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In the fruit fly, Drosophila melanogaster, a known deletion on the X chromosome removes both a gene involved in the fly's development and the gene for eye color. The deletion is lethal when homozygous or hemizygous. Flies that are heterozygous for the mutation are viable, but have indentations at the wing margin. Therefore, the deletion is named the Notch mutation. The allele for wild type eyes is dominant over the allele for white eyes. Predict the phenotypic ratio of the offspring from a cross between a white-eyed notched female and a wild type male.

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Answer:

1 wild-type eyed notched female : 1 wild-type female : 1 white-eyed male

Explanation:

There's a known mutation in the X chromosome that involves the deletion of a developmental gene and the eye color gene.

  • The deletion is lethal in homozygous or hemizygous individuals.
  • Heterozygous individuals are viable, but have a "Notch" phenotype.
  • The allele for wild type eyes is dominant over the allele for white eyes.

A cross between a white-eyed notched female and a wild type male can be written with their genotypes:

Xᵈᵉᴵ Xʷ    x   Xʷ⁺Y

From the Punnett Square, we obtain the following genotypic proportions and phenotypes of the offspring:

  • 1/4  Xᵈᵉᴵ Xʷ⁺  wild-type eyed notched female
  • 1/4  Xʷ Xʷ⁺  wild-type female
  • 1/4  Xᵈᵉᴵ Y  deletion is hemizygous, lethal
  • 1/4  Xʷ Y  white-eyed male

The phenotypic ratios therefore are:

1 wild-type eyed notched female : 1 wild-type female : 1 white-eyed male

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