Mendel's Laws - gene interactions and epigenetics Flashcards
what are the alleles of the A gene Agouti in mouse and their functions?
this gene determines distribution of pigment in the hair
A = hair lighter in middle (has little patch of yellow)
a = solid back
what type of mutation leads to the mouse being completely black?
LOF mutation
homozygous for loss of function allele
what are the alleles of the B gene Agouti in mouse and their functions
this gene determine the colour of the pigment
B = black
b = brown
LOF mutation = homozygous for bb = fully brown
what are the alleles of the C gene Agouti in mouse and their functions
this gene permits colour expression
C = colour is expressed
c = no colour (albino)
LOF mutation = homozygous for cc = albino
what are the alleles of the W gene Agouti in mouse and their functions
controls distribution of pigment
W = dominant white/white spotting (this is the abnormal one, unlike the others)
w = normal (this is the wild type gene)
what is epistasis
interaction between 2 or more genes that control a single phenotyp
what is recessive epistasis
when one homo rec is ‘epistatic’ to another homo rec
so for example, a gene can be masked by another
recessive epistasis example in mice
homozygous cc (albino) is EPISTATIC to brown
if mouse had bbC_ = it would be brown (so the recessive mutation wins over the Cc/CC)
but if mouse had bb and cc
the cc would win over the brown gene and make it albino
so brown is HYPOSTATIC to albino
why does this change the 9:3:3:1 ratio to 9:4:3?
cuz any that are cc will always be expressed as albino, regardless of if the brown is dominant
remember that 9:4:3 is always recessive epistasis
see onenote for the genetic cross thing
which enzyme is required for melanin synthesis?
tyrosinase
whats the name of the cells that produces melanin
melanocytes (found in skin and bulb of hair)
what’s the 2 types of melanin produced?
eumelanin (black)
pheomelanin (yellow)
both need tyrosinase to be produced
so why is albino allele epistatic to all other coat colour genes?
it controls production of the enzyme tyrosinase
so doesn’t matter if colour determining genes are dom or not
if theres no enzyme, colour won’t be produced
example of dominant epistasis in mice
heterezygous Ww (dominant white) is epistatic brown
bb ww = will be brown
but
bb W_ = will be white
whats the ratio of phenotype in dominant epistasis and why
12:3:1
any of the genotypes that have the W will be white regardless of the other alleles
see onenote for genetic cross
what is this dominant white gene epistatic towards?
all other coat colour genes except albino
what is a pleiotropic effect of dominant white mutation?
hearing impairment
what is special about this white mutation
normally, LOF genes are recessive
this one is dominant
which receptor is responsible for the division and migration of melanocytes
transmembrane growth factor receptor
how does this receptor work
forms a dimer
this enables receptor to send signal which causes ploriferation and migration from the neural crest
how does the W allele affect this recptor
inactive dimer is formed
fails to signal for ploriferation and migration
but not ALL of them will be impaired, some will be normal receptors
this causes the white colour
cuz melanocytes wont make it to the skin (whereas in albino, melanin not produced at all)
how would the W gene affect hearing then?
small subpopulation of the melanocytes will usually migrate to the inner ear to aid in function of the ear
so if the W gene causes the melanocytes to not be able to migrate then it will cause loss of function in ear
what is epigenetics
heritable changes in gene expression that dont involve alteration of DNA sequence
wxplain epigenetic regulation of gene expression
- an envornmental factor switches on expression of 2 genes (e.g. A and B)
- gene A is transient (so doesn’t last and isn’t passed down) not expressed in daughter cells
- gene B persists tho and is passed down through multiple cell divisions = an epigenetic effect
see onenote for diagram
why is chromatin structure important in this
chromatin structure affects gene expression
so any mods in meythlation or histone mods will alter the structure
and this altered structure can be passed down to daughter cells
what are the 2 types of epigenetic tags
5-methylcytosine
modification of histones
what is 5 methylcytosine
cytosine base has a methyl group added to it
this is reversible
this will still be read as a C
~70-80% of CpG dinucleotides (basically a pair of C and G) are methylated in vertabrates
associated with silencing/switching off of genes (so in order to express a gene you’d need to get rid of it)
this methylation acts as epigenetic tag
histone modification
histones are proteins
so they can get modified post translationally
e.g. additions of different residues to the histone
these act as epigenetic tags
genomic imprinting: paternal imprinting
paternal allele imprinted - silences its expression
maternal allele is preferentially expressed in the embryo
genomic imprinting: maternal imprinting
maternal allele imprinted - silences its expression
paternal allele is preferentially expressed in the embryo
what is igf2 and does it show mat or pat imprinting
stands for insulin-like growth factor 2
- required for normal growth
- only the paternal copy is expressed
- maternal copy is imprinted and silenced
how was the imprinting effect discovered in the igf2 gene
parent-of-origin effect
basically
mouse is heterozygous for the LOF mutation
if mutant inherited from mother
and dad has normal allele
then the mutant LOF wont be expressed
cuz maternal imprinting
and mouse will be normal sized
but
mouse is heterozygous for the LOF mutation
if mutant expressed from dad
and mum has normal allele
then the mutancy will be expressed
and mouse will be dwarf
so even tho its heterzygous for the mutation, itll still be dwarf as the normal allele is hidden (imprinted)
see onenote for diagram
human genomic imprinting: Prader Willi syndrome
happens at chromosome 15
deletion is of paternal origin
ie it happens during spermatogenesis
caused by lack (deletion) of several of maternally imprinted genes
because its lack of several genes, this is a polygenic disease
see this in onenote diagram
human genomic imprinting: Angelman syndrome
happens at chromosome 15
deletion is of maternal origin
ie it happens during oogenesis
caused by lack of paternally imprinted UBE3A gene
monogenic cuz its the lack of singualr gene
again, see this in onenote
both of these diseases are linked, it just depends which parent the microdeletion occurs in
many imprinted genes involve foetal growth,
which of the parents’ genes promote growth and which supresses growth usually>
paternally expressed genes usually promote growth#
maternally expressed genes usually supress growth
what is the genetic conflict hypothesis
female may mate with several males, so if the famle has a bunch of babies in her then the father will only be related to a subset of these foetuses
father wants to increase the fitness of his offspring, so he would want to promote growth in his offspring only
mother is obvs related to all her babies tho, so she’d want to divide resources equally between them, hence the supression so one isnt too much stronger than another
