Unit 7 - Genetics, Populations, Evolution & Ecosystems Flashcards
define dominant allele
allele that has the same effect on the phenotype whether it is present in homozygous or heterozygous state
allele that is always expressed in the phenotype
define gene
a section of DNA that codes for the production of a particular polypeptide –> determines one characteristic in an organism
define allele
one of a number of different forms of a gene that result in different polypeptides being made
define genotype
the genetic make up of an organism/what alleles the organism has
define phenotype
the physical characteristics of an organism - a result of the expression of the genotype & its interaction with the environment
define diploid
having 2 copies of each chromosome
define haploid
having only 1 copy of each chromosome
define homologous
a pair of chromosomes from an organism with identical genes
(but not necessarily identical alleles)
define locus
the specific position on homologous chromosomes of a gene
define recessive
an allele that only affects the phenotype when present in the homozygous state/when both recessive alleles are present
define codominant
pairs of alleles that both affect the phenotype when present in a heterozygote
define homozygous & heterozygous
homozygous: having 2 identical alleles of a gene
heterozygous: having 2 different alleles of a gene
define carrier
an individual that has one copy of a recessive allele that causes a genetic disease in individuals that are homozygous for this allele
define mutation & what does it often lead to
a permanent change in DNA base sequence in a gene
often leads to a new allele being formed –> genetic variation
define monohybrid cross
inheritance of 1 gene
define dihybrid cross
inheritance of 2 genes
lots of traits are controlled by many genes
polygenic
for a characteristic controlled by a single gene, what will the individual’s gametes contain?
one allele of each gene
there is an = probability that it will contain the maternal or paternal allele
what are the 4 methods for genetic crossing & the types of alleles inherited?
dominance - dominant, recessive
co-dominance - 2 codominant alleles (heterozygous is a blend)
sex linkage - sex-linked dominant or recessive, sex-linked co-dominant
multiple alleles - ‘multiple’ bc more than 2 alleles of the gene exist in the population
describe dominance & expected phenotype ratio for 2 heterozygous parents
- dominant allele impacts the phenotype when only one is present
- recessive allele only affects the phenotype in the absence of the dominant allele
2 possible phenotypes
expected phenotype ratio from heterozygous cross 3:1
describe co-dominance
both alleles are codominant
both alleles are expressed in the phenotype of a heterozygous individual, creating an intermediate/blend phenotype b/w the homozygous phenotypes
expected phenotype ratio from heterozygous cross 1:2:1
describe inheritance of sex
determined by which 2 sex chromosomes the individual has
females have XX - all eggs contain X
males have XY - sperm have either X or Y
–> sex of offspring is determined by the sex chromosome carried by the sperm
describe the inheritance of sex-linked genes
females have 2 alleles/copies of a sex-linked gene
males have 1 allele/copy of a sex-linked gene = can’t be heterozygous
males are more likely to have sex-linked phenotypes than females bc males only need one allele for it to be expressed
a male can only inherit a sex-linked allele (only on X chromosome) from his mother
do genetic cross for sex-linked genes
see booklet
describe the inheritance of multiple alleles
definition: inheritance of a gene which exists as more than 2 alleles within the population’s gene pool
sometimes multiple alleles show dominance hierarchy with each allele being dominant to the alleles after it
do genetic cross for multiple alleles
see booklet
describe test cross
performed to distinguish b/w homozygous dominant & heterozygous individuals
(same phenotype different genotype)
each unknown individual is crossed with a homozygous recessive
if any of the offspring show recessive phenotype, the unknown parent must be heterozygous
do genetic cross for test cross
see booklet
why might a test cross not be conclusive?
if the sample is small, there is an element of chance
why can a test cross give different observed phenotypic ratios than expected?
small sample size
random fertilisation of gametes
linked genes/crossing over/sex linkage
epistasis
lethal genotypes
how do you prove traits are caused by dominant alleles in pedigree analysis?
parents have the trait with unaffected offspring –> the parents must be heterozygous carriers of the recessive allele
how do you prove traits are caused by recessive alleles?
unaffected parents have affected offspring –> parents must be heterozygous carriers of the recessive allele
how do you prove traits are caused by sex-linked genes?
different inheritance patterns b/w sexes
unaffected mother & affected offspring - males can’t be carriers of sex-linked genes
(don’t assume male = XY & female = XX bc depends on species)
describe dihybrid cross (inheritance of 2 genes simultaneously)
there are 4 possible combinations of 2 characteristics
for each characteristic there are 2
what is the expected phenotype ratio for dihybrid cross of 2 heterozygous parents?
