Download 2. gene interactions

Gene interactions
SLIDE 1 Gene interactions Phenotype is produced as a result of the interaction among several
genes plus the environment.
SLIDE 2 Epistasis – Labrador colors We demonstrate the phenomenon of epistasis with the
determination of hair color of Labrador dog breed. The dominant version of gene „B” (allele B)
determines black color, while the recessive version (allele b) determines brown. Another gene (E)
is responsible for the sedimentation of pigment into the skin. If in an individual at least one copy of
E gene is dominant (EE or Ee; E; it is the normal allele), then B gene inheritance follows the normal
route (BB and Bb: black; bb: brown). However, if E gene occurs in homozygote recessive form (ee; e is
the malfunctioned allele), then the dog will be white independently of E gene genotype.
SLIDE 3 Pleiotropy – a gene affects several phenotypes Pleiotropy occurs when a single gene
influences multiple phenotypic traits. Consequently, a new mutation in the gene will have an effect
on several traits simultaneously. The underlying mechanism is that the gene codes for a product that
is, for example, used by various cells, or has a signaling function on various targets, or cause the
alteration of gene expression of several other genes through complex multistep interactions. The
easiest way to explain the mechanism is to use an example. A classic example of pleiotropy is the
human disease phenylketonuria. This disease causes mental retardation, and reduced hair and skin
pigmentation. The cause is a mutation in a single gene that codes for an enzyme (phenylalanine
hydroxylase) that converts the amino acid phenylalanine to tyrosine, another amino acid. The
mutation results in a no or reduced conversion of phenylalanine to tyrosine, and phenylalanine
concentrations increase to toxic levels, causing damage at several locations in the body. The term
pleiotropy comes from the Greek pleio, meaning "many", and tropo, meaning "changes".
SLIDE 4 Gene networks A gene network is a cluster of functionally-linked genes, which
participate in the same molecular processes and in the formation of a particular phenotype. The one
gene network, one phenotype paradigm means, that, in contrast to the one gene, one phenotype
principle, a group of interacting genes contribute to the formation of a particular phenotype. The
interaction among genes is dynamic, which means that the amounts of participating gene products
are dependent on each other. The relationship among the components is hierarchical, that is certain
genes have a more pronounced effect on the other than conversely. The members of ontogenic gene
networks operate according to a pre-programmed timetable. The steps of ontogenesis are controlled
by transcription factors* exerting stimulating or inhibitory effects on each other. Several gene
networks operate in an individual in different time and location. The interactions are more frequent
within a gene network, than between gene networks. A given component can be the member of
more gene networks. For example, oxitocin plays a role in more physiological process: uterine
contraction, blood pressure, parental care, etc.
SLIDE 5 Interaction among gene products
Genes interact with each other through gene products …
1. at the level of gene expression: transcription factors
2. via protein – protein interaction: transcription factors, subunits phosphorylation, etc.
3. via mediators: messengers (hormones, neurotransmitters, cAMP, etc)
SLIDE 6 Messages:
- The expression of a gene depends on other genes (epistasis)
- A single gene contributes to several phenotypes (pleiotropy)
- Genes are organized to gene networks
 Living organisms are systems based on gene interactions
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The effect of environment
Sex determination In many species sex is determined by environment, such as the position in
Crepidula snakes (see slide), or the actual number of sex partners in fishes, which means that if
there are much more females than males, they change sex, and vica versa.
Weight, height, studying The effect of training and diet on body structure is self evident. The
height of modern man was lower than those of Cro Magnon’s as long as until recently. Today, the
increase of height (called acceleration) is observable in a single generation. We do not know the
reason for this. Learning is largely independent from our genetic blueprint.
Genes or environment? Ontogenesis follows a strict program; it is protected by environmental
perturbations, which is indicated by the very high similarity of identical twins. It seems to be
contradictory that embryogenesis is controlled by a series of dynamic molecular processes, while
the course of ontogenesis is very rigid. The major reason for this is that the underlying mechanisms
exhibit self-correcting properties and that they resist to environmental and genetic perturbances (this
process is called robustness).
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