I pulled this from another thread....
"the 50-50 thing is based on the assumption that only one parent carries the gene on only one chromosome. There will however be cases where both parents carry the gene or one parent carries two copies of the gene. If both parents are hd positive then 75% of their children will on average be positive and 25% of their children will carry two copies of the hd gene. 100% of the children of a parent with two copies will be hd positive. "

I have never heard of someone having "two copies" of the hd gene before. When someone is tested, does this show on the test results? If not, how can someone find out if they have two copies of the gene. I have concerns. If my husband and I ever decide to do PDG IVF, this would mean that no matter what, there would be no egg that would be HD free.

I hope that makes sense. HELP!!!!

I think you could only get 2 copies of the HD gene if both of your parents had HD.

Where exactly did you pull this info from? I have always been under the understanding that everyone has an hd gene--
if a parent has a mutated hd gene then there is a 50/50 of passing hd on to thier child -
I would like to read more about what you found and read

thanks

DCB
*Living for today*

Everyone has two copies of the huntingtin's protein gene. When you get your test results they will tell you the number of CAG repeats on both genes. Most people in the U.S. who have HD have just one copy of the HD gene along with a normal gene. In the Lake Maracaibo area of Venezuela where there are a lot of people with HD, it is much more common for there to be two copies of the HD gene, one inherited from the father and one from the mother.

When you got your results, the CAG count on the HD gene was probably given with the statement that you were positive for the HD gene. The doctor would have seen the normal allele number on the lab report and not bothered to mention it since it wasn't significant. Just to be sure, you can call and ask what it was.



Edited 1 time(s). Last edit at 08/11/2008 08:11PM by Marsha.

Most people in the US who inherit the mutated HD gene inherit it from one parent - thus the "probability" of inheriting the gene will be 50%

In areas where HD is much more prevalant (say in Lake Maracaibo, where Wexler did her studies) - it's entirely possible that more than one parent carried the gene.

When that's the case, one of two scenarios would exist.

If one of their two parents carried the "heterozygous" version of the gene - i.e. inherited it from only one of THIER parents - then the probability of their passing on the affected gene would be 75%


If both of the parents had the "homozygous" version of the gene i.e. they inherited bad copies of the gene from BOTH parents - then the probability of passing it on would be 100%.

But again, this would just be a matematical probability, based on the basic laws of autosomal dominant inheritance. The reality could vary.

One exception to that, I believe would be the offspring of two homozygous parents - I think that person would irrevocably have a 100% chance of inheritance.

Barring some chance gene-mutation..........



Edited 2 time(s). Last edit at 08/11/2008 08:25PM by JL.

I think what the poster was asking is that if both parents were PHD, with one "bad" gene each, what chance does the child have of not having HD. My math says that if 2 parents, each with one "bad" HD gene conceive, the child has a 75% chance of having HD (one or 2 HD "bad" genes) and 25% chance of not having HD it all.

I'd think that would be correct!

75% of having HD.

Matt, it depends on how you define "bad gene".


Most of us - if we've inherited the defective gene at all have inherited only one "bad" copy (heterozygous). Since it only takes one "bad" copy of the gene, they will most likely develop symptoms in their lifetime. They will in turn (like THEIR affected parent) - have a 50% chance of passing the gene onto THEIR children.

When we're talking about BOTH parents being gene-positive for HD - then it gets down to how many defective copies of the gene THEY have. That can skew the odds, considerably!

Matt also said: "I think what the poster was asking is that if both parents were PHD, with one "bad" gene each, what chance does the child have of not having HD. My math says that if 2 parents, each with one "bad" HD gene conceive, the child has a 75% chance of having HD (one or 2 HD "bad" genes) and 25% chance of not having HD i"

Matt, your math would technically be correct, were it not for one factor.

You are equating "both parents being HD positive " with both parents having "one 'bad' gene each". This is simply not a given (since both parents have TWO copies of the huntingtin gene).

It is quite possible for one (or both) of the parents to have two ABNORMAL copies of the gene!



Edited 1 time(s). Last edit at 08/12/2008 03:58PM by JL.

This is why Nancy Wexler and associates were able to study HD so much more quickly and successfully in Lake Maracaibo, Venezuela.

The combination of the presence of an autosomal-dominant disease - and a very limited population (both in terms of numbers and access to the outside world) - had exactly the effect one might expect.

An adequate-but-manageable number of subjects - with an EXTRAORDINARILY HIGH instance of HD!



Edited 2 time(s). Last edit at 08/12/2008 04:20PM by JL.

Also they could see the affect on disease progression where both huntingtin protein genes are HD+. Interestingly, with two HD+ genes, the disease developed on about the same late-onset schedule as with one HD+ gene. Progression after onset did show some acceleration however.

dcb - I got this information from the post title "50-50" that someone added to again.

Marsha - Thanks. You answered my question. I called today and found out that my husband's count is 43 & 17. In Canada, they don't normally give you the results. We just find out if the gene is present or not. They don't want anyone to know in fear of what they may do. We had found out previously through another doctor, but I told them when I wanted to know and the lady told me. I was quite surprised actually. Thanks again as usual for all your information!

Sally, are you serious? I'm in canada, and i was told my exact results, both genes, 39/19, what it meant, and was given my copy of the results. You are entitled to that, but i am glad you have now at least been able to find out the numbers, that helps a lot.

That is what they told us. However, one of his specialists told us later so we knew the 43 count.