Final answer:
The incorrect explanation for a dominant disorder is a change in chromosome number (B), which usually leads to conditions such as Down syndrome, not autosomal dominant conditions which result from mutations affecting a single gene's function.
Step-by-step explanation:
B. A change in chromosome number. This typically leads to conditions such as Down syndrome rather than autosomal dominant disorders.
Autosomal dominant disorders are conditions where just one copy of the mutant allele (gene variant) can cause the disorder. Examples include Huntington's disease, where a gain-of-function mutation leads to a defective protein causing neurological degeneration. Haploinsufficiency is a mechanism in which a single functional copy of a gene does not produce enough protein to maintain normal function, leading to a dominant disorder.
A gain-of-function mutation might enhance the activity of a protein, or give a protein a new and uncontrolled function, leading to a dominant effect. A dominant-negative mutation interferes with the function of the normal protein product, essentially dominating the normal function and leading to a dysfunctional state. Conversely, changes in chromosome number, such as having an extra chromosome as in Down syndrome, are not a common cause of dominant disorders, as these typically result in widespread developmental issues that are not limited to a single gene's malfunction.