Flashcards in Genomic Imprinting Deck (87)
What is genomic imprinting?
When alleles are expressed differently based on their parental origin.
Which groups does GI most commonly occur in?
Mammals and flowering plants.
How did studying human genetic disorders help us understand GI?
In disorders that involve gene deletions, the phenotype is radically different if either the maternal or paternal copy is deleted.
Which genetic disorders does deletion of the Snrpn gene on chromosome 15 cause (in humans)?
If the maternal X is deleted, Angelman syndrome results.
If the paternal X is deleted, Prader-Willi syndrome results.
What are the symptoms of Angelman syndrome?
Hyperactivity, manic laughter, repetitive movement and a large tongue
What are the symptoms of Prader-Willi syndrome?
Hypotonia, short stature and small gonads
What mechanism causes imprinting?
Acetylation or methylation as it represses transcription of a gene
Does imprinting alter the DNA molecule?
No, epigenetic markers are added/removed to alter expression patterns
Which nucleotides attract methyl groups?
CdG nucleotides (cytosines next to guanines)
Are epigenetic markers conserved during somatic cell division? Why, why not?
Yes; enzymes that recognise methylation patterns maintain them during DNA replication before cell division
Generally all DNA across a proliferating tissue carries the same epigenetic markers. What causes exceptions?
Ageing and specific tissues
What happens to epigenetic markers during gametogenesis?
They are erased and re-set depending on the sex of the organism, i.e. egg DNA displays maternal markers whilst sperm DNA displays paternal.
In autosomal genes why does imprinting occur?
Due to relatedness asymmetry among offspring in a brood.
What is the name for the relatedness asymmetry hypothesis of imprinting?
Who proposed conflict theory and when?
David Haig in the 1990s
Why is there relatedness asymmetry among sibs in a brood?
Polyandry, where females mate with multiple males, means that offspring in a brood may be sired by different fathers. Therefore some are full sibs and some are half sibs.
Do offspring have the same mother?
What is the relatedness of the maternal alleles in a brood?
When taken at random from a brood in a polyandrous system, what is the relatedness of the paternal alleles?
0, as offspring more than likely have different fathers.
Why can we assume that maternal alleles act more altruistically than paternal?
Because they are more related within a brood
During feeding (in the womb), why is there conflict of interest between the mother and her offspring?
The mother wishes to save resources for herself and future offspring, whereas the offspring wish to maximise feeding for their own growth.
Why is there a cut-off point for offspring demand in feeding?
All current and future offspring share a mother and so do not want to completely deplete her.
How can Hamilton's rule be used to explain the cut-off point in offspring demand in feeding?
rb > c
r = relatedness of sibs
b = benefit to sibs
c = cost to self
The cost of foregoing maximal feeding as an individual is less than the benefit of sharing resources among sibs
Why does Hamilton's rule apply to maternal and paternal genes differently?
Because r is different.
r among maternal genes is 0.5, whilst among paternal genes it is 0.
Due to this relatedness asymmetry, what does Conflict Theory predict about the behaviour of maternal and paternal alleles in resource allocation?
Maternal alleles act altruistically to make lesser demands of the mother to save more resources for sibs.
Paternal alleles act selfishly to make more demands of the mother to take more resources for the self.
What is the outcome of conflict theory for maternal and paternal alleles?
An evolutionary arms race between them that results in the maternal allele being switched off and the paternal allele being switched on.
What evidence can be found for Conflict Theory? Who was it by and when?
Igf2 in murine systems
Haig and Graham, 1991
What is Igf2?
An embryonic growth promoter that signals to the placenta when the embryo needs feeding
When is Igf2 switched on?
When it is paternally inherited