GAA repeats means that the number of repeats EXPANDS (increases) or CONTRACTS (decreases) as the gene is passed down from parent to child. Whether the repeats are more likely to expand or contract depends on which parent the repeat comes from. GAA repeats tend to expand when passed from a female parent to a child. GAA repeats tend to contract when passed from a male parent to a child.4,5  Currently, there is not enough research to predict how likely a GAA repeat is to expand or contract based on the size of the parent's repeat. 

There may be a relationship between the repeat size and age of onset with a larger number of repeats leading to an earlier age of onset.4  So, the affected children of a mother with SCA27B may start having symptoms at an earlier age than their mother did. On the contrary, the affected children of a father with SCA27B may start having symptoms at a similar or later age than their father did.

The chart and image carousels below summarize the possible outcomes when a copy of the FGF14 gene is passed from a parent to a child based on the sex and repeat size of the parent. Click the ">" and "<" buttons to scroll through the image carousels.