A female Drosophila from a stock that breeds true for blue eyes is crossed with a male from a...


A female Drosophila from a stock that breeds true for blue eyes is crossed with a male from a true-breeding wild-type red-eyed stock, all offspring have wild-type red eyes. Which is correct?

a. The allele for blue eye color is dominant.

b. The gene for eye color is autosomal.

c. The gene for eye color is sex-linked.

d. None of the above.

Modes of Inheritance:

When the mode of inheritance of a trait is unknown, clues have to be elucidated from the crosses and results. A dominant trait is one that is capable of masking a recessive trait. Autosomal traits are those for which the causative genes are not on the sex chromosomes, while an sex-linked trait is one for which the causative gene is on the X chromosome. Sex-linked traits tend to appear more often in males than females, while autosomal traits affect both sexes equally.

Answer and Explanation:

There are a few clues in this cross that lead us to the right choice. When the blue female is crossed to the red male, all the offspring are red. This tell us right away that red is dominant to blue, as the red allele from the male is able to mask the presence of the blue allele donated by the female. Therefore, (a) cannot be the correct answer.

Second, all the offspring are red-eyed - males AND females. If the gene that controls eye color was on the X chromosome, that would mean that the male would have a single red allele: X(R)Y. The female is purebreeding for the blue allele, so if the gene was sex-linked, she would be homozygous for blue: X(r)X(r). If this was the cross performed, the male would give his dominant red allele to all his female offspring, which would make all female offspring red. However, whenever he passes on his Y allele to his offspring, the female would pass on a blue allele, and therefore all male offspring would be blue (X(r)Y). This didn't happen here, so the gene cannot be sex-linked, so (c) cannot be true.

Therefore, the only option that is true is b: The gene for eye color is autosomal.

Learn more about this topic:

Pedigree Analysis in Human Genetics: Inheritance Patterns

from Biology 102: Basic Genetics

Chapter 6 / Lesson 3

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