mutations and gene pools Flashcards
evolution
change in characteristics of a species over time. it is gradual change that occurs over a number of generations, rather than the change of a particular individual or generation.
Phenotypes
set of characteristics of individuals are a result of the alleles
genotypes
the combination of alleles for a gene
mutation
A sudden and random change in a gene or chromosome leading to new characteristics in an organism. Occurs purely by chance.
- source of all new alleles
mutant
an organism that possesses a characteristic resulting from a mutation.
causes of mutation
- spontaneous
- induced
spontaneous mutations
- arise from errors in replication
- different genes mutate at different rates
induced mutations
mutations can be induced by mutagens (environmental factors that cause a change in DNA)
mutagen
agents that are known to increase the rate at which mutations occur
mutagen examples
saif:
- sulfur dioxide
- some antibiotics
- ionising radiation e.g. UV rays, X-rays and fallout from nuclear explosions
- formaldehyde
location of mutations
- somatic mutations
- germline mutation
somatic mutations
- Somatic (body) cells are affected.
- Only the person with the mutation is affected - cannot be passed on.
- Each time a mutant somatic cell replicates after the point of mutation it passes the mutation to the daughter cells.
- When the individual dies, the mutation is lost.
- Involved in many cancerous growths
germline mutation
- Reproductive cells (gametes) are affected.
- The person with the mutation is NOT affected – but it can be passed on.
what happens if conception occurs with a mutated gamete?
it usually naturally aborts. However, some conditions such as PKU (cannot break down proteins and usually results in brain development retardation) can arise from mutated gametes.
effects of mutations (not amino acid related)
- harmful mutations
- neutral mutations
- beneficial mutations
- lethal recessive
harmful mutations
these mutations are harmful because they alter the DNA sequence, thereby upsetting the structure and function of the protein they code for
neutral mutations
these often produce little or no change in the phenotype, neutral mutations are hard to defect
beneficial mutations
these mutations are best observed in species with short generation times
lethal recessive mutation
Lethal recessives occur when a recessive trait can cause death if there is two recessive alleles inherited (in other words, not masked by a dominant allele).
Most gene mutations produce…
recessive alleles (to do with protein production)
two types of mutations
- gene mutation
- chromosomal mutation
gene mutation
changes in a single gene so that the traits usually produced by that gene are changed or destroyed. Occur during the replication of DNA before cell division. Even subtle changes to DNA can have significant effects. If a mistake occurs at this stage, each time the cell replicates, it will replicate the mistake too.
point mutation
a type of gene mutation which affects only one base (on DNA)
chromosomal mutation
all or part of a chromosome is affected. Types of chromosomal mutations next slide.
aneuploidy
loss or gain of whole chromosomes
polyploidy
loss of gain of complete set of chromosomes
point mutations
- Point mutations change the sequence of bases in DNA for a single gene and may produce a new allele of a gene.
- Single gene mutations involving a single nucleotide are usually called point mutations.
- The new DNA sequence will result in a new sequence of amino acids making up a protein.
- Because of the degeneracy in the genetic code not all changes in a DNA sequence will result in a new sequence of amino acids.
- Even with a change in amino acid sequence, protein function may not be affected.
change in DNA
Change in the DNA
Mutations vary in the change in the DNA. Point mutations are due to changes in a single nucleotide; therefore, only one base is changed. These mutations may be due to a nucleotide being:
- insertion
- substitution
- deletion
insertion
a new nucleotide is added to the DNA strand
substitution
an existing nucleotide is replaced with another one, with a different base
deletion (single gene)
a nucleotide is removed from the DNA strand
single gene mutation types
- missense
- nonsense
- neutral
- silence
missense mutation
- a single base is substituted by another
- causes a change in the amino acid, and therefore in the protein produced
nonsense mutation
- a single base is substituted by another
- this results in a new triplet that doesn’t code for an amino acid, changes base sequence to the code to STOP
- means that the synthesis of the protein will stop, and so a shorter protein is produced that is unlikely to be able to fulfil its function
neutral mutations
- cause a change in the amino acid; however, the amino acid is of the same type and doesn’t change the structure enough to change its function
silent mutation
- do not cause any change in the amino acid, and therefore in the protein produced
- this is possible as most amino acids are coded for by more than one base sequence
sickle cell mutation
point substitution mutation
- autosomal recessive mutation which results in the substitution of a single nucleotide
cystic fibrosis
- single gene deletion mutation
autosomal recessive, over 500 different recessive mutations
chromosomal/block mutation types
- inversion
- translocation
- duplication
- deletion
inversion
middle piece of chromosome falls out, rotates 180˚ and then rejoins, gene appears in reverse order
translocation
piece of one chromosome breaks off and joins onto another chromosome
duplication
Homologous chromosomes: a segment is lost from one chromosome and is added to its homologue
- pieces of chromosomes are repeated so there are duplicate segments
deletion
break occurs at two points on the chromosome and the middle piece falls out, two ends then rejoin to form a chromosome deficient in some genes
effects of chromosomal mutations
- Could be larger effect than gene mutation as the mutation can effect all or part of a chromosome. Therefore, multiple genes.
