Answer:
By stopping the translation process, leading to a non-functional protein.
Step-by-step explanation:
The genes of organisms are the protein-coding regions of the genetic material (DNA). They are expressed in a two step process to produce functional products e.g proteins that bring about certain phenotypes in the organisms. The two stages involved in gene expression are viz: transcription and translation.
During transcription, the gene (DNA) is transcribed into RNA molecule which is read in a group of three nucleotides called CODON during translation. Translation synthesizes an amino acid from each codon. A STOP CODON i.e. UUC, UUG and UUA signals the end of the translation process and brings it to an halt.
Hence, if a mutation i.e. a change in the nucleotide sequence, causes a stop codon to be produced during the expression of Lucy's ADA gene, the stop codon will signal the end of the translation process. Because the ADA protein was truncated, it will be non-functional i.e. not work in Lucy's body.