Genes are the unique set of instructions in every cell which make each of us individual. There are many thousands of genes, each carrying a different instruction. If a gene is altered, it can cause a genetic condition or disease. This gene alteration is known as a mutation.

Some conditions are inherited in a way that is called recessive. Individuals who have two altered copies of a gene are affected with the condition. Individuals who have only one altered copy of gene are usually completely healthy.

They are known as carriers because they carry one altered copy of a gene. Their normal copy of the gene keeps them healthy and compensates for the altered copy.

If both healthy parents carry the same altered recessive gene, then there are four possible outcomes for each pregnancy they have, regardless of the sex of the child (see Diagram 1):

• A one in four (25%) chance of inheriting the altered gene from both parents and being affected
• A one in two (50%) chance of inheriting the altered gene from one parent and therefore being a healthy carrier
• A one in four (25%) chance of inheriting the normal gene from both parents and being neither a carrier nor affected

If only one parent is a carrier of the altered gene, then each of their children has a one in two (50%) chance of being a healthy carrier, but will not be affected (see Diagram 2). If one parents is affected and the other is a carrier, then each of their children will have a 50% chance of being affected and 50% chance of being a carrier (see Diagram 3). If one parent is affected with the condition, then each child will be a carrier but not affected, once the other parent is not a carrier (see Diagram 4)

Couples who are closely related to each other (eg: first cousins) are more likely to share a copy of the same altered gene, thereby increasing the likelihood of having a child with a recessively inherited condition.

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