- Can cause mutations so severe that miscarriage occurs in the early stages of pregnancy
down’s syndrome
a relatively frequent chromosomal mutation resulting from non-disjunction. An extra chromosome 21 is inherited (Often referred to as trisomy – three chromosomes instead of two)
patau syndrome
extra chromosome 13. results in mental retardation and physical deformities such as extra fingers, cleft palate/lip, deformed eyes and ears…
monosomy
can also occur – missing a chromosome (one instead of two) such as Turner’s syndrome where an X chromosome is missing. Females with this condition would be infertile, short in stature and lack secondary sexual characteristics.
variation
diversity of genetic and phenotypic traits within and between populations
variation gives. species..
greater opportunity to adapt to and survive in dynamic environments
Populations of highly variable species include…
individual with different fitness in that environment
- This variability → offers chance of reproductive advantage if environments change
ultimate source of variation
Mutations are the ultimate source of variation introducing new alleles into a population; new alleles may be favourable or unfavourable to survival
Different genotypes produce…
a variety of phenotypes, which are acted on differently by factors in the environment, producing different rates of survival
Mutations in genes and chromosomes can result from…
errors in DNA replication, cell division or from damage caused by mutagens
Species with low variation (pop. of clones)
highly successful in stable conditions
species
group of individuals that share many characteristics; able to interbreed under natural conditions to produce fertile offspring
alleles
alternative forms of a gene
geneticists
study populations, they consider the characteristics of the population as a whole rather than individuals
population
group of organisms of the same species living together in a particular place at a particular time
gene pool
sum of all the alleles in a given population
allele frequencies
how often each allele of a gene occursin a population
Populations with different characteristics are likely to have…
different frequencies of the various alleles in their gene pools
example of allele frequency
- If the frequency of Cystic Fibrosis in a given population is 5% then this means 5 in every 100 members of that population carry the Cystic Fibrosis allele.
- Cystic fibrosis is carried on chromosome 7. Therefore 95 out of 100 chromosome 7’s will have the regular allele and 5 out of 100 will have the cystic fibrosis allele.
example of frequency of various alleles in gene pools
Scandinavians commonly have blue eyes, black Africans commonly have brown eyes. Therefore, the frequency for the blue eye allele would be much higher in the Scandinavian gene pool compared to the African gene pool.
what also changes when allele frequencies for a population change?
gene pool would also change
what chances can cause changes in allele frequency?
- Pure chance: mutations
- Natural means: changes to environment
what processes cause changes in allele frequency?
- mutation
- gene flow (immigration and emigration)
- small population size and genetic drift
- natural selection
- non-random mating
selection pressures
External agents which affect an organisms ability to survive in a given environment
how can selection pressures be negative?
decrease occurrence of trait
how can selection pressure be positive?
increase proportion of trait
three examples of selection pressures
- Resource availability
- environmental conditions
- biological factors
selection pressure: resource availability
Presence of sufficient food, habitat (shelter/territory) and mates
selection pressure: environmental conditions
Temperature, weather conditions or geographical access
selection pressures: biological factors
Predators and pathogens (diseases)
density dependent selection pressures definition
density-dependent - affected by population size
density independent selection pressures definition
density-independent - unaffected by population
selection pressures acronym
PANDA PAW
density dependent selection pressures examples
PANDA
Predators
Availability of resources (e.g. shelter water)
Nutrient supply (i.e. food source)
Disease/pathogenic spread
Accumulation of wastes
density independent selection pressures examples
PAW
Phenomena (e.g. natural disasters)
Abiotic factors (e.g. temperature, CO2 levels)
Weather conditions (e.g. floods, storms, etc.)
variation
the natural differences that are present between individuals or species in a given species
what are the causes of variation?
- random assortment
- crossing over
- non-disjunction
- random fertilisation
- mutations
random assortment
random assortment of chromosomes in meiosis resulting in many different combinations of chromosomes originally from each parent.
crossing over
crossing over of chromatids in meiosis resulting in recombination’s of alleles.
non-disjunction
results in incorrect numbers of chromosomes.
random fertilisation
sperm and egg combinations are random
mutations
mutations can result in new characteristics.
- Mutations result in brand new alleles to the gene pool therefore are considered the most important source of variation. If a mutation is beneficial to survival, it can result in change to the whole gene pool!
changes to allele frequencies in gene pools
- Natural selection
- Random genetic drift (including Founder Effect)
- Migration
- Barriers to gene flow
- Genetic diseases
natural selection
the process by which a species becomes better adapted to its environment; those individuals with favourable characteristics have a survival advantage and so pass those characteristics on to subsequent generations
what is a major cause of change to allele frequencies in a gene pool
natural selection
evolution
a gradual change in the characteristics of a species
who put forward the theory of evolution
Charles Darwin and Alfred Russel Wallace in 1858
darwin’s three main observation
- Variation: differences in members of the same species
- Birth rate: living organisms reproduce at a faster rate than which their food supply would increase – therefore causing overcrowding.
- Nature’s balance: although birth rate was high, each species tended to maintain its numbers at a relatively constant level
struggle for existence
there is such due to high birth rate and limited resources
Survival of the fittest
because there was a range of variations in a species, those with characteristics best suited to their environment were more likely to survive and those with the least favourable characteristics were likely to die before passing on those characteristics to offspring.
Natural selection in humans
The environment of early humans had a big effect on the characteristics that were selected as the most suitable for survival in the region where they lived.
natural selection example: body shape
Body shape/stature correlates with resistance to the cold weather. Shorter limbed statures have less surface area to body volume therefore less heat can escape via radiation. As a pose to areas with a hotter climate tending to a have higher frequency of tall, slender body shapes which has a higher surface area to body volume ratio.
natural selection example: sickle cell
Sickle-cell trait in areas where malaria is prevalent. Malaria acted as a selective agent for the sickle-cell allele.
principles of evolution through natural selection
- There is variation of characteristics within a species
- More offspring of a species are produced than can possibly survive to maturity
- Due to excessive birth rate, and limited resources, there is a struggle for existence-competition for survival
- The individuals with characteristics best suited to the environment have more chance of surviving and reproducing-survival of the fittest
- Favourable characteristics (those with survival value) are passed on to the next generation
- In the gene pool, the proportion of alleles that produce favourable characteristics gradually increases.
sickle cell anaemia cause
caused by mutation of the gene that makes haemoglobin → distorts the shape of the red blood cell
what causes incidence of sickle cell anaemia in diff parts of the world?
natural selection operating in human populations
what causes incidence of malaria to increase
- As humans began to clear forests of Africa for agriculture → changed environment → created additional breeding areas for mosquito
- Increased food supply from agricultural production → human population to increase → more bodies for mosquitos to feed
what genotype is required for sickle cell anaemia?
homozygous for a particular recessive allele
heterozygous sickle cell anemia
- show no ill effects unless oxygen in short supply
- RBC show mild sickling
- Carriers → have sickle-cell trait
anaemia definition
Anaemia = condition in which where is a reduced amount of haemoglobin (Hb) in the blood; or reduced no. of RBC’s
why is it called sickle cell anaemia?
Cells are inflexible → become stuck in the blood vessels → blockages → reduced amount of Hb
complications of sickle cell anaemia
Fatigue, jaundice, organ damage, high blood pressure, heart failure
effect on survival SCA
- If person with SCA dies before reproducing → allele that causes disease is not passed onto the next generation
- Expect that over many gens. the frequency of SCA allele would gradually decrease until eliminated from the population/removed from gene pool
sickle cell allele occurs only in areas where…
malaria is prevalent
discovery about SCA
- Observations = malarial patients who were ‘sicklers’ had fewer malarial parasites than ‘non-sicklers’
- Sickling allele had highest frequency in areas where the risk from malarial parasites was greatest
- individuals with one sickle-cell allele were more resistant to malaria than those with normal Hb in their RBC
heterozygotes SCA
- Heterozygotes were less susceptible to infection from malaria than individual homozygous for normal Hb
- Individuals heterozygous for SC allele have a survival advantage in areas where malaria is prevalent
heterozygote advantage
genotype has a higher chance of survival than homozygous genotype
gene flow
the transfer of the alleles from one population to another through migration
migration
the movement of people from one area to another with the intention of settling permanently
- gene flow from one population to another population
how are gene pools affected by migration
- If immigrants to a certain population bring alleles that are not already there in that population, the allele frequencies will therefore be altered.
