What are the chances that two carriers of an autosomal recessive disorder will have an affected child?

When two individuals who are both carriers of an autosomal recessive disorder conceive a child, there is a 25 percent chance that the child will be affected by the condition.123

In this scenario, each parent is a heterozygote, meaning they possess one normal gene and one mutant gene but remain phenotypically unaffected. For each pregnancy, the statistical probability of the child's genetic outcome is as follows:412

• 25 percent chance of inheriting two mutant genes, resulting in an affected child.1
• 50 percent chance of inheriting one mutant gene and one normal gene, making the child an asymptomatic carrier.1
• 25 percent chance of inheriting two normal genes, meaning the child is neither affected nor a carrier.1

Because these disorders require two mutant genes to manifest symptoms, they are often classified as inborn errors of metabolism when they involve mutations in key metabolic enzymes. While these conditions are generally rare in the population, the risk is higher among couples who are related (consanguineous), as they are more likely to share the same recessive mutant gene.21