Drosophila melanogaster is the scientific name for the common fruit fly. The wild type (natural) eye color of these fruit flies is red. However, mutations can occur that cause the eyes to develop without pigment and appear white.
One gene controlling eye pigmentation is found on the sex chromosome of the fly. Sex chromosomes determine the sex of an organism. In fruit flies, females have two X chromosomes (one from their mother and one from their father) whereas males have one X chromosome (from their mother) and one Y chromosome (from their father).
The pigmentation gene in question is located on the X chromosome. Wild type flies have the R (red eyes) form of the gene, while mutants possess the r (white eyes) form. The mutated form is recessive to the wild type, meaning that an individual with an R and an r will have red eyes.
Genotype refers to the combinations of the various forms an individual possesses for a certain gene. Table 1 lists the possible genotypes and the corresponding eye colors.
Students were studying the effects of inheritance of eye color. In each of the experiments they conducted, 50% of the offspring were females and 50% were males.
Experiment 1
100 red-eyed females with genotype XRXR were mated with 100 white-eyed males. The results were as follows:
Females: 100% red-eyed
Males: 100% red-eyed
Experiment 2
100 female offspring and 100 male offspring from Experiment 1 were mated together. The results were as follows:
Females: 100% red-eyed
Males: 50% red-eyed; 50% white-eyed
Experiment 3
100 female offspring from Experiment 1 were mated with 100 white-eyed males. The results were as follows:
Females: 50% red-eyed; 50% white-eyed
Males: 50% red-eyed; 50% white-eyed