- Migration can also bring new disease to areas which previously did not have the disease and thus cause rapid decrease in populating numbers – affecting the gene pool.
example of migration with disease
Many Australian Aborigines died from diseases such as chickenpox after European immigration and thus any alleles they possessed were also lost from the gene pool
what happens when a population is divided by a form of barrier
the two environments will not be exactly the same
types of barriers to gene flow
- geographical
- sociocultural
geographical barrier
includes oceans, mountain ranges, large lake systems, deserts and expansive ice sheets
sociocultural barrier
such as economic status, religion, educational background and social barriers are barriers to interbreeding
Changes due to genetic disease
- An allele that causes an inherited fatal disease to be expressed is expected to gradually be eliminated from the population (such as lethal recessives). Therefore in these cases, the allele frequency reduces.
- If an individual or individuals with a genetic disease migrates to a population which previously did not have that inheritable disease, then the allele frequencies could increase.
- If the disease provides an advantage to a particular population, it can increase in frequency
examples of changes due to genetic disease
- Sickle-cell anaemia (one allele only) provides resistance to malaria and is common in African countries where Malaria is high
- Tay-Sachs disease (one allele only) provides resistance to Tuberculosis (TB – bacterial infection of lungs) which can be common in small populations where it can be passed on easily.
Random genetic drift
In small populations there is often random, non-directional variation in allele frequencies that occurs purely by chance.
The founder effect
- A phenomenon similar to random genetic drift which occurs when a small group moves away from its homeland to a new area and establishes a community which eventually expands.
- Being a small sample of the original population, the alleles they possess wouldn’t be representative of the whole population but only a selection of it – reducing the gene pool and changing the common characteristics.
genetic drift
Genetic drift is the random fluctuation of allele frequencies in a population from one generation to the next.
consequence of a genetic bottleneck
Genetic drift is often a consequence of a genetic bottleneck i.e. it results from inbreeding brought about by the limited mating possibilities in a small community.
effects of genetic drift
The effects of genetic drift can be amplified by differences in the number of children raised by couples, or individuals dying prematurely.
Genetic drift can result in:
- traits being lost from small populations.
- unusual traits, not commonly found in the parent population, and that are often non-adaptive, becoming established.
population bottlenecks
Anything that creates a sudden drop in population size (e.g. wars, natural disasters or migration), or prevents individuals from breeding, reduces mating possibilities and can cause a genetic bottleneck.
Founder effect
- Founder effect is an example of a genetic bottleneck.
- Founder effect occurs when a small number of people migrate and settle in a new area.
- The founding population carry only a small fraction of the original population’s genetic variation. As a result, they may differ both genetically and in appearance, compared with the parent population.
tay sachs as a example of founder effect
- Ashkenazi Jews an example (high incidence of Tay Scahs)
- Small original population
- Some individuals carrying the allele for Tay-Sachs disease
- Restricted breeding with gene pool/cultural isolation
- Frequency of allele increases over time
Species
group of individuals that share many characteristics; able to interbreed under natural conditions to produce fertile offspring
All humans are the same species that means…
have the capacity to interbreed to produce fertile offspring
Speciation
process of new species developing
speciation over time
Over time → allele frequencies of each gene pool will change (depending on which characteristics are favoured for survival)
speciation over generation
Over generations → populations will become less and less alike → develop characteristics that better suit them to respective environments
what happens when two populations are isolated for long enough
environmental influences are different enough → major changes in allele frequencies within each pop could occur
what happens when members of those populations may become so different
interbreeding no longer possible
steps involved in speciation
- VARIATION = b/w individuals of a species
- ISOLATION = populations of same species are isolated without gene flow
- SELECTION = each population subjected to different selective agents, subspecies begin to form
- SPECIATION = allele frequency changes until they become so different that the two groups are no longer able to interbreed
