Download Inheritance and the muscular dystrophies

Inheritance and the muscular
dystrophies
This leaflet provides a brief summary of the genetics of the muscular dystrophies. An
understanding of their inheritance patterns makes it possible for families to know what the
risk is of the dystrophy happening again.
If, after reading this, you feel that any member of your family may be an unrecognised
carrier of muscular dystrophy, or may have the condition themselves, do get expert advice
from a genetic centre. New genetic tests can help trace, within families, those members
with the abnormal gene for many muscular dystrophies.
How are the muscular dystrophies inherited?
The way in which our bodies grow and work is controlled by structures called genes.
Genes are parts of chromosomes, which are strands of biochemical material transmitted
from every cell to its offspring. We have two genes for every hereditary characteristic and
function, one inherited from each parent. Sometimes a gene is faulty in part of its structure
with the result that a certain bodily process is impaired and this fault may then be passed
on to the children.
There are three main types of inheritance, and when a gene is faulty the fault is inherited
in one of these ways with differing result.
1) X-linked or sex-linked recessive
Including:
Duchenne muscular dystrophy
Becker muscular dystrophy
Scapuloperoneal muscular dystrophy
These conditions are determined by genes which are carried on one of the chromosomes
which control the sex of a child. The result is that only boys are affected and they inherit
the disease from their mothers who are known as carriers. Carriers usually show no sign
of the disease (though sometimes trivial or even unrecognised symptoms are present) but
they are capable of passing the condition on to their own sons. Each son of a carrier has a
50% chance of having muscular dystrophy and each daughter has a 50% chance of being
a carrier (see figure 1):
This factsheet is under review, due for updating later in 2017. If you have any queries, please contact us.
Version 2 / Date published: 03/01 / Original author: Muscular Dystrophy UK / Updated: 04/04 / Updated by: / Date of review: 2016
Registered Charity No. 205395 and Registered Scottish Charity No. SC039445
Page 1 of 7
This factsheet is under review, due for updating later in 2017. If you have any queries, please contact us.
Version 2 / Date published: 03/01 / Original author: Muscular Dystrophy UK / Updated: 04/04 / Updated by: / Date of review: 2016
Registered Charity No. 205395 and Registered Scottish Charity No. SC039445
Page 2 of 7
Unaffected boys cannot transmit the disease: however, the daughters of a man with any Xlinked dystrophy are all carriers (see figure 2):
This factsheet is under review, due for updating later in 2017. If you have any queries, please contact us.
Version 2 / Date published: 03/01 / Original author: Muscular Dystrophy UK / Updated: 04/04 / Updated by: / Date of review: 2016
Registered Charity No. 205395 and Registered Scottish Charity No. SC039445
Page 3 of 7
Genetic mutation
This is an unpredictable change in the structure of a gene causing a different characteristic
to appear for the first time in a family. In about a third of those who have Duchenne
muscular dystrophy, it is due to genetic mutation.
2) Autosomal recessive
Including:
Most 'Limb girdle' types
Scapulohumeral muscular dystrophy
Recessive childhood muscular dystrophy
Congenital muscular dystrophy
and most types of Spinal muscular atrophy.
These conditions become apparent only if both parents carry a faulty gene but the parents
themselves do not manifest any symptoms. Each child of such parents has a 25% chance
of carrying both abnormal genes and therefore of being affected. Either sex can have the
condition (see figure 3):
This factsheet is under review, due for updating later in 2017. If you have any queries, please contact us.
Version 2 / Date published: 03/01 / Original author: Muscular Dystrophy UK / Updated: 04/04 / Updated by: / Date of review: 2016
Registered Charity No. 205395 and Registered Scottish Charity No. SC039445
Page 4 of 7
The children of an affected individual are usually unaffected but cousin marriages between
affected or unaffected members of such families greatly increase the risk that they will
have affected children. Limb girdle dystrophy is a term that is becoming out-dated because
it is thought to include several different forms of muscular dystrophy inherited in different
ways. Some people who have been diagnosed as having limb girdle muscular dystrophy in
the past have turned out later to have entirely different non-genetic disorders.
Furthermore many cases of the Becker type have been called Limb girdle muscular
dystrophy in the past and occasionally women with 'limb girdle muscular dystrophy' turn
out really to be carriers of the Duchenne gene. If concerned about the inheritance of their
limb girdle muscular dystrophy, it is important people are reassessed to make sure of the
diagnosis, this would be done as part of a referral for genetic counselling.
3) Autosomal dominant
Including:
Facioscapulohumeral muscular dystrophy
Myotonic dystrophy
Some oculopharyngeal/ocular muscular dystrophy
and some limb girdle dystrophies
This factsheet is under review, due for updating later in 2017. If you have any queries, please contact us.
Version 2 / Date published: 03/01 / Original author: Muscular Dystrophy UK / Updated: 04/04 / Updated by: / Date of review: 2016
Registered Charity No. 205395 and Registered Scottish Charity No. SC039445
Page 5 of 7
Here the condition becomes apparent even though the affected person has only one
abnormal gene. Either sex can have the condition and each child of an affected parent has
a 50% chance of being affected (see figure 4):
However, the severity of the condition may vary considerably in different individuals.
Some people are so mildly affected that they may not have recognised that they have the
condition but they may then have a more severely affected child. So examination of all the
family members is important as part of genetic counselling.
Can muscular dystrophy be identified in the unborn child tested for
in pregnancy?
Research on gene probes has made the diagnosis of some forms of muscular dystrophy
possible in early pregnancy. Although these tests of the unborn child can be done from the
tenth week of pregnancy onwards, investigations of some family members may be
required before the prenatal tests are done. (Please see the leaflet in this series entitled
Carrier detection and prenatal diagnosis of inherited dystrophies).
This factsheet is under review, due for updating later in 2017. If you have any queries, please contact us.
Version 2 / Date published: 03/01 / Original author: Muscular Dystrophy UK / Updated: 04/04 / Updated by: / Date of review: 2016
Registered Charity No. 205395 and Registered Scottish Charity No. SC039445
Page 6 of 7
Where can advice be obtained?
The various tests can be carried out and genetic advice given at a number of genetic
centres throughout the country. Your family doctor can make the appropriate referral for
you and any of your relatives who are worried. You can also contact the Information
Officers at the Muscular Dystrophy Campaign headquarters for a list of such centres.
Disclaimer
Whilst every reasonable effort is made to ensure that the information in this document is complete, correct
and up-to-date, this cannot be guaranteed and Muscular Dystrophy UK shall not be liable whatsoever for any
damages incurred as a result of its use. Muscular Dystrophy UK does not necessarily endorse the services
provided by the organisations listed in our factsheets.
If you have feedback about this factsheet please email [email protected].
Here for you
The friendly staff in the care and support team at the Muscular Dystrophy UK’s London
office are available on 0800 652 6352 or [email protected] from 8.30am to
6pm Monday to Friday to offer free information and emotional support.
If they can’t help you, they are more than happy to signpost you to specialist services
close to you, or to other people who can help.
www.musculardystrophyuk.org
This factsheet is under review, due for updating later in 2017. If you have any queries, please contact us.
Version 2 / Date published: 03/01 / Original author: Muscular Dystrophy UK / Updated: 04/04 / Updated by: / Date of review: 2016
Registered Charity No. 205395 and Registered Scottish Charity No. SC039445
Page 7 of 7