Chapter 14 Outline
From Gene to Phenotype
Complementation – the same mutant phenotype does not necessarily mean that the same
gene is affected.
Determine this by complementation test. Cross mutants together so resulting organism
has one copy of each mutant allele. If the phenotype is wild type, the mutations are in
different genes (they complement each other). If the phenotype is mutant, the mutations
are in the same gene and form part of the same “complementation group”.
Complementation analysis can be done in haploids, such as Neurospora, by means of
heterokaryon analysis.
Note that in haploid genetics, typically dominance has no meaning. For instance, if two
uridine auxotrophs are crossed together, where mutations are in different genes, showing
independent assortment, you would get 4 possible haploid outcomes (one with the
mutation in gene 1; one with the mutation in gene 2; one with mutations in both genes;
and one with two wt alleles). The phenotypic ratio would be 3 auxotrophs: 1 prototroph.
Allele Variation
Incomplete Dominance: where the phenotype of the heterozygote is intermediate
between the two homozygotes.
Co-Dominance: where the phenotypic effects of two alleles can be seen concurrently
Eg. ABO blood system IA and IB are co-dominant and both are dominant to i. IA and IB
code for enzymes that add single monosaccharides to the H tri-saccharide precursor.
There exists a rare mutation that causes a defect in the synthesis of the H precursor and is
thus epistatic to IA and IB.
Describing an allele can depend on the vantage point. HbS, is recessive with respect to
the sickle cell anemia phenotype, it shows incomplete dominance with respect to red
blood cell morphology, and it is co-dominant at the molecular level.
Gene interaction can be inferred from modified di-hybrid ratios. For instance, if two
dominant genes are required to achieve a phenotype, the F2 ratio would be (9:7).
Another example would be when two genes affect a phenotype, and the presence of one
gives an intermediate phenotype (eg. Blue-eyed Mary). A non functional allele early in a
pathway would have an epistatic effect on later steps of the pathway (i.e. if an early step
is prevented, the allele composition of genes in later steps is irrelevant). The ratio in this
example is 9:3:4. Note that these ratios are based on the 9:3:3:1 phenotypic ratio of a
Mendelian di-hybrid cross.
If the interacting genes are in different pathways, an out-come with a 9:3:3:1 ratio
involving 4 distinct phenotypes is possible (eg. Corn snake skin pigmentation). Another
example of genes in different pathways is that of suppression, where a mutant phenotype
is reversed due to a mutation in a second gene.
Complicating factors in genetic analysis include cases where penetrance is not complete.
Penetrance is the percentage of individuals where phenotype corresponds with genotype.
In addition there can be significant variation in phenotypic expression or expressivity.
Finally it is possible that a mutation is pleiotropic (has multiple phenotypic effects) and
some effects may not be fully penetrant and/or exhibit variable expressivity.
SUGGESTED PROBLEMS
Chapter 14: 2, 3, 5, 7, 9, 11, 12, 14, 16, 17, 19, 23-25