Final answer:
In Drosophila, males express an X-linked recessive disease if they inherit the recessive allele from their mothers, because males have only one X chromosome and thus express whatever allele is present on it.
Step-by-step explanation:
In fruit flies, known as Drosophila, the male is the heterogametic sex, possessing one X and one Y chromosome (XY), while females have two X chromosomes (XX). For X-linked recessive diseases, male fruit flies will express the disease if they inherit the recessive disease allele from their mother. This is because males only have one X chromosome, and thus only one allele for any X-linked trait. This allele will always be expressed, whether it is recessive or dominant, because there is no second allele to potentially mask it, as would be possible in females.
As for X-linked dominant diseases, they are not typically seen in recessive forms, and the question does not apply as such. Dominant alleles would always be expressed, whether in males or females, provided that one dominant allele is present. Hence, in the context of X-linked recessive diseases, it is only when male fruit flies inherit the recessive disease allele from their mothers that they will express the disease.
Similar principles apply to human genetics, where disorders such as color blindness, hemophilia, and muscular dystrophy are also linked to the X chromosome. Because human males are XY, inheriting a recessive allele on the X chromosome from their mothers will result in expression of the disease, while females must inherit recessive alleles from both parents to express the disorder.