CH 6: GENE INTERACTION
Oct/18/2011 – pg 221-239, 247-249
1. Biosynthetic pathway: an essential molecule is produced by a series of chemical
interconversions catalyzed by gene-encoded enzymes
2. Signal transduction pathways: transmit instructions from an extracellular signal such as an
environmental chemical or a hormone from another part of the body. The signal activates a
cascade that activates a regulatory protein that turns transcription on or off.
3. Developmental pathways processes controlled by genes that promote the growth and
differentiation of the body of an organism.
6.1 Interactions Between the
Alleles of a Single Gene:
Variations on Dominance - The known mutants of a gene are referred to as multiple alleles or an allelic series
Complete Dominance and
- A fully dominant allele will be expressed when only one copy is present – i.e. heterozygote
o Ex. The P wild type allele in PAH is haplosufficient, thus the person doesn’t have
- In fully dominant mutations, the wild-type allele of a gene is haploinsufficient, where one wild-
type does isn’t enough to achieve normal levels of function.
Null mutations - In null mutations, a non-functional protein is produced
Dominant negative - Dominant negative mutations act as ‘spoilers’ where the gene product is a unit of a
homodimeric protein, a protein composed of 2 units of the same type.
- In the heterozygote (+/M), the spoiler binds to the wild-type polypeptide and distorts it or
interferes with its function
- The same type of spoiling can also hinder the functioning of a herterodimer composed of
polypeptides from different genes. The gene product is a monomer and the mutant binds the
substrate so the wild-type protein can’t act on it.
- Occurrence of an intermediate phenotype – pink , red, and white – suggests incomplete
Incomplete dominance dominance, when the phenotype of a heterozygete is intermediate between those of 2
- The number of doses determines the concentration of a chemical made by the protein, such as
pigment, one dose produces less pigment, so the petals are pink.
Codominance - The expression of both alleles of a heterozygote is codominance
o Ex. ABO blood groups where there’s codominance of antigen alleles.
o Sickle cell anemia: In HB /HB there is codominance at molecular level because
when you do a gel, both alleles migrate to different positions
Recessive Lethal Alleles
- An allele capable of causing the death of an organism is a lethal allele
- Lethal alleles are useful in determining the developmental stage at which the gene normally
o Ex. Normal wild-type mice have dark coats, a mutation called yellow shows a 1:1
ratio of yellow to wild type if yellow mice are mated to homozygous wild type
o Shows that yellow mice always heterozygous for the yellow allele and the yellow
allele is dominant over wild type.
o Whenever yellow x yellow 2/3 yellow, 1/3 wildtype
o Yellow allele is lethal when homozygous
o The A allele produces effects on 2 characters : coat colour and survival. Pleiotropic - Pleiotropic: when an allele affects several properties of an organism
- Whether an allele is lethal or not depends on the environment the organism develops, certain
alleles are lethal in virtually any environment, others are viable in one environment but lethal in
- Sublethal alleles are recessive alleles that give a 60:40 ratio (or something similar) instead of a
50:50 ratio when you have a A/a x a/a cross. Lethality is expressed in some but not all of the
- Lethality may range from 0-100% depending on the gene itself, the rest of the genome, and the
Temperature-sensitive - Heat-sensitive lethal alleles are useful because they are members of a class of temperature-
mutations sensitive mutations
Permissive temperature - At the permissive temperature (often room temp), their phenotype is wild type but they`re
Restrictive temperature mutant at a restrictive temperature (higher)
- High temperatures might cause the protein to twist or bend its shape to an inactive
conformation at the restrictive temperature.
- Temperature sensitive dominant lethal mutations are expressed even with a single dose, but
only when the experimenter switches the organism to the restrictive temperature.
6.2 Interaction of Genes in
Biosynthetic Pathways in - Beadle and Tatum irradiated Neurospora cells to produce mutations then tested cultures for
Neurospora interesting mutant phenotypes.
- Found numerous mutants with defective nutrition – auxotrophic mutants – the mutants were
defective for synthesis of nutrient step
- They found that t