In fruit flies, eye color is a classic example of a sex-linked trait that is controlled by genes located on the X chromosome. The dominant red-eye allele (X^R) suppresses the recessive white-eye allele (X^w) in heterozygous individuals. Since males have only one X chromosome, their eye color phenotype is solely determined by the allele present on their single X chromosome.
XRX² female: This female is homozygous dominant for the red-eye allele and will have a red eye phenotype.
Ry male: This male is hemizygous and carries the recessive white-eye allele. He will have a white eye phenotype.
xixi female: This female is homozygous recessive for the white-eye allele and will have a white eye phenotype.
fe fe male: This male is homozygous dominant for the red-eye allele and will have a red eye phenotype.
XRXR female: This female is homozygous dominant for the red-eye allele and will have a red eye phenotype.
xrx male: This male is hemizygous and carries the recessive white-eye allele. He will have a white eye phenotype.
XTY: This individual is a male with one X chromosome and one Y chromosome. Since the Y chromosome does not carry the eye color gene, the eye color cannot be determined from the sex chromosomes alone.
Xry male: This male has a white-eye phenotype and carries one copy of the recessive white-eye allele (X^w) on his single X chromosome. His genotype is X^wY.
White-eyed female: This female has a white-eye phenotype and is hemizygous for the recessive white-eye allele (X^w). Her genotype is X^wX^w.
XRX² red-eyed male: This male has a red-eye phenotype and is homozygous dominant for the red-eye allele (X^RX^R). His genotype is X^RX^R.
The white-eyed female is homozygous recessive for the eye color gene (X^wX^w) and will only produce gametes carrying the X^w allele. The red-eyed male is hemizygous for the eye color gene (X^RY) and will produce gametes carrying either the X^R or Y allele.
The Punnett square for this cross would be:
| X' | X'
--|---|---
XR|XRX'|XRX'
Y |X'Y|X'Y
The predicted offspring are:
50% red-eyed females (X^RX^w)
50% white-eyed males (X^wY)