Drosophila melanogaster is a species of fruit fly. [Figure 1.1] shows a female fruit fly and a male fruit fly. A typical fruit fly is 3 mm in length. Scientists have studied the inheritance patterns of many genetic traits in fruit flies. To carry out genetic crosses with fruit flies: • a specimen tube is prepared with food for the fruit flies, as shown in [Figure 1.2] • adult male and female fruit flies are added to the specimen tube to allow mating to take place • the specimen tube is kept in warm conditions for several days • the eggs laid by female fruit flies develop into pupae • adults are removed from the specimen tube before pupae mature into adult fruit flies • offspring emerge as adult flies 10-15 days after eggs are laid. One of the genetic traits studied in fruit flies is eye colour. The normal (wild type) eye colour of D. melanogaster is red. Eye colour in D. melanogaster is controlled by several genes, including two genes that are located on separate chromosomes, A/a and D/d. • Allele A is dominant to allele a. • Allele D is dominant to allele d. Table 1.1 summarises eye colour in D. melanogaster for these two genes. Table 1.1 eye colour phenotype | genotypes --- | --- red | AADD, AaDD, AaDd, AADd brown | AAdd, Aadd scarlet (bright red) | aaDD, aaDd white | aadd