Autosomal recessive inheritance in carrier parents
Autosomal recessive conditions (gene on a non-sex chromosome) affect a person only with two altered copies, so two unaffected carriers, each with one altered copy, can have an affected child.
Remember
Two unaffected carrier parents of an autosomal recessive condition have a 1 in 4 chance of an affected child at each pregnancy, so the disease can appear in a family with no history.
An autosomal recessive condition is caused by a faulty gene on one of the non-sex chromosomes (autosomes). A person is affected only when both of the two copies of the gene are altered, one from each parent.
Someone with one altered and one normal copy is a carrier and is usually healthy, because the normal copy does enough. Cystic fibrosis is a typical example of an autosomal recessive condition.
When two unaffected carriers have a child, each parent passes on the altered copy half of the time, so the chance that both do so is half of a half, which is 1 in 4 (25%).
A Punnett square (a grid that lists the possible combinations of the parents' alleles) shows the equally likely combinations. One gives two altered copies and an affected child (25%), two give one altered copy and an unaffected carrier (50%), and one gives no altered copy (25%).
The same 1 in 4 risk applies afresh to every pregnancy. A son or a daughter has the same risk, because the gene is on an autosome, so males and females are affected equally.
Because carriers are unaffected, the condition can seem to skip generations, and several children of the same parents can be affected while both parents are unaffected. On a pedigree (a family tree), carriers are often not drawn, so a recessive disease may seem to come out of nowhere.
Key facts
- Affected child
- needs two altered copies of the gene, one from each parent.
- Carrier
- has one altered and one normal copy and is usually healthy, because the normal copy does enough.
- Two carrier parents
- each child has a 25% chance of being affected, a 50% chance of being an unaffected carrier and a 25% chance of having no altered copy, at every pregnancy.
- Sex
- males and females are affected equally, because the gene is on an autosome (a non-sex chromosome).
- Family pattern
- the condition can skip generations, because carriers are unaffected and are often not shown on a pedigree.
- Related parents
- if the parents are relatives (consanguineous), the chance of both carrying the same rare allele rises.
- Genetic counselling
- a couple who are both carriers can ask for it: a conversation with a specialist who can give detailed information about the risks and their options.
Exam traps
- Being unaffected does not mean being free of the altered gene: a carrier has one normal and one altered copy, so two unaffected carriers can each pass on the altered copy and have an affected child.
- Counting every combination that includes an altered copy gives 75%, but only the combination with two altered copies causes disease; the others are carriers or have two normal copies.
- About half of the children of two carriers are unaffected carriers with one altered copy, so a 50% chance is the chance of being a carrier, not of being affected.
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