24) Population genetics and epigenetics

Question 1

Natural selection increases _ in a population and alters _

Answer 1

the frequency of beneficial alleles

the gene pool

Question 2

The allele that causes sickle cell anaemia is a _ as produces both positive and negative effects

Answer 2

gene variant

Question 3

Explain how sickle cell anaemia can cause blockages in capillaries and organ damage

Answer 3

substitution mutation -> abnormal haemoglobin

at low concentrations of oxygen, haemoglobin clumps together and deforms red blood cells into a sickle shape

Question 4

State the symbols for the co-dominant alleles for normal and abnormal haemoglobin production

Answer 4

H^A and H^S

Question 5

Why is there a link between the distribution of malaria and the frequency of the H^S allele?

Answer 5

H^S has a protective effect as slightly increases CO2 production during the breakdown of red blood cells and therefore prevents development of the disease

Question 6

Give the possible genotypes for H^A and H^S

Answer 6

H^A H^A = normal haemoglobin
H^S H^S = sickle cell anaemia (unlikely to survive long enough to reproduce, allele expected decrease)
H^A H^S = sickle cell trait (symptoms only at low O2 levels)

Question 7

DNA mutation alters the _ of a _ and can therefore, change _ of a _ including _

Answer 7

amino acid sequence
polypeptide
tertiary structure
protein
active site of enzymes

Question 8

What do the alleles H^C and H^H represent?

Answer 8

H^C = breakdown of some red blood cells
H^H = high affinity for O2, little released at tissues

Question 9

Define genetic bottleneck

Answer 9

a drastic reduction in population number, with an accompanying reduction in genetic diversity, where survival is often due to chance

Question 10

Define founder effect

Answer 10

a type of genetic bottleneck in which a group of organisms leave an original population to form a new smaller population / colony

Question 11

Describe Ellis-van Crevald syndrome

Answer 11

recessive allele more common in Amish that has symptoms of polydactyly, dwarfism, short ribs and cleft palates

Question 12

Describe blood group distribution

Answer 12

ABO, human migration / each new population has the potential to filter the gene pool and proportion of each allele present

Question 13

Define the Hardy-Weinberg principle

Answer 13

the proportion of alleles will remain the same from one generation to the next provided...
no new mutations
no migration (flow of alleles)
no natural selection for/against alleles
large population
random mating

Question 14

Give two equations relating to the Hardy-Weinberg principle for genes with two variants (one recessive, one dominant)

Answer 14

p + q = 1
p = frequency of dominant allele; q = frequency of recessive allele
p^2 + 2pq + q^2 = 1
p^2 = frequency of homozygous dominant genotype; 2pq = frequency of heterozygous genotype; q^2 = frequency of homozygous recessive genotype

Question 15

Define speciation

Answer 15

the formation of new species (when groups of individuals of the same species evolve in different ways due to a form of isolation; gene flow has effectively stopped between populations

Question 16

Outline how speciation by geographical isolation takes place (8 steps)

Answer 16

1. population becomes isolated geographically (how)
2. different environmental conditions (name)
3. different selection pressures (examples)
4. mutations occur in the population leading to different phenotypes/advantageous alleles in population
5. phenotype gives a survival advantage
6. organisms more likely to survive to breed and pass on advantageous alleles
7. advantageous allele increases in gene pool
8. over time (many generations) become genetically different and unable to interbreed to produce fertile offspring

Question 17

Define geographical isolation

Answer 17

when a physical barrier separates members of a species

Question 18

When does reproductive isolation arise?

Answer 18

eventually in geographically isolated populations or between organisms whose geographically distributions overlap by isolation mechanisms / barriers

Question 19

Name 3 isolation mechanisms / barriers which could result in reproductive isolation

Answer 19

temporal - different mating seasons among animals / flowering seasons in plants
behavioural - different mating rituals
mechanical / anatomical - incompatible reproductive systems

Question 20

As well as isolation mechanisms / barriers which could result in reproductive isolation, what could stop successful reproduction?

Answer 20

post-mating barriers e.g. gametes being unable to meet or hybrid offspring being sterile

Question 21

Define epigenetics

Answer 21

the study of changes to gene expression without changing the genetic code (non-mutational changes)

Question 22

How can gene expression change without changing the genetic code?

Answer 22

genes can be switched ‘on’ or ‘off’, the extent to which they are expressed is altered (inherited/influenced by environment)
DNA methylation
histone modification

Question 23

Explain how DNA methylation can result in changes to gene expression

Answer 23

methyl group (-CH3) added to cytosine bases of DNA
reducing gene expression by preventing transcription

Question 24

Explain how histone modification can result in changes to gene expression

Answer 24

histones can be chemically modified (addition of an acetyl, methyl or phosphate group) to activate or deactivate a gene - making it more or less accessible to transcription factors

Question 25

Define histones

Answer 25

proteins in eukaryotes that the DNA wrap around in supercoiling

Question 26

Give 3 conditions of a gene switched on

Answer 26

unmethylated cytosine
acetylated histones
transcription occurs

Question 27

Give 3 conditions of a gene switched off

Answer 27

methylated cytosine
deacetylated histones
transcription does not occur

Question 28

Norrbotten studies observed years of famine and abundance which led to women with reduced life expectancies as foetuses. Also, descendants of men over-nourished during puberty (sperm-forming) had reduced life expectancies. What was the conclusion made?

Answer 28

environment can produce epigenetic changes that are passed on to subsequent generations

Question 29

Dutch Hunger Winter studies observed wartime winter famine and malnourishment of mothers during early pregnancy. Children conceived and grandchildren had a higher risk of diabetes, obesity and heart disease. What was the conclusion made?

Answer 29

epigenetic alterations to genes associated with these diseases is perhaps the reason for health problems (regardless of environment in early lives)

Question 30

Twin studies observed identical twins separated early I life and raised in different environments. Significant differences in middle age and those who spent less of their lifetime together showed the greatest epigenetic changes. What conclusion was made?

Answer 30

the influence on genetic and environmental factors on phenotypes

Question 31

Name 3 diseases caused by a substitution mutation

Answer 31

sickle cell anaemia
cystic fibrosis
PKU

Question 32

Suggest why a substitution mutation on a DNA molecule changing a triplet from AGA to AGG did not result in a genetic disease

Answer 32

genetic code is degenerate
similar R group therefore, doesn’t change tertiary structure
codes for the same amino acid
some amino acids have more than one triplet

Question 33

Changes to the p53 gene have been associated with many different types of cancer. Suggest whether the p53 gene is activated or inactivated when a cell becomes cancerous and explain why.

Answer 33

inactivated because...
tumour-suppressor gene
failure of DNA repair
cell cycle not stopped at check points
apoptosis not initiated

Question 34

An anticancer vaccine would lead to the production of antibody molecules that recognise Tumour Associated Antigens in the host. Explain how the structure of the antibody molecule would enable it to recognise only TAAs.

Answer 34

variable region complementary to antigen

different antigens would have different 3D shapes

Question 35

Explain why the blue-headed wrasse and the rainbow wrasse are described as different species

Answer 35

different phenotypes
different mating behaviour
unable to reproduce to make fertile offspring