Mendel's Laws - gene interactions and epigenetics

Question 1

what are the alleles of the A gene Agouti in mouse and their functions?

Answer 1

this gene determines distribution of pigment in the hair

A = hair lighter in middle (has little patch of yellow)

a = solid back

Question 2

what type of mutation leads to the mouse being completely black?

Answer 2

LOF mutation
homozygous for loss of function allele

Question 3

what are the alleles of the B gene Agouti in mouse and their functions

Answer 3

this gene determine the colour of the pigment

B = black

b = brown

LOF mutation = homozygous for bb = fully brown

Question 4

what are the alleles of the C gene Agouti in mouse and their functions

Answer 4

this gene permits colour expression

C = colour is expressed

c = no colour (albino)

LOF mutation = homozygous for cc = albino

Question 5

what are the alleles of the W gene Agouti in mouse and their functions

Answer 5

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)

Question 6

what is epistasis

Answer 6

interaction between 2 or more genes that control a single phenotyp

Question 7

what is recessive epistasis

Answer 7

when one homo rec is ‘epistatic’ to another homo rec

so for example, a gene can be masked by another

Question 8

recessive epistasis example in mice

Answer 8

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

Question 9

why does this change the 9:3:3:1 ratio to 9:4:3?

Answer 9

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

Question 10

which enzyme is required for melanin synthesis?

Answer 10

tyrosinase

Question 11

whats the name of the cells that produces melanin

Answer 11

melanocytes (found in skin and bulb of hair)

Question 12

what’s the 2 types of melanin produced?

Answer 12

eumelanin (black)

pheomelanin (yellow)

both need tyrosinase to be produced

Question 13

so why is albino allele epistatic to all other coat colour genes?

Answer 13

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

Question 14

example of dominant epistasis in mice

Answer 14

heterezygous Ww (dominant white) is epistatic brown

bb ww = will be brown
but
bb W_ = will be white

Question 14

whats the ratio of phenotype in dominant epistasis and why

Answer 14

12:3:1

any of the genotypes that have the W will be white regardless of the other alleles

see onenote for genetic cross

Question 15

what is this dominant white gene epistatic towards?

Answer 15

all other coat colour genes except albino

Question 16

what is a pleiotropic effect of dominant white mutation?

Answer 16

hearing impairment

Question 17

what is special about this white mutation

Answer 17

normally, LOF genes are recessive
this one is dominant

Question 18

which receptor is responsible for the division and migration of melanocytes

Answer 18

transmembrane growth factor receptor

Question 19

how does this receptor work

Answer 19

forms a dimer
this enables receptor to send signal which causes ploriferation and migration from the neural crest

Question 20

how does the W allele affect this recptor

Answer 20

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)

Question 21

how would the W gene affect hearing then?

Answer 21

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

Question 22

what is epigenetics

Answer 22

heritable changes in gene expression that dont involve alteration of DNA sequence

Question 23

wxplain epigenetic regulation of gene expression

Answer 23

• 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

Question 24

why is chromatin structure important in this

Answer 24

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

Question 25

what are the 2 types of epigenetic tags

Answer 25

5-methylcytosine

modification of histones

Question 26

what is 5 methylcytosine

Answer 26

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

Question 27

histone modification

Answer 27

histones are proteins

so they can get modified post translationally
e.g. additions of different residues to the histone

these act as epigenetic tags

Question 28

genomic imprinting: paternal imprinting

Answer 28

paternal allele imprinted - silences its expression

maternal allele is preferentially expressed in the embryo

Question 29

genomic imprinting: maternal imprinting

Answer 29

maternal allele imprinted - silences its expression

paternal allele is preferentially expressed in the embryo

Question 30

what is igf2 and does it show mat or pat imprinting

Answer 30

stands for insulin-like growth factor 2

• required for normal growth
• only the paternal copy is expressed
• maternal copy is imprinted and silenced

Question 31

how was the imprinting effect discovered in the igf2 gene

Answer 31

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

Question 32

human genomic imprinting: Prader Willi syndrome

Answer 32

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

Question 33

human genomic imprinting: Angelman syndrome

Answer 33

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

Question 34

many imprinted genes involve foetal growth,
which of the parents’ genes promote growth and which supresses growth usually>

Answer 34

paternally expressed genes usually promote growth#

maternally expressed genes usually supress growth

Question 35

what is the genetic conflict hypothesis

Answer 35

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