C24 - Population Genetics and Epigenetics

Question 1

What’s the role of natural selection?

Answer 1

To increase the frequency of beneficial alleles in a population and alter the gene pool. (E.g. sickle cell anaemia.)

Question 2

What is sickle cell anaemia?

Answer 2

A common genetic disease, resulting from a substitution mutation which produces abnormal haemoglobin.

At low oxygen concentrations, the abnormal haemoglobin clumps together and deforms red blood cells into sickle shapes.

This can cause blockages in capillaries and damage to organs.

Question 3

What is the allele for normal and abnormal haemoglobin production?

Answer 3

The allele for normal haemoglobin production is Hᴬ.
The allele for abnormal haemoglobin is Hˢ.
They are co-dominant.

Question 4

How do the alleles for normal/abnormal haemoglobin show co-dominance?

Answer 4

The allele for normal haemoglobin production is Hᴬ. The allele for abnormal haemoglobin is Hˢ. (Co-dominance).

People with the genotype HᴬHᴬ produce only normal haemoglobin.
All haemoglobin produced by people with the HˢHˢ genotype is abnormal and they have sickle cell anaemia.

However people with the genotype HᴬHˢ have sickle cell trait - half is normal, half is abnormal. (They’re often symptomless but may experience symptoms when short of oxygen).

Question 5

How has natural selection led to the increase of the sickle cell trait in certain areas?

Answer 5

Heterozygotes HᴬHˢ who have the sickle cell trait are protected against malaria.

(Since children with sickle cell anaemia HˢHˢ are unlikely to survive long and reproduce, it would be expected that the frequency of the Hˢ allele would decrease).

This has led to an increase in the % of population that have sickle cell anaemia in regions like West Africa.

Question 6

How does an allele variation / DNA mutation change the function of a protein?

Answer 6

It alters the sequence of amino acids of a polypeptide, ultimately resulting in the change of the tertiary structure of the protein.

This will lead to different interactions between R groups, changing the proteins 3D shape and active site.
This affects how well the enzyme can bond with its substrate.

Question 7

What are the 3 types of haemoglobin gene variations?

Answer 7

Haemoglobin S

Haemoglobin C

Haemoglobin H

Question 8

What changes to haemoglobin have resulted in the formation of the gene variation, haemoglobin S?

Answer 8

Mutation:
There’s a substitution in the beta polypeptide gene

Change to protein structure:
Valine replaces glutamic acid

Change to protein function:
Sickle cell disease - haemoglobin clumps at low oxygen levels

Question 9

What changes to haemoglobin have resulted in the formation of the gene variation, haemoglobin C?

Answer 9

Mutation:
There’s a substitution in the beta polypeptide gene

Change to protein structure:
Lysine replaces glutamic acid

Change to protein function:
The effect is less severe than sickle cell disease. It can cause some red blood cells to break down.

Question 10

What changes to haemoglobin have resulted in the formation of the gene variation, haemoglobin H?

Answer 10

Mutation:
There are severe mutations to alpha polypeptide genes

Change to protein structure:
Unstable haemoglobin is constructed from 4 beta polypeptides due to lack of alpha polypeptides

Change to protein function:
It has a higher affinity for oxygen than haemoglobin A, resulting in little oxygen being released in tissues.

Question 11

What’s a genetic bottleneck?

Answer 11

A drastic reduction in population size.

This can be caused by a natural event e.g. volcanic eruption or hunting by human population.

Question 12

What’s the founder effect?

Answer 12

A genetic bottleneck which occurs when a small group breaks away from the original large population to form a new colony.

Question 13

How is the Ellis-van Creveld syndrome an example of the founder effect in human populations?

Answer 13

When Amish people migrated to Pennsylvania (1744), 2 of the members of the new colony possessed the recessive allele for Ellis-van Creveld syndrome.

Symptoms include dwarfism, extra fingers, short ribs and cleft palates.

After several generations of inbreeding within the new generation, the Ellis-van Creveld allele had increased in frequency. It’s now more common among the Amish than the general population.

Question 14

How is blood group distribution an example of the founder effect in human populations?

Answer 14

Over thousands of years, global migration resulted in many small populations being established.

Each time a new population splintered off, it would have the potential to filter the gene pool and change the proportion of each allele present.

The original human population that migrated to South America was almost entirely blood group O. When they first migrated to Asia, the original population had a high proportion of the Iᴮ allele.

Question 15

What’s the Hardy-Weinberg principle?

Answer 15

The proportion of alleles will remain the same from one generation to the next provided that the following conditions are met:

• no new mutations
• no migration in or out of the population, and therefore no flow of alleles
• no natural selection for or against alleles
• the population is large
• mating is random

Question 16

Under what conditions can the Hardy-Weinberg principle be used?

Answer 16

The proportion of alleles will remain the same from one generation to the next provided that the following conditions are met:

• no new mutations
• no migration in or out of the population, and therefore no flow of alleles
• no natural selection for or against alleles
• the population is large
• mating is random

Question 17

What are the two calculations for calculating allele frequency?
What do they represent?

Answer 17

1) p + q = 1.0
Where p = frequency of dominant allele and q = frequency of recessive allele. If only 2 alleles exist, their frequencies must add up to 1.0.

2) p² + 2pq + q² = 1.0
Where p² is the frequency of the homozygous dominant genotype, 2pq is the frequency of heterozygous genotypes and q² is the frequency of homozygous recessive genotype.
This must add to 1.0 since no other genotype is possible.

Question 18

What do the calculations for allele frequency show?

Answer 18

1) p + q = 1.0
Where p = frequency of dominant allele and q = frequency of recessive allele. If only 2 alleles exist, their frequencies must add up to 1.0.

2) p² + 2pq + q² = 1.0
Where p² is the frequency of the homozygous dominant genotype, 2pq is the frequency of heterozygous genotypes and q² is the frequency of homozygous recessive genotype.
This must add to 1.0 since no other genotype is possible.

Question 19

What’s speciation?

Answer 19

The formation of new species.

Question 20

What are the (2) mechanisms of speciation?

Answer 20

Geographical isolation

Reproductive isolation

Question 21

What occurs in geographical isolation?

Answer 21

• Something causes a population to become geographically isolated
• Environmental conditions differ between the 2 places
• Selection pressures encourage evolution
• Mutations lead to different phenotypes / advantageous alleles in the population which gives survival advantage
• Organisms are more likely to breed and pass on alleles
• Advantageous allele increases in the gene pool
• Over time, the two groups become very different/unable to successfully interbreed

Question 22

How does reproductive isolation occur?

Answer 22

It arises eventually in geographically isolated populations due to different mechanisms (temporal, behavioural, mechanical/anatomical or post mating barriers).

Question 23

What are the mechanisms for reproductive isolation?

Answer 23

Temporal (e.g. different mating or flowering seasons)

Behavioural (e.g. different mating rituals)

Mechanical/anatomical (e.g. incompatible reproductive systems)

Post mating barriers (e.g. sperm destroyed by chemicals in females reproductive system)

Question 24

How do the primate, Guenon species, demonstrate reproductive isolation?

Answer 24

26 species of guenon monkey live in Africa. Several have ranges with geographical overlap, but do not interbreed.

Some scientists have concluded that the different facial features of guenon species act to strengthen reproductive isolation.

Distinctive visual differences e.g. colour variations appear to prevent breeding between species, reducing the risk of producing in fertile offspring.

Question 25

How do the primate, Hominid species, demonstrate geographical isolation?

Answer 25

Chimpanzees (Pan troglodyte) and bonobos (Pan paniscus) share a genus and have very similar genomes.

They became separated and geographically isolated by the Congo river.

They will mate in captivity and their hybrid offspring are thought to be fertile.
The behavioural and anatomical differences between them, which have involved during their geographical isolation, insufficient to stop the mating.

Question 26

What is epigenetics?

Answer 26

The study of alterations in a gene expression that are not a result of changes to DNA sequence.

The structure of the gene remains in tact however genes can be switched on and off by epigenetic changes.

Question 27

What are the (2) examples of epigenetic mechanisms?

Answer 27

DNA methylation

Histone modification

Question 28

What is DNA methylation?

Answer 28

An epigenetic mechanism where methyl groups (CH3-) can be added to DNA, (specifically cytosine bases).

This tends to decrease gene expression by preventing transcription.

Question 29

What’s histone modification?

Answer 29

An epigenetic mechanism.
In eukaryotes, DNA is packaged and ordered by histones. DNA molecules are wrapped around histones.

Histones can be chemically modified (e.g. by the addition of an acetyl, methyl or phosphate group).
These changes either activate or deactivate the gene, depending on the modification.

Modification can make the gene more or less accessible to transcription factors (proteins controlling the rate of transcription).

Question 30

What are the (3) examples of epigenetic studies in human populations?

Answer 30

Norrbotten studies

Dutch Hunger Winter

Twin studies

Question 31

What were the Norrbotten studies of epigenetics in human populations?

Answer 31

People of Norrbotten (Northern Sweden) experienced famine in 5 particular years, interspersed with some years where crop were abundant.
Scientists discovered that this not only affected the population of the time but also subsequent generations.

The descendants of women who experienced famine when fetuses had reduced life expectancy.
Descendants of men who ate too much and were over nourished during puberty, when the sperm were forming, also had reduced life expectancies.
This shows the environment can produce epigenetic changes that are passed on to subsequent generations.

Question 32

What was the Dutch Hunger Winter study of epigenetics in human populations?

Answer 32

Children conceived during the 1944/45 wartime winter famine in the Netherlands have an increased risk of diabetes, obesity, and heart disease.

This may be due to epigenetic alterations to genes associated with these diseases.

The malnourishment of their mothers during early pregnancy appears to have produced an effect on the children lasting there entire lives. Grandchildren of the malnourished women also tend to have the same health problems.

Question 33

What were the twin studies of epigenetics in human populations?

Answer 33

Identical twins have the same DNA sequences.
However, if scientists are able to find twins that have been separated early in life and have been raised in different places, any differences between them will be a result of their environments.

Scientists have found that identical twins raised part show for you at the genetic differences in their early years but significant differences in middle age.
Twins who spent less of their lifetime together showed the greatest epigenetic differences.

Question 34

What are the 2 examples of the founder effect?

Answer 34

Ellis-van Creveld Syndrome (from Amish)

Blood group distribution