Human Biology - Mutations / Gene Pools and Evolution

What is the definition of a species?

Group of individuals that share many characteristics, able to interbreed under natural conditions to produce fertile offspring

What is the definition of alleles?

Alternative forms of a particular gene

What is the definition of a population?

Group of organism of the same species living together in a particular place at a particular time

What are gene pools

Sum of all alleles in a given population
- Considers the characteristics of the whole population (not individuals)
- consists of genotypes of the individuals capable of reproducing

What are allele frequencies

How often each allele of a gene occurs in the gene pool

What are populations that vary in characteristics likely to have?

Have different frequencies of alleles and therefore different gene pools

What may happen to gene pools over time?

Overtime, frequency of particular alleles in population may change

Why does the frequency of particular alleles in a population change (genepool) and what does it allow?

Chance events (Mutations, Changes in Environment)
- Changes allow population to be compared at different times/ locations

What are mutations?

Permanent change in a gene or chromosome
- May occur suddenly and purely by chance
- Not all are harmful and sometimes leads to adaption to environment
- Offspring shows variation that do not resemble either parent nor family history

What is a mutant

Organism with a characteristic resulting from a mutation

What are the two types of mutations?

Gene Mutation
Chromosomal

What is Gene mutation ( type of mutation )

Changes in a single gene, so traits normally produced by that gene are changed / absent
- Usually occur during DNA replication
eg cystic fibrosis, sickle cell anemia

What are mutagens?

since Mutations can be spontaneous and have no cause
- Some substances can increase rate of mutations
eg Mustard gas, X-rays, radiation

How do mutagens cause mutations?

-Can resemble proteins and be incorporated into DNA
-Can trigger DNA replication errors
-Can cause DNA breakages/ lengthening
- Can block DNA replication / damage DNA structure
- Can chemically react and modify DNA

What is somatic mutation?

When mutation occurs in the somatic (body cells) of a person
- only that individual affected

What is germline mutation?

Mutation occurs in gamete (sex cells - egg/sperm)
- individual not usually affected
- can be passed into offspring

→ trait then expressed in offspring

What are effects of mutations?

A gene is a sequence of nucleotides in DNA or RNA that codes for a molecule that has a function ( usually protein / enzyme )

Sequence of 3 bases in DNA is code for amino acids used to build a protein
- Just 1 missing protein can have a huge effect eg Albinism

What is point mutation

A change in one base in a dna sequence - > Could alter a protein, have no effect at all or prevent protein from being produced

How can point mutations occur

Can occur when a nucleotide is
- Inserted (new nucleotide is added)
- Substituted ( an existing nucleotide is replaced w a different one)
- Deleted ( a nucleotide is removed from the strand )
* Typically occurs during DNA replication

What are the mutation effects

Missense
Nonsense
Neutral
Silent

What is the mutation effect of missense

Change the amino acid produced = changed protein

What is the mutation effect of nonsense

Changes the sequence to stop = shorter protein → unlikely to function properly

What is the mutation effect of neutral

Changes amino acid, however new amino acid is the same type = no significant change to protein

What is the mutation effect of silent

No change to amino acid = no change to protein

What will some mutations result in

Frameshift mutation
(occurs with insertions or deletions)

What are frameshift mutations

Affects the reading frame → the way which nucleotides are clustered in groups of 3 to make translatable codons
- Affects all the amino acids / nucleotides that occur after them

What does adding or deleting nucleotides do

Affects the groups of all subsequent nucleotides → they will not be translated into the correct protein sequence

What does substitution mutations do

Affect just one amino acid

What are lethal recessives

Recessive genes can be lethal if not masked by dominant allele
- Causes death of embryo / foetus / young child

what does lethal recessives do to the gene pool

Changes the composition of the gene pool
- People who inherit lethal recessives usually die before alleles can be passed onto next generation
- Thus freq. of these alleles in gene pools are reduced

What is an example of a lethal recessive

Tay sachs disease - disorder of lipid metabolism inherited in autosomal recessive pattern.
- Causes fatty material to build up in the nervous system, destroying the neurons