9:3:3:1
do a dihybrid genetic cross for one heterozygous & one homozygous recessive parent
see booklet
describe the use of Chi Squared tests in genetics
determines whether there is a statistically significant difference b/w observed & expected results in phenotype ratios
autosomal linkage is a big factor for why observed phenotype ratios often don’t match up with expected ratios
what is the formula for the Chi Squared test? (don’t need to learn)
Σ (O-E)^2/E
explain how meiosis results in cells that have a haploid number of chromosomes & show genetic variation
- homologous chromosomes pair up on equator of cell
- there is crossing over b/w homologous chromosomes
- this prouces new combinations of alleles
- independent segregation of homologous chromosomes
- homologous chromosomes separate in meiosis 1
- produces varying combinations of chromosomes/genes/alleles
- sister chromatids are separated in meiosis 2
how to do Chi Squared test Qs with genetics
number of categories - 1 = degrees of freedom
x^2 value = less than critical value –> P value greater than 0.05 –> no dif.
vice versa
explanation:
1. yes/no significant difference b/w observed & expected ratios
2. any differences are due to chance
3. accept/reject null hypothesis
describe epistasis
the allele of one gene has an effect on the expression of another gene
do epistasis genetic cross & what are the expected ratios?
see booklet
9:3:4
12:3:1
13:3
9:7
when does autosomal linkage occur?
when genes are located on the same chromosome
so alleles do not separate during meiosis when gametes are formed
alleles are inherited together as they do not sort independently
what is the degree of autosomal linkage determined by?
how close the genes are physically
the closer the genes, the less likely crossing over takes place
crossing over during meiosis –> formation of recombinants
define linkage group
a set of genes at different loci on the same chromosome that, except for crossing over, tend to be inherited together
what are the letters for linked genes with/without crossing over & what do most gametes contain?
without: AB or ab = same as parent genotype = parental gametes
with: Ab or aB = new combination of alleles = recombinant gametes
most gametes formed are the same as the parent genotype
describe what happens when crossing over does occur b/w genes
crossing over of homologous chromosomes happens at a point b/w genes as they are not linked so new combinations of gametes are formed
this will affect phenotype ratios
define allele frequency
number of times an allele occurs within the gene pool
define genotype frequency
the # of individuals with a particular genotype in a population
calculating allele frequency by multiplying genotype by 2…
see booklet
what is the Hardy-Weinberg equation?
p^2 + 2pq + q^2 = 1
dominant phenotype is p^2 + 2pq
recessive phenotype is q^2
p+q=1
what does the H-W principle predict?
that the proportion of dominant & recessive alleles of a gene in a population remains the same from one generation to the next if certain conditions are met
what conditions must be met for H-W principle to be valid/what are the H-W assumptions?
- the population is large
- the organisms are diploid & reproduce sexually
- mating is random
- no new mutations occur
- no natural selection occurs (all alleles are equally likely to be passed to next generation)
- the population is isolated (so no flow of alleles into or out of population)
only 1 & 2 can be met
H-W exam Q tips
q^2 is the key
q^2 allows you to calculate q which unlocks allele frequency equation p+q=1
what can the trend in population growth be if uncontrolled by selection pressures?
exponential
what are the 2 types & e.gs of selection pressures?
biotic: predation, competition, disease
abiotic: temp., sunlight, pH
what are the causes of variation?
genetic:
mutations create new alleles - gene mutations (new alleles), chromosome mutations
sexual reproduction creates new combinations of existing alleles - independent segregation, crossing over - in meiosis & random fertilisation
environmental - not passed on to offspring
what are the different types of variation?
discontinuous variation - not influenced by the environment - controlled by one or a few genes, categoric
continuous variation - influenced by the environment - often controlled by many genes & alleles = polygenic
what is reproductive success?
passing on alleles to the next generation
what is a selection pressure?
a factor that limits a population
what are the different types of selection?
directional, stabilising & disruptive
describe directional selection
definition: a type of natural selection in which a single phenotype is favoured, causing the allele frequency to continuously shift in one direction
there is a selection pressure to move the mean phenotype in one direction/way
the advantageous allele will increase in frequency & will become fixed/the only allele
directional selection will change the characteristics/phenotype of the population over time
be specific to Q
what does the graph for directional selection look like?
see booklet
describe how antibiotic resistance evolves as an example of directional selection
- variation in population of bacteria due to random mutation of DNA, leading to resistance allele
- when antibiotic is used, bacteria with resistance allele survive & reproduce, passing on beneficial resistance allele
- frequency of resistance allele increases in population over time
how do antibiotics work?
stop binary fission by:
- preventing cell wall synthesis
- inhibiting metabolism
why do we not say bacteria are immune to antibiotic?
immune to pathogen
resistant to drug
describe stabilising selection
extremes from the mean are selected against so the characteristics of a population are maintained
extreme characteristics at both ends are affected by selection pressures
what does the graph for stabilising selection look like
see booklet
describe how human birth weight evolves as an example of stabilising selection
lighter & heavier infants have higher infant mortality rate
define disruptive selection
individuals with values close to the mean are selected against –> often leads to speciation
what does the graph for disruptive selection look like?
see booklet
define gene pool
all of the alleles of all the genes in all individuals in a population at a given time
how does natural selection affect H-W principle?
allele/genotype frequencies are altered
e.g. dominant phenotype favoured so decrease q & q^2
or
heterozygous genotype is favoured so decrease q^2 & p^2
define species
a group of organisms that can potentially interbreed to produce fertile offspring
define population
all organisms of the same species present in a particular area at a particular time
define speciation
formation of more than one species from an original species
describe speciation overview
gene pools must be isolated so there is no flow of alleles b/w the 2 populations (this allows the genes pools to diverge until they become sufficiently different)
divergence can be due to natural selection if 2 populations have different selection pressure, mutations or genetic drift
describe the different types of speciation
1- allopatric speciation - creating a new species when the 2 populations are separated by a physical/geographical isolation
2- sympatric speciation - creating a new species when the 2 populations are in the same area but have reproductive isolation
describe the process of allopatric speciation
- single population of a species with individuals that can interbreed to produce fertile offspring
- physical/geographical barrier separates 2 different areas & individuals in them forming 2 separate populations
- different conditions in each area = different selection pressures
- so alleles & phenotypes & gene pool of the 2 populations become increasingly different
- the differences are so great that individuals can’t/won’t interbreed with each other to produce fertile offspring so a new species has been formed
apply this framework to Q
what types of isolation are involved in sympatric speciation
pre-zygotic isolation
temporal isolation - breeding seasons do not coincide
behavioural isolation - related to courtship behaviour
mechanical isolation - anatomical differences prevent mating
leads to reproductive isolation - gene pools are isolated & no gene flow
describe temporal isolation
2 different species produce gametes at different times of year –> very unlikely to fertilise each other
random mutation caused different timing of gamete production
–> disruptive selection
hybrid infertility
e.g. 2 parents w different chromosome # - mule
= post-zygotic mechanism
define evolution
a change in the allele frequencies in a population over time
what are the major contributing factors to evolution?
mutations & selection
define genetic drift
change in allele frequency that is due to chance
in a small population, the chance differences in which individuals reproduce & which die impact alleles passed on to offspring even if the alleles are neutral
this impacts allele frequency & so evolution
what can genetic drift cause?
alleles being lost from a population randomly (not due to selection pressure or natural selection)
as a population gets larger,
the chance differences in the frequency of alleles passed on gets smaller & the impact of selection is larger
define ecology
the study of how organisms interact with each other & the environment
define population
all the individuals on a species living in an area
define community
all the organisms of all species living in an area
there are interactions within & b/w these communities
define habitat
where an organism lives
define ecosystem
the interaction of a community of living organisms (biotic) with the non-living (abiotic) parts of the environment
define biotic
something living that affects a population
define abiotic
something non-living that affects a population
define interdependence
where within a community each species depends on the others, so if one is removed it can affect the whole community
define stable community
a community where all the species & environmental factors are in balance so population sizes remain fairly constant
define niche
the role of a species in its habitat
how an organism fits into the environment where it lives & what it does
which consists of its biotic & abiotic interactions with the environment
what is the competitive exclusion principle?
no 2 species occupy the same niche
what levels of organisation are included in ecology?
organisms > populations > communities > ecosystems > biomes > biosphere
what is the distribution & abundance of organisms in a habitat controlled by?
biotic (e.g. disease & competition) & abiotic (e.g. light intensity, soil pH, temperature) factors
define carrying capacity
the maximum stable population size of a species that an ecosystem can support
can be sustained over long period which is determined by limiting factors
describe variation in population size
at first a population may grow quickly bc there are no limiting factors
but generally there are many biotic & abiotic factors that affect size of population & rate of pop. growth
see booklet for graphs of:
exponential growth
logistic/restricted growth - sigmoid shape - rate slows down bc of limiting factors & tends towards carrying capacity
state 7 explain the effect of 4 abiotic factors
- pH: affects enzyme action - each enzyme has optimum pH, pop. larger when pH is optimum
- light: increased light intensity - increased PS so increased plant growth so higher carrying capacity of ecosystem
- water & humidity: affects transpiration rates - if water is scarce, populations are limited
- temperature: optimum temp. for enzymes - the closer to the optimum enzyme temp., the greater the carrying capacity so the greater the population size
what causes competition?
to survive & reproduce, organisms require a supply of materials from the surroundings & other organisms, which leads to competition
what do animals compete for?
food & water
mates
territory
what do plants compete for?
light
water & mineral ions
space
define intraspecific competition
competition within a species
define interspecific competition
competition b/w different species
what type of relationship is a predator-prey relationship?
interspecific
define predation
when one organism is consumed by another
define predator
an organism that feeds on another organism, known as its prey
draw the graph of population size of predator & prey vs time
see booklet
describe how predator & prey population sizes change over time
- predators eat their prey, which decreases the pop. of prey
- fewer prey available = greater intraspecific competition b/w predators
- so predator pop. decreases as there are not enough prey for the predators to survive & reproduce
- fewer predators = fewer prey eaten so more prey survive & reproduce
- prey population increases
- more prey = more food for predators so