Inheritance and genetics

Inheritance and genetics d.doran@muscul… Wed, 05/19/2021 - 16:12

We’d recommend reading these pages together with our pages relating to your muscle-wasting condition.

This factsheet provides a brief summary of genetics and typical inheritance patterns.

Understanding these patterns within a family tells us about the likelihood of another family member being affected or not.

This information is particularly important in family planning. If you have any concerns or questions about inheritance and genetic testing, do discuss these with your GP who can refer you to your local genetics counsellor.

DNA, chromosomes and genes

Deoxyribonucleic acid (DNA) contains instructions for the functioning of living organisms. Inside our cells, DNA is organised into structures called chromosomes. These are found in the cell nucleus.

Imagine your DNA as a recipe book, and your genes as the recipes. Our cells use these [genetic] recipes to make proteins, which are essential for a wide range of biological processes in the body.

Almost all of our cells contain 23 pairs of chromosomes (46 in total). These include one pair of sex chromosomes (XX for females, XY for males) and 22 pairs of non-sex chromosomes (autosomes).

For each pair of chromosomes, we inherit one from our mother and one from our father. The rest come from one parent or the other.

For example, some genes involved in energy production are inherited from the mother only. These genes are known to be associated with mitochondrial diseases.

What is a genetic mutation?

A genetic mutation is a change in the DNA code. This can happen by complete chance, for example if the cell makes a ‘mistake’ during DNA replication.

Sometimes this happens as a result of environmental factors, such as smoking, excessive exposure to sunlight or exposure to radiation.

Our cells have natural mechanisms for recognising mutations and correcting them, but these are not always 100 percent effective.

Depending on where a mutation occurs, and the type of mutation it is, the effect could be harmless. Or it could ‘disrupt’ a gene and result in a genetic condition such as muscular dystrophy.

How are muscle-wasting conditions inherited?

Muscle-wasting conditions have different inheritance patterns, depending on the type of condition and which gene is mutated.

Where can I get advice?

There are a number of specialist genetic centres throughout the country that do genetic tests and offer advice. Your GP can arrange a referral to a clinical geneticist or genetic counsellor at your local genetics centre.

 

 

 

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X-linked recessive inheritance

X-linked recessive inheritance d.doran@muscul… Wed, 05/19/2021 - 16:28

These conditions are caused by a mutation in a gene on the X chromosome, which is one of the sex chromosomes. Males have one X chromosome and one Y chromosome; females have two X chromosomes.

If a male’s X chromosome has the mutated gene, then he will have the condition. He has only one copy of the gene and the mutation isn’t on the Y chromosome.

If a female has the mutated gene on one of her X chromosomes, she still has a healthy copy of the gene on her other X chromosome. This makes her a ‘carrier’ and she will not usually have symptoms of the condition.

If a carrier of an X-linked condition and a non-carrier have children, there is a 50 percent chance that each son will have the condition and a 50 percent chance that each daughter will be a carrier (see first diagram).

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This diagram shows what would be a likely outcome if a carrier mother of an X-linked recessive condition had children with an unaffected father. They have 4 offspring, 1 unaffected daughter, 1 unaffected son. They also have a carrier daughter and an affected son.

Diagram 1

This probability is the same for each child born. So, for instance, if this couple have two sons, each son will have a 50 percent chance of inheriting the condition.

Unaffected males cannot pass on an X-linked condition. Affected males cannot pass an X-linked condition to their sons, but all of their daughters will be carriers (see second diagram)

Image
This diagram shows the likely outcome if a non-carrier mother and an affected father had children. They have 4 children; two carrier daughters and two unaffected sons.

Diagram 2

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Autosomal recessive inheritance

Autosomal recessive inheritance d.doran@muscul… Wed, 05/19/2021 - 17:03

‘Autosomal’ means the mutation is in a gene on the non-sex chromosomes, so children of either sex can inherit the condition. ‘Recessive’ means both copies of that gene have to be mutated for someone to have the condition. If only one copy of the gene is mutated, that person is a carrier and will have no symptoms.

If mother and father are both carriers, (see first diagram), there is a:

  •  25 percent chance that each child will be unaffected
  •  50 percent chance that each child will be a carrier
  •  25 percent chance that each child will have the condition.
Image
This diagram shows what would happen if two carrier parents of an Autosomal Recessive condition had children. Of the 4 children they have; 1 would be unaffected, 1 would be affected, and 2 others would both be carriers.

Diagram 1

Parents with autosomal recessive conditions will pass on one copy of the mutated gene to their children (see second diagram). It is very unlikely their children will be affected (unless the other parent is a blood relative).

Image
This diagram shows what would happen if an affected parent with a recessive condition and a non-carrier parent have children. All 4 of their children will be carriers.

Diagram 2

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Autosomal dominant inheritance

Autosomal dominant inheritance d.doran@muscul… Wed, 05/19/2021 - 17:18

‘Autosomal’ means the mutation is in a gene on the non-sex chromosomes, so either sex can inherit the condition.

‘Dominant’ means the mutation has to occur in only one copy of the gene for someone to inherit the condition.

If one parent has the condition and the other doesn’t (see diagram), there is a:

  •  50 percent chance that each child will have the condition
  •  50 percent chance that each child will be unaffected.
Image
This diagram shows what would happen if an affected parent with an automsomal dominant condition and an unaffected parent had children. Two of the four children would be affected and the other 2 would be unaffected.

Are there ways of not passing the condition on to my children?

There are options available for some couples affected by muscle-wasting conditions to have an unaffected child. You can read about these in our Genetic counselling and family planning pages

We recommend speaking to your local clinical geneticist or genetic counsellor about what options might be available to you.

 

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