Chapter 18, Problem 5P

In 1932, H. J. Muller suggested a genetic test to determine whether a particular mutation whose phenotypic effects are recessive to wild type is a null (amorphic) allele or is instead a hypomorphic allele of a gene. Muller’s test was to compare the phenotype of homozygotes for the recessive mutant alleles to the phenotype of a heterozygote in which one chromosome carries the recessive mutation in question and the homologous chromosome carries a deletion for a region including the gene.

In a study using Muller’s test, investigators examined two recessive, loss-of-function mutant alleles of rugose named rg⁴¹ and rg^γ3 . The eye morphologies displayed by flies of several genotypes are indicated in the following table. Df(1) JC70 is a large deletion that removes rugose and several genes to either side of it.

a.	Which allele (rg41 or rgγ3) is stronger (that is, which causes the more severe mutant phenotype)?
b.	Which allele directs the production of higher levels of functional Rugose protein?
c.	How would Muller’s test discriminate between a null allele and a hypomorphic allele? Suggest a theoretical explanation for Muller’s test. Based on the results shown in the table, is either of these two mutations likely to be a null allele of rugose? If so, which one?
d.	Explain why an investigator would want to know whether a particular rg allele was amorphic or hypomorphic.
e.	Suppose that a hypermorphic rg allele exists (rghyper) that causes rough eyes due to an excess of cone cells. Could you use Muller’s genetic method to determine that the dominant allele is hypermorphic? Explain.
f.	Suppose an antimorphic rg allele exists (rganti). Can you think of a way to determine if a dominant mutation is antimorphic? (Hint: Assume that in addition to the chromosome with a deletion that deletes rg, a chromosome with a duplication that includes the wild-type rg gene is available.)