Susie Aged 65
Typically people with the Huntington's Disease (HD) gene get symptoms from late 30s to mid-50s, although any age of onset is possible from childhood to old age. The size of the CAG repeat count does not reliably predict the age of onset in most cases.
The phenomenon of anticipation occurs when the size of the CAG repeat expansion increases at conception, that is when egg meets sperm. The reason why it happens is not understood. Most often anticipation occurs when the gene positive parent is the father rather than the mother, but vary rarely anticipation occurs when the mother is gene positive.
Many men with the gene for HD have children without the CAG count increasing.
Anticipation can result in either a boy or a girl child having a bigger CAG repeat length than the gene positive parent . The important thing is the sex of the parent rather than that of the child.
When the size of the CAG repeat length does increase then on average the child is likely to develop symptoms of HD earlier than the parent. However, this is not invariable as the CAG length doesn't reliably predict age of onset of symptoms in most cases
Dr Andrew