Genes Lecture 3

Question 1

epistasis

Answer 1

the interaction between two or more genes that control a single phenotype

Question 2

types of epistasis

Answer 2

-one mutation affects the phenotype of the other mutation e.g. linear signaling pathways or metabolic cascades

-either mutation in two different genes present no phenotype but the double mutant has a phenotype (Reveals buffering mechanisms that prevent phenotypic manifestations i.e. cancers and developmental diseases)

-either mutation has the same phenotype but the double mutant presents with a single phenotype

Question 3

In mice, what do the A, B, C, W alleles control

Answer 3

A- distribution in pigment in the hair
B- colour of pigment in the hair
C- expression of pigment
W-distribution of pigment in skin and hair

Question 4

A allele in mice

Answer 4

A= wt allele
AA= agouti, Aa= agouti, aa= black
melanocytes are responsible for pigmentation: eumelanin-dark, pheomelanin- lighter yellow
when the Mc1R receptor is activated, the melanocytes produce eumelanin but when the A gene is present, A codes for the ASP which binds to the Mc1R causing a shift from the production of eumelanin to pheomelanin

Question 5

B allele in mice

Answer 5

B allele is wt and black
b=loss of function allele and is brown
B codes for tyrosine- related protein 1 which is essential for the production of eumelanin therefore small b is a lighter production giving brown

Question 6

C allele in mice

Answer 6

C allele= wild type
C codes for tyrosinase to produce melanin therefore a mutation (small C) means no melanin production leading to white fur and red eyes ( absence of pigment)

Question 7

W allele in mice

Answer 7

W allele= dominance in white/ white spotting (mild mutation)
ww= wild type
W codes for the Kit protein which controls the distribution of pigment in the hair and skin
Kit is essential for survival and migration of melanoblasts (precursors of melanocytes)

Question 8

recessive epistasis

Answer 8

recessive phenotype prevails against all other phenotype

Question 9

recessive epistasis phenotypic ration

Answer 9

9:4:3

Question 10

why is the albino (c) allele epistatic to all other coat colour genes

Answer 10

albino allele is a recessive loss of function mutation in the enzyme tyrosinase required for melanin synthesis in melanocytes
doesn’t matter the variation in coat colour; these won’t manifest due to lack of tyrosinase

Question 11

dominant epistasis

Answer 11

dominant allele prevails over other phenotypes

Question 12

dominant epistasis phenotypic ratio

Answer 12

12:3:1

Question 13

how can dominant white b epistatic towards all other coat colour genes

Answer 13

W is a dominant loss of function mutation in a transmembrane growth factor receptor (c-kit) required for proliferation and migration of melanocytes therefore if these can’t reach the skin there will be no pigment and white fur will be produced

Question 14

epigenetics

Answer 14

heritable changes in gene expression that don’t involve alterations of the DNA sequence of the genome

Question 15

Mitotic and epigenetic inheritance of patterns of gene expression

Answer 15

1. cell exposed to stress
2. change in pattern of gene expression
3. daughter cells although not directly exposed to this stress, daughter and grand daughter cells keep on expressing the affected gene
4. not caused by a DNA mutation and it is inherited to daughter cells via mitosis

Question 16

Meiotic and epigenetic inheritance of patterns of gene expression

Answer 16

environmental factors switch on the expression of a particular gene in oocyte
-pattern of gene expression can be inherited by the next generation
*rare but does happen

Question 17

Chromatin-based modifications causing epigenetic effects

Answer 17

changes in patterns of gene expression modified by chromatin
-altered chromatin structure can be passed on to daughter cells
-histones and DNA methylation can act as epigenetic tags to dictate whether specific genes will be expressed or not

Question 18

genomic imprinting

Answer 18

genes are tagged with epigenetic markers causing certain genes to be switched off

Question 19

paternal imprinting

Answer 19

genes from the fathers sperm are tagged and this switched off meaning embryo will only inherit the mothers copy

Question 20

maternal imprinting

Answer 20

genes from the mother’s egg are tagged and turned off meaning that in the embryo only the fathers copy of the gene will be active

Question 21

Maternal imprinting of the Igf2 gene

Answer 21

Insulin-like growth factor 2 (Igf2)
required for normal growth
maternal copy of the gene is imprinted & silenced
only the paternal copy of the gene is expressed
-if the father had a mutant Igf2, mice would expressed dwarfism as mothers functional gene is silenced

Question 22

examples of genomic imprinting in humans

Answer 22

Prader-Willi syndrome
Angelman syndrome
-both result from microdeletions in a region of chromosomes

Question 23

Prader-Willi syndrome

Answer 23

Deletion is of paternal origin
Maternally genes are intact but imprinted (orange) by a methylation (CH3) mechanism

Question 24

Angelman syndrome

Answer 24

Angelman syndrome:
Deletion is of maternal origin
Paternally UBE3A is intact but imprinted (red) by a non-coding RNA acting as an anti-sense that silences its expression

Question 25

why are the Prader-Willi syndrome and Angelman syndrome not inherited

Answer 25

microdeletions have occurred during meiosis in the sperm or oocytes

Question 26

how was genomic imprinting selected in evolution

Answer 26

1. Affects a limited number of genes (~80 in humans)
2. Many imprinted genes involved in foetal growth
3. The pattern observed is:
   -Paternally expressed genes promote growth
   -Maternally expressed genes suppress growth

Question 27

Haig and Westoby suggestions

Answer 27

genomic imprinting evolved due to conflict between maternal and paternal genes over how much maternal resources go to offspring creating a tug of war where:
Paternal genes push for larger offspring to maximize the father’s success.
Maternal genes aim for smaller offspring to save resources for future siblings.
Key points:
Without imprinting: An embryo could take more resources, growing faster, but this would harm its siblings by reducing their chances of survival.
With imprinting: A balance is achieved between the needs of one embryo and the overall success of the mother’s other offspring. This balance reflects the conflicting evolutionary interests of the mother and father.