Final answer:
The Bar phenotype in Drosophila is caused by a duplication mutation of the Bar gene on the X chromosome, leading to 'bar-shaped' eyes. It is an example of the effect of genetic mutations on morphology, which can also be seen in Hox gene mutations resulting in legs instead of antennae. Wild-type alleles often show complete dominance, explaining the lack of mutant phenotype expression when they are present.
Step-by-step explanation:
The mutation that causes the Bar phenotype in Drosophila is a duplication of the Bar gene on the X chromosome. This results in a narrowed or "bar-shaped" eye. The Bar phenotype is an example of how genetic mutations can affect morphological traits such as eye shape in fruit flies.
Eye color in Drosophila was one of the first X-linked traits to be identified, with red eye color being the wild type and other colors resulting from mutations. For instance, the white-eye mutant allele (w) is recessive to the wild-type red-eye allele (W), as shown in Figure 8.16. Related to the provided information, mutations in Hox genes, such as Antennapedia, result in dramatic morphological changes, like legs growing where antennae should be. The dominance of wild-type phenotypes, such as in the case of Drosophila eye color or rabbit fur color, often explains why certain mutant phenotypes are not expressed when wild-type alleles are present.