Certain conditions are passed down through recessive inheritance. In recessive inheritance, both copies of the same gene (the copy from each parent) need to have a mutation for a person to be affected with a condition.
Transcript
For recessive inheritance both copies of the same gene — the copy from mom and the copy from dad — need to have a mutation for a person to be affected with a condition, instead of just one, like in dominant inheritance. The affected person would have inherited one copy of a mutated gene from each parent.
Parents of someone with a recessive condition are called “carriers.” Carriers usually have no symptoms, because they have one normal copy of the gene, which is enough to support typical growth and development.
Pregnancy outcomes for carriers of the same recessive condition include affected (25% risk, or 1 in 4), unaffected but a carrier like mom and dad (50% chance, or 1 in 2), or unaffected and not a carrier (25% chance, or 1 in 4).
We are ALL carriers for something. Most people don’t even know they are carriers, unless there is a positive family history, or they have had genetic testing.