How do autosomal recessive metabolic disorders pass from parents to children?

Autosomal recessive metabolic disorders occur when an individual inherits two copies of a mutant gene—one from each parent. Because these conditions are recessive, a single functional gene is typically sufficient to maintain normal enzyme activity, meaning parents who carry only one mutant gene (heterozygotes) are generally asymptomatic carriers.123

When two carriers produce offspring, the inheritance patterns for each pregnancy are as follows:
* 25% chance of the child inheriting two mutant genes and being affected by the disorder.12
* 50% chance of the child inheriting one mutant gene, making them an asymptomatic carrier.1
* 25% chance of the child inheriting two normal genes, being neither affected nor a carrier.1

If one parent is a carrier and the other is affected, the probability of having an affected child increases to 50%. While these mutations are individually rare in the general population, the risk of inheriting the same mutant gene from both parents is higher in cases of consanguinity, where parents share a common genetic background.321