Final answer:
X linkage refers to genes located on the X chromosome, creating distinct inheritance patterns in males and females due to their different sex chromosomes. An example in Drosophila is the white eye color, a recessive X-linked trait. Males only need one copy of the recessive allele to express the trait, while females need two.
Step-by-step explanation:
X linkage refers to the inheritance of traits located on the X chromosome. This creates a unique pattern because males (XY) and females (XX) have a different number of X chromosomes. A recessive X-linked allele is always expressed in males because they only have one X chromosome. For females to express a recessive X-linked trait, they must have two copies of the allele, one on each of their X chromosomes.
An example of an X-linked trait in Drosophila melanogaster (fruit flies) is the white eye color, which is a recessive trait. The inheritance pattern of this trait shows that if a female fruit fly is heterozygous for eye color (XwX+, where Xw is the allele for white eyes and X+ for red eyes), she will have red eyes since the red eye allele is dominant. However, if she produces a male offspring with a Y chromosome inherited from the father, this male offspring will have white eyes (XwY) if he inherits the Xw from his mother. This demonstrates that males are hemizygous for X-linked traits, meaning they will express whatever allele is present on their single X chromosome.