Name: Jacob
Age: 16
Question: Do fraternal/identical twins still have a 50/50 chance of testing positive for HD - or is it a case of if one has it, the other definitely will too?
If twins are TRULY identical, then if one is gene positive the other will have the gene also.
If twins are non-identical, then each twin is at an independent 50/50 risk of inheriting the gene. That is, one could have a different gene test result than the other. For example, one might be gene positive & the other gene negative. It might also be the case that both might be gene positive but have different sized CAG repeat expansions. Both might of course be gene negative.
In terms of predictive testing, if one member of an identical twin pair tested positive for HD then I would recommend that the other considered getting counselling to help them work out what the result might mean to him or her.
There has been one case, of Australian identical twins, who presented differently. That is, identical twins cannot assume that HD will behave in exactly the same way in both twins. Also, the psychological impact of being gene positive might be different in each twin. These subtleties are why I'd recommend that the second identical twin might see a counsellor
Andrew Churchyard




