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**In fruit flies, eye color is a sex linked trait. Red is dominant to white.

1. What are the sexes and eye colors of flies with the following genotypes?
XRX²femalex Ry malexixi feteigle
XRXR female xrx male
XTY
2. What are the genotypes of these flies:
Xry
white eyed, male
white eyed, female X RX RX red eyed, male
3. Show the cross of a white eyed female X'X' with a red-eyed male XR
red eyed female (heterozygous)
y
47x

1 Answer

5 votes

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)

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