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How is Myotonic Dystrophy inherited?

• Men and women are equally likely to pass on Myotonic Dystrophy to their children.
• Both sexes are equally likely to inherit the disease.
• An affected woman is much more likely to have a severely affected child than an affected man.
• On average there is a 50% chance that the child of an affected individual will inherit Myotonic Dystrophy.
• Normally the symptoms of the disease become more severe and the age at onset becomes earlier in successive generations of a family.

Myotonic Dystrophy is a genetic disease and so can be inherited by the child of an affected parent if they receive the mutation in the DNA from the parent.

The disease can be passed on and inherited equally by both sexes.

Congenital Myotonic Dystrophy, the most severe form of the disease is almost exclusively passed on by maternal transmission. The disease is an Autosomal dominant disorder meaning that inheritance is not affected by sex of the parent or child and that only one copy of the mutation is required to cause the disease.

Every cell in the human body has two copies of each chromosome. Myotonic Dystrophy is caused by a mutation on chromosome 19 so an affected individual will have one normal copy of chromosome 19 and one that carries the mutation. When the sex cells are produced (either sperm in males or eggs in females) they will only have one copy of each chromosome. One copy of each chromosome is passed to the child by each parent.

There are four possible combinations of the parental chromosomes. As can be seen there is a 50% chance that the chromosome carrying the mutation will be inherited. Therefore when one parent has Myotonic Dystrophy there is a 50% chance that the child will inherit the disease.

If the father was affected the combination of chromosomes would be the same and the chance of a child being affected would still be 50%.