Passage II

Most fruit flies have red eyes; some have white eyes. One gene that affects eye color is located on the X chromosome. Female offspring receive 1 X chromosome from both their mother and their father, so they have 2 copies of this gene. Male offspring receive 1 X chromosome from their mother and no X chromosomes from their father (instead, they receive 1 Y chromosome from their father), so they have 1 copy of this gene.

There are 2 alleles (forms) of this gene: the red-eye allele (X^R) and the white-eye gene (X^r). X^R is dominant to X^r, and X^r is recessive to X^R. This means an individual with 1 X^R and X^r will be red-eyed.

Genotype refers to the combination of alleles an indi­vidual has at a gene. The table lists each genotype and the corresponding eye color.

Students did 4 experiments to investigate the inheri­tance of eye color in fruit flies. In each experiment, 50% of the offspring were females and 50% were males.

Experiment 1

Fifty red-eyed females with Genotype X^RX^R were mated with 50 white-eyed males. The results were as follows:

Females: 100% red-eyed.

Males: 100% red-eyed.

Experiment 2

Fifty female offspring and 50 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

Fifty female offspring from Experiment 1 were mated with 50 white-eyed males. The results were as follows:

Females: 50% red-eyed; 50% white-eyed.

Males: 50% red-eyed; 50% white-eyed.

Experiment 4

Fifty white-eyed female offspring from Experiment 3 were mated with 50 red-eyed males. The results were as follows:

Females: 100% red-eyed.

Males: 100% white-eyed.