What are conditional lethal recessives

Some lethal recessive mutations only cause death under certain conditions
- Only cause death if individual is exposed to certain environmental factors

What is an example of a conditional lethal recessive

Favism - Person will develop haemolytic anaemia (abormal breakdown of erythrocytes) but only if they eat fava beans

how are recessives maintained

, a mutated recessive allele is only expressed when two is present (rr)

Thus neutral recessive alleles and even harmful recessive alleles may continue to exist in the population as heterozygotes (carriers)

What is chromosomal mutations ( type of mutation )

All or part of a chromosome is affected
- Often occurs during meiosis / cell division, less often in mitosis
eg down syndrome

How do chromosomal mutations occur

Deletion - part of a chromosome is lost

Duplication - section of chromosome occurs twice, Can occur if part of chromatid breaks off and then joins onto wrong chromatid

Inversion - breaks occur in chromosome, broken piece joins back in but wrong way around

Translocation - part of chromosome breaks off and is rejoined to wrong chromosome

Non-disjunction - during meiosis , chromosome pair does not separate and one daughter cell has an extra chromosome and the other has one less

What may chromosomal mutations be

May be unique but characteristic disorders may occur

What is monosomy (chromosomal mutation)

When an individual is missing a chromosome
Two types - Partial and Total Monosomy

What is partial monosomy

Part of a chromosome is missing

What is total monosomy

If autosome (non-sex chromosome) is totally missing → severe malformations/ miscarriage

What is trisomy (chromosomal mutations)

Conditions caused by an extra chromosome
eg Down syndrome (trisomy 21) → has 3 chromosome 21 not 2

What is the differences between Gene and Chromosomal Mutations (gene)

Gene mutations are more common than chromosomal
Gene changes are common errors during DNA replication → results in substitutions, insertions, deletions and frameshifts

What is the differences between Gene and Chromosomal Mutations (chromosomal)

While chromosomal changes tend to occur during cell division - mitosis and particularly meiosis in crossing over

What is the similarity between Gene and Chromosomal Mutations

Both can result from DNA replication / Cell division and exposure to mutagens

What are the evolutionary mechanisms that can affect the frequency of alleles in a gene pool

Mutation
Differing selection Pressures
Random genetic drift, including the founder effect
Changes in gene flow between adjoining groups

What is the definition of variation?

The natural differences that are present in a given species

What is variation caused by?

Random Assortment
Crossing over
Non-disjunction
Random fertilisation
Mutations

What is random assortment (cause of variation)

The chromosome given to each daughter cell is meiosis is random
- Random assortment of chromosomes during meiosis results in gametes (large number of possible combinations)

What is crossing over (cause of variation)

The genes exchanged between parent chromatids in meiosis is random

- Crossing over of chromatids during meiosis may result in chromatid being broken off and attaching to a different chromatid- results in changed sequence / recomnination

What is non disjunction (cause of variation)

Chromosomes may not seperate correctly during meiosis
- Incorrect number of chromosomes divided into gametes

What is random fertilisation (cause of variation)

the genes / alleles the egg and sperm contain are random
- Large number of different sperm / eggs, any sperm can fertilise an egg - large combination of alleles

What is mutation (cause of variation)

can result in new characteristics / alleles and considered most important source of variation

- Permanent changes in DNA - introduce new / different alleles into gene pool

Who is Charles Darwin

Biologist
- Founded the theory of evolution through natural selection
→ Collected evidence of similarities and different between animals in different places and different times until its conclusion
saw that Species change over time → evolution

What is evolution

A gradual change in the characteristics of a species

What were Darwin’s three main observations

Variation
Birth rate
Nature’s balance

What was Darwins observation of Variation

differences in members of the same species

What was Darwins observation of birth rate

Living organisms reproduce at a faster rate than which their food supply and other resources would increase → overcrowding

What was Darwins observation of nature’s balance

Although birth rate was high, each species tended to maintain its numbers at a relatively constant level as equal numbers would die

What was Darwin’s conclusions from his observations

There is a struggle for existence : due to high birth rate and limited resources

- Survival of the fittest as those with characteristics best suited to their environment were more likely to survive and pass on those characteristics to offspring. Those with the least favourable characteristics likely died and never passed those characteristics.

What must natural selection have to occur

Variation

Selective agent : the environmental factor acting on the population that will favour particular characteristics

What is natural selection

Viewed as selection of those alleles in a population that give an organism a greater survival advantage
- where the environment is gradually changing, characteristics that enhance survival allow succeeding generations to gradually adapt to it

What are the principles of evolution through natural selection

1. There is a variation of characteristics within a species.

2. More offspring of a species are produced than can possibly survive to maturity.

3. Due to excessive birth rate, and limited resources, there is a struggle for existence - competition for survival.

4. The individuals with characteristics best suited to the environment have more chance of surviving and reproducing - survival of the fittest.

5. Favourable characteristics (those with survival value) are passed on to the next generation

6. In the gene pool, the proportion of alleles that produce favourable characteristics gradually increases -> evolution.

What is the format for a Natural Selection Question

1. Variation in population due to mutation
   

2. Some had (relevant characteristics)
   

3. Struggle to Survive as these indiv had to compete with each other when exposed to the selective pressure of _____
   

4. Those with the favourable characteristic _________ had selective adv and survived and reproduced
   

5. Passing on the favourable allele to their offspring, overtime increasing the allele frequency for that particular trait

What is the heterozygote advantage

Alleles with a negative effect r expected to disappear from the population over time.
- In some cases the allele frequency for a negative allele is much higher in a population than expected because heterozygote advantage

def :Individuals heterozygous for a trait exist at a higher frequency in a population than expected, due to providing some survival advantage, in certain environment

What is sickle cell anemia?

Recessive point mutation ( one amino acid change ) in allele for one of the beta chains in haemoglobin
- Causes blood cells to distort (sickle shaped) and die early, leading to anaemia and blockages in blood vessels in homozygous recessives → early death
Leads to expectation that allele to decrease in frequency

Where is sickle cell anaemia most common

Most common in areas where malaria is endemic and places where malaria isnt → sickle cell anaemia is rare.

Why is sickle cell anaemia most common in areas where malaria is endemic

Malaria is the selective agent : favours individuals with sickle cell trait, (Rr) as they are resistant to malaria (for no. of reasons)
- Also don’t suffer the full effects of sickle cell anaemia (single dominant allele compensates unless oxygen is low)
- Hence heterozygote advantage - advantage to having one copy of allele only

What is random genetic drift?

Random, non - directional change in allele frequency between generations
- Occurs purely by chance
- Most common and occurs faster in small populations
- Compared to natural selection in which population size has no effect

what is the founder effect?

A sub type of genetic drift
- Occurs when a small group moves away from its homeland to a totally new area and establishes a population, which later expands

What does the founder effect lead to

Decreased genetic variation / decrease alleles in gene pool
Different allele frequencies to original population ( not representative of original population )

What is the bottle neck effect

A sub - type of genetic drift
An event such as a natural disaster , suddenly and severely reduces the size o the population

- Allele frequencies of the new, smaller population may be different / not representative to the population before the event

What is migration / gene flow?

The movement of genetic material from one population to another
- Can result from the movement of a single individual
- If the population does’t contain the allele, then it is likely the allele will increase in frequency

What are barriers to gene flow?

Opposite of gene flow
- Geographical barriers
- Sociocultural barriers
Populations are isolated → no interbreeding → seperate gene pools form

What are examples of geographical barriers

e.g Oceans, mountain ranges , large lake systems , deserts , expansive ice sheets

What are examples of sociocultural barriers

economic status , educational background , religion , social position

How does the introduction of genetic diseases occur

If carrying one copy of the affect allele confers a selective advantage → increase allele frequency in the population
e.g sickle cell trait in areas where malaria is prevalent

If the disease is fatal (eg lethal recessive) → decrease allele frequency in the population
- If a fatal disease is introduced into a population who doesn’t have the allele, it may initially increase in frequency before decreasing

What is speciation

The process of producing two species from one, due to long term isolation, different environmental influences and major changes in allele frequencies

→ there is also Variation in Pop due to Mutation

What is differential selection

a difference in how traits are favoured in one condition compared to another
selective agents r different

What are the steps to speciation

4 steps
- Variation
- Isolation
- Selection
- Speciation

What is the step of variation in speciation (1st step)

Individuals of a species possess different characteristics
V in P due to M

What is the step of isolation in speciation (2nd step)

Populations of the species are isolated without gene flow (due to barriers to gene flow (geo or sociocult) as it prevents interbreeding

What is the step of selection in speciation (3rd step)

Each population is subject to different selective agents
Natural selection acts differently on each population (different characteristics are favourable in each environment and gene pools change) → populations become less and less alike as their allele frequencies start to become different

Struggle to Survive

What is the step of speciation in speciation (4th step)

the populations gene pools change until they become so different that the groups can no longer interbreed (to produce fertile offspring)

start of knowledge for the test

HAHA YAY

What is evolution?

Gradual change in physical characteristics of a species over many generations

What is DNA

Chemical compound that contains our genes → determines types of protein a cell makes

DNA is universal - same in all living things but the sequences is different between species
Allows for comparisons

What happens to DNA when speciation occurs

New species have very similar DNA
- The more closely related two species are, the greater proportion of DNA they share

DNA contains some non-coding sequences (junk dna)
More closely related species have more junk sequences in common

What is DNA hybridisation

Molecular biology technique that measures the degree of genetic similarity between DNA sequences.
- Used to determine the genetics distance between two organism

What is DNA melting

DNA of one organism is labelled, then mixed with the unlabelled DNA to be compared against
- The mixture is heated to allow DNA strands to break apart and then cooled to form renewed hybrid double - stranded DNA

Hybridised sequences with a high degree of similarity will bind more firmly , and require more energy to seperate them → they seperate when heated at higher temps than less sequences

What are Endogenous Retroviruses

Occurs when a viral DNA sequence becomes a part of an organism/s genome
- Retroviruses will enter a cell and copy its RNA sequence into the DNA (reverse transcription)
- Only becomes endogenous if it inserts its DNA into chromosomes of sex cell which can be passed on through generations
- Once this happens , the ERV DNA appears in the same place in the chromosome in every single cell and in all subsequent generations

How much ERVs make up the human genome

8%
- Other primates possess same ERVs in same locations in their genomes (common ancestor)
- More ERVs in common - more closely / recently related

What are Mitochrondrial DNA (mtDNA)

Small Circular molecules (5-10)
- 37 genes → 24 genes code for making tRNA, 13 genes have code for enzymes for cellular respiration
- Most cells have numerous amounts of mitochondria → thus 500-1000 copies of mtDNA molecule
- Alot easier to extract than DNA

Where are mtDNA found

Human eggs / sperm contain mitochondria
- After fertilisation , mitochondria in sperm are destroyed

Nuclear DNA comes from egg / sperm, mtDNa only comes from egg.

What is the rate of Mutation in mtDNA compared to nuclear DNA

mtDNA has a higher rate of mutation than nuclear DNA

Means that human mtDNA slowly diverging from mtDNa from original female ancestor

What correlation is there between number of mutations

More time has passed → more mutations → more difference → less closely related

What does similarities in mtDNA mean in two individuals

Provides estimate of closeness of their relationship through maternal ancestors
used to map relationship between species

what does it mean if mtDNA is identical

Closely related / siblings

What does it mean if mtDNA is different

last common maternal ancestor lived long ago

What are protein sequences

Proteins consist of long chains of amino acids, the sequence of which is determined by the DNA code
- We can compare types of proteins and sequence of amino acids in proteins in different species

Animals of same species have identical amino acid sequences (diff species diff sequences

What does the degree of difference between proteins show between animals

Gives estimate of how much evolution has occurred since they shared a common ancestor → longer the period of time involved, the greater number of amino acids that are different