Before gene therapy, Arabella Smygov suffered from spinal muscular atrophy due to a mutation in an allele of a gene that produces a protein critical for muscle function. A similar disorder can occur when a genetic mutation occurs on chromosome 2 leading to intellectual disability and delayed development, among other traits. Sometimes this can occur even when neither of the parents carry any mutations on chromosome 2, and the affected child has the mutation on only one copy of chromosome 2. Which of the following would describe such a disease?
A. sex-linked
B. heterozygous
C. dominant
D. recessive