Explanation of autosomal recessive inheritance

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    06-Jan-2017

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Passing on genetic information from parents to children

Passing on genetic information from parents to childrenAn explanation of autosomal recessive inheritance

Genetics and genomics for healthcarewww.geneticseducation.nhs.uk 2013 NHS National Genetics and Genomics Education CentreThe many thousands of instructions (which are also known as genes) for making a new human being are inherited from a persons parents. Most medical conditions occur because of the interaction of several genes and environmental influences, but some really important conditions are the result of an alteration in just one or a pair of the thousands of genes. These single gene conditions as they are called are important to understand because they have a high probability of other family members inheriting the same condition.

Here are some PowerPoint teaching slides which demonstrate the transmission of an autosomal recessive condition from parent to child, please feel free to use these within your teaching

Some parts of the animations run automatically; others require a mouse click

Most slides have notes with further information

Genetics and genomics for healthcarewww.geneticseducation.nhs.uk 2013 NHS National Genetics and Genomics Education Centre

Hundreds of genes are located on each chromosome (The dark bands on the chromosomes are due to a special staining technique and are not genes as these are too small to see)The 46 human chromosomes seen down the microscope

Genetics and genomics for healthcarewww.geneticseducation.nhs.uk 2013 NHS National Genetics and Genomics Education CentreThe genes are physically located on structures called chromosomes. Genes are too small to be seen down the microscope, but chromosomes can be, and this is a picture of human chromosomes in the nucleus of a cell as seen down the microscope. Nearly all normal cells in the human body contain 46 chromosomes (apart from eggs, sperm and red blood cells).

The 46 human chromosomes arranged in their 23 pairs (these are the chromosomes of a normal male)

Genetics and genomics for healthcarewww.geneticseducation.nhs.uk 2013 NHS National Genetics and Genomics Education Centre4Having applied a special technique to identify each chromosome, we can sort them into their 23 pairs as shown here one of each pair of chromosomes is inherited from our mother and the other from our father. The man whose chromosomes are shown here will pass on into his sperm one or other chromosome of each pair. Similarly, a womans eggs will contain one or other of each pair of her chromosomes.

Understanding how chromosomes are passed on from parent to child through each generation of a family can be clinically important, as it allows us to determine the chance of someone developing a condition or of being a carrier for a condition.

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Position of particular geneChromosomepair

Genetics and genomics for healthcarewww.geneticseducation.nhs.uk 2013 NHS National Genetics and Genomics Education Centre5We will now concentrate on just one pair of the 23 pairs of chromosomes, as shown here. Each chromosome contains many genes, arranged in order along the chromosome. We have shown the position of just one of the genes on this particular chromosome.

Has the conditionUnaffected carrier for the condition

Altered geneAltered geneAltered geneUsual gene

Genetics and genomics for healthcarewww.geneticseducation.nhs.uk 2013 NHS National Genetics and Genomics Education Centre6For some inherited conditions, people have to have an alteration in both genes of a particular pair to show signs of the condition. Such a condition is called an autosomal recessive condition and these can affect males or females (examples of conditions demonstrating autosomal recessive inheritance include cystic fibrosis and sickle-cell disease). Where a condition is an autosomal recessive condition, having an alteration in just one gene of the particular pair does not result in the person having the condition. A person with one copy of the altered gene and one copy of the usual gene usually shows no signs of the condition, and is called a carrier for the condition. But when two parents who are carriers for the same autosomal recessive condition have children, a child can inherit two copies of the altered gene and so develop the condition.

This is explained in the following animation - it shows how parents who are carriers for an autosomal recessive condition can pass on either the usual or the altered gene of a particular pair when eggs or sperm are formed.

ParentsSperm or eggsAutosomal recessive inheritance where both parents are carriers

Carrier for the conditionCarrier for the condition

Genetics and genomics for healthcarewww.geneticseducation.nhs.uk 2013 NHS National Genetics and Genomics Education CentreIn this animation, each parent is a carrier for the same autosomal recessive condition.

A parent passes on one or other of each pair of their chromosomes to a child - which one they pass on is due to chance. There are therefore four possible combinations of chromosomes which the children of this couple could inherit.7

ParentsSperm or eggsAt conceptionAutosomal recessive inheritance where both parents are carriers

Carrier for the conditionCarrier for the condition

Genetics and genomics for healthcarewww.geneticseducation.nhs.uk 2013 NHS National Genetics and Genomics Education CentreThe first combination shows a child who has inherited a chromosome with the usual gene from each parent: he or she will not have the condition and will not pass on the altered gene for the condition to his or her children. When a child inherits one copy of the usual gene and one copy of the gene alteration, he or she is a carrier for the condition like their parents. When a child has inherited a gene alteration from both parents, he or she will have the condition.

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ParentsSperm or eggsAt conceptionAutosomal recessive inheritance where both parents are carriers

Carrier for the conditionCarrier for the condition

Does not have the condition, non-carrierCarrier for the conditionHas the conditionCarrier for the condition

Genetics and genomics for healthcarewww.geneticseducation.nhs.uk 2013 NHS National Genetics and Genomics Education CentreSo where both parents are carriers, at conception each child has a 1 in 4 (25%) chance of inheriting two copies of the usual gene and of being neither a carrier nor having the condition. There are two chances out of four (50%) that at conception a child will inherit one copy of the usual gene and one copy of the altered gene and so will be a carrier for the condition. There is a 1 in 4 (25%) chance that a child will inherit a copy of the altered gene from each parent, and so inherit the condition.

As each one of the four possible outcomes shown here is equally likely, in each pregnancy there is a 1 in 4 (25%) chance that a child will inherit the autosomal recessive condition. It can sometimes be important to explain to parents that as chance has no memory, the 1 in 4 probability of inheriting two copies of the altered gene for the condition applies each time two parents who are carriers decide to have a child, irrespective of how many children they may already have who have or do not have the condition.9

This animation is part of a series describing modes of inheritance

To access these and other resources for teaching and learning genetics, please visit:

www.geneticseducation.nhs.uk

Genetics and genomics for healthcarewww.geneticseducation.nhs.uk 2013 NHS National Genetics and Genomics Education Centre

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