Primates lack the enzyme needed to complete the conversion of simple sugars into vitamin C, yet their DNA contains the gene that controls this conversion. It has been "turned off" due to a mutation. An ancestral gene that carries a mutation, which prevents it from making the products it would normally make, is know as a:

Pseudogene

Explanation:

A segment of the DNA that is non-functional but its sequence resembles that of a functional gene is known as a pseudogene. Pseudogenes can arise in the genome due to several reasons such as DNA duplication, reverse transcription of the mRNA, mutation, etc. The reason why these genes are nonfunctional is that either they lack regulatory sequences or they have frameshift mutations. Therefore, in the given case the ancestral gene that is carrying a mutation that makes it non-functional and thus, makes it unable to carry out the conversion of simple sugar into vitamin C is an example of a pseudogene.