Re: How exactly is myotonic dystrophy inherited?

Hi!

Mutations in the DMPK and ZNF9 genes cause myotonic dystrophy.

The two forms of myotonic dystrophy, type 1 and type 2, have similar 
clinical features but are caused by mutations in different genes (DMPK 
and ZNF9). The mutation that causes myotonic dystrophy is a DNA segment 
that is abnormally repeated within the DMPK or ZNF9 gene. The repeats 
form an unstable region in the gene. Researchers believe that the large 
number of repeats disrupts cells' ability to make the DMPK and ZNF9 
proteins and the proteins of other genes. This disruption prevents cells 
from functioning normally, leading to the signs and symptoms of myotonic 
dystrophy.

Myotonic dystrophy is inherited in an autosomal dominant pattern, which 
means one copy of the altered gene is sufficient to cause the disorder. 
In most cases, an affected person has one affected parent.

As type 1 myotonic dystrophy is passed from one generation to the next, 
the disorder generally begins earlier in life and signs and symptoms 
become more severe. This phenomenon is called anticipation. In type 1 
myotonic dystrophy, anticipation is caused by an increase in the length 
of the unstable region in the DMPK gene. In type 2 myotonic dystrophy, a 
longer unstable region in the ZNF9 gene does not appear to influence the 
age of onset of the disorder.

Since you don't have the disease, it means that you don't have the 
affected gene(s) (since its a dominant gene and if you had it, you sure 
would have had the disorder). Hence, your children is not likely to have 
the disease. Please consider the following diagram (taken from  http://ghr.nlm.nih.gov/info=inheritance/show/inheritance_patterns;
jsession
id=21DBB877B61419F63F1891CAD352FAE5):



A dominant parent provides 50% chances for his/her child to have the 
disorder. One of your parents is dominant, so your brother fell on the 
wrong side of the 50% and you did not. So when you have children, there 
won't be this 50% probability.


As for your brother, you may use the following links:
 http:/
/www.ninds.nih.gov/health_and_medical/disorders/md.htm
 http://ww
w.nlm.nih.gov/medlineplus/musculardystrophy.html
 http://www.ncbi.nlm.nih.
gov/books/bv.fcgi?
call=bv.View..ShowSection&rid=gnd.section.164
 http://ghr.nlm.nih.gov/condition=myotonicdystrophy/show/Educationa
l+resour
ces;jsessionid=21DBB877B61419F63F1891CAD352FAE5
 http://ghr.nlm.nih.gov/condition=myotonicdystrophy/show/Patient+su
pport;js
essionid=21DBB877B61419F63F1891CAD352FAE5

To find a genetic professional in your area, use this link:  http
://rarediseases.info.nih.gov/html/resources/info_cntr.html

Well, that's all. If you need any more help, just drop me an email to 
adeydas4@yahoo.com.

Regards,
Abhishek Dey Das.