Final answer:
A single-gene mutation that has effects at multiple stages of development is described as A)pleiotropic. This is one way a gene's influence can manifest in various aspects of an organism's phenotype.
Step-by-step explanation:
When a single-gene mutation affects multiple stages of development, it is referred to as pleiotropic. Pleiotropy occurs when one gene influences two or more seemingly unrelated phenotypic traits.
This contrasts with other inheritance patterns such as incomplete dominance, where the phenotype is intermediate between the phenotypes of the two parents, or codominance, where both alleles express their effects fully within a heterozygote.
In the context of the question, the correct answer is A. pleiotropic.