A scientist wants to determine whether a mutant phenotype is due to the loss of gene C or gene D, which are both deleted in a mutant fly with abnormal eyes. Mutant flies with that have a transgenic gene C have wild-type eyes, whereas mutant flies that have a transgenic gene D have abnormal eyes. What can be concluded from these results

a transgenic copy of the gene C is sufficient to restore normal eye development

Explanation:

In genetics, there are diverse approaches to determining a gene's function. For example, it is possible to use a complementation test to determine loss-of-function recessive mutations when it is unknown if such mutations fall in the same or in different genes. Similarly to the generation of loss-of-function phenotypes, it is possible to insert a gene and thus produce a gain-of-function mutation that restores normal gene function (i.e., to restore the wild phenotype). Moreover, transgenic organisms refer to genetic engineering techniques by which any foreign or modified gene is inserted in the genome of an organism, which can also be used to study gene function. In this case, the restoration of the normal phenotype (wild-type eyes) is associated with the expression of the transgenic gene C, thereby evidencing that the gene C is required for normal eye development. In the last years, transgenic models have shed light on developmental pathways and on gene function.