Final answer:
The correct answer for dosage compensation in a species with ZW sex determination is (3) ZW females have one copy of each gene on their Z chromosome. Mechanisms would ensure equal gene expression levels between ZW females and ZZ males, rather than changing the gene copy numbers.
Step-by-step explanation:
In birds and some other species, sex determination is controlled by the Z and W chromosomes, where the male has ZZ (homozygous) chromosomes and the female has ZW (heterozygous) chromosomes. For dosage compensation to be achieved in a species with ZW sex determination, there must be a balance in the expression of the genes on the Z chromosome between males and females. Since males have two Z chromosomes and females have only one, either the females would need to duplicate the gene expression from their single Z chromosome, or the males would need to halve it from each of their Z chromosomes.
The correct answer to the question is 3) the ZW females have one copy of each gene on their Z chromosome. This is because ZW females naturally have one copy of the Z-linked genes, and thus to achieve dosage compensation, mechanisms would work to equalize gene expression levels between the sexes, rather than altering the number of gene copies.