Question

In Drosophila (fruit flies) the genes how, dumpy and ebony are located on chromosome 3. LOF = loss of function.

• Flies homozygous for a LOF mutation (no gene product made) in ebony have dark black bodies.
• Flies homozygous for a LOF mutation (no gene product made) in dumpyhave truncated (short) wings.
• Flies homozygous for a partial LOF mutation (some gene product made but significantly less than normal) in how have wings that will not fold down (held out wings; that's where the gene name is coming from, how).

Use the following allele/phenotype designations:

For how

• allele h+ = wild type/normal
• allele h = partial loss of function mutation
• h+ > h
• phenotype h+ = wild type = wings that fold, which is dominant to held out wings = phenotype h

For dumpy

• allele dp+ = wild type/normal
• allele dp = LOF mutation
• dp+ > dp
• phenotype dp+ = wild type = normal wing size, which is dominant to truncated wings = phenotype dp

For ebony

• allele eb+ = wild type/normal
• allele eb = LOF mutation
• eb+ > eb
• phenotype eb+ = wild type = normal body colour, which is dominant to black body = phenotype eb

You cross a wild type female fruit fly that is a heterozygous carrier for all three mutant alleles with a black bodied male who has truncated wings that do not fold down.

You observe their 504 offspring and note their phenotypes and the number of offspring with each phenotype:

• wings that do not fold - 129
• truncated wings - 83
• black body - 6
• wings that do not fold and a black body - 80
• truncated wings that do not fold - 10
• truncated wings and a black body - 135
• truncated wings that do not fold, and a black body - 15
• wild type - 14

Note: that non-mutant phenotypes are generally ignored. For example a phenotype of "black body" means a fly with a black body, normal wing length and wings that fold.

Flies homozygous for a partial LOF mutation (some gene product made but significantly less than normal) in how have wings that will not fold down (held out wings; that's where the gene name is coming from, how).

But there is another mutation in how that is a complete loss of function (no gene product made). The complete LOF mutation is represented as h-; h- is recessive lethal. Flies homozygous for the complete loss of function mutation (h-/h-) die when trying to emerge from their pupal cases. Basically, you won't see h-/h- adult flies because they are all dead.

You mate two wild type flies who are both carriers for the dumpy LOF mutation (dp) and the complete LOF how mutation (h-). They both have the same genotype: h- dp / h+ dp+.

Q. How many of the resulting larvae (offspring) will be h- / h-, if the genotype at dumpy doesn't matter? Remember, the complete loss of function mutant dies when coming out of its pupal case, the larval stage is before the pupal stage.

Select one:

a. 1/4

b. 0

c. 1/2

d. 1/500

Answer 1