September 26, 2011
Allelic Relationships: Dominance and Recessivity
Dominance and Recessivity describe the relation of alleles of a gene to each other.
Crossing two alleles and examining the phenotype of the heterozygote tells you which is
dominant and which is recessive based on which parent the heterozygote looks like.
How do we tell if a mutant is dominant or recessive?
Add back a copy of the wildtype allele and see if it restores wild-type function.
-Yes- the mutant allele was recessive to wild-type
-No- the mutant allele was dominant to wild-type.
Loss of function
Mutation
Type
Common
dominance
relation
Hypomorph
Amorph
Usually recessive to wildtype. Can be dominant in
the case of
haploinsufficiency.
Gain of function
Antimorph
hypermorph
neomorph
Usually dominant to wild-type or
incompletely dominant.
Refer to the following definitions from the concept list (on the course website):
Gain of function – any mutation that results in the gene product to be over-expressed or to
gain a novel function. These are often, but not always, dominant mutations. They include:

Hypermorph – a mutant that produces increased activity or levels gene product.

Neomorph – a mutant with a novel gain of function phenotype.

Dominant-negative – a mutation which results in a gene product that neutralizes
the wild-type gene product. Antimorph is a mutant with a dominant-negative
phenotype.
Loss of function – any mutation which results in the gene product having less activity or no
function. These are often, but not always, recessive mutations. Haplo-insufficiency is an
exception, elimination of a function of one allele results in a mutant phenotype; in which a
loss of function condition causes a dominant phenotype. It occurs when a diploid organism
only has a single working copy of a wild-type gene, and the single functional copy of the
gene does not produce enough of a gene product to bring about a wild-type condition

Null or Amorph– a mutant where the function of a gene has been completely
eliminated.


Hypomorph – a mutant with partial loss of function. These are often, but not
always, associated with missense mutations.
Complementation Test Practice Problem:
You isolate a new phage that destroys the Contagion, a horrible bacteria that is infecting the
world and selectively killing beautiful celebrities. In order to map the genes essential for
lytic growth in your attempt to save the beautiful people, you perform mutagenesis of your
phage and do a pilot screen of 100 plaques and identify 7 temperature sensitive mutants.
You do a complementation test and get the following results, where + indicates growth at
the restrictive temperature and – indicates no growth:
1
2
3
1
2
3
4
5
6
7
-
+
+
+
-
-
-
-
+
-
+
-
+
-
+
+
-
-
-
+
-
+
-
-
-
-
-
4
5
6
7
What’s up with mutant number 6?
It’s a dominant negative because it kills all the other phage it is grown with.
OR
It has mutations in all of the above genes
What would you do to validate your hypothesis?
Do a co-infection with wildtype to be sure that it is not a mutant in all of the other genes
isolated.
If it is dominant negative co-infection with wild-type will not rescue the mutant
phenotype of #6.
Mutant 7 does not complement mutant 1 or 3, while they complement each other. Can
you give an explanation for why this would happen?
1 and 3 can complement each other, so they are mutant in different genes. It is possible
that mutant 7 has mutations in genes represented by both 1 and 3.
How would you test your hypothesis?
Co-infect E.coli with #1, #3 and #7 at MOI=5 and see whether the WT phenotype is
rescued.
How many complementation groups do you have in total?
1 and 5- mutant in gene “A”
2 and 4- mutant in gene “C”
3- mutant in gene “B”
7- mutant in gene “A” and “B”
6- dominant negative.