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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