Which statement best describes autosomal recessive inheritance?

• A. Affected individuals can be born to unaffected parents if both are carriers
• B. Affected individuals must have affected parents
• C. Autosomal recessive traits affect only males
• D. Traits show complete dominance when heterozygous

Answer: (D)

Autosomal recessive traits are expressed only when an individual has two copies of the recessive allele. Carriers with one copy are usually unaffected, so two unaffected-looking parents can have an affected child if both carry the recessive allele. Because the gene is on an autosome, males and females are affected equally. When two carriers mate, about a quarter of offspring are affected, half are carriers, and a quarter are unaffected non-carriers. The idea that heterozygotes show complete dominance describes autosomal dominant inheritance, where a single copy of the dominant allele is enough to express the trait, which isn’t the case for autosomal recessive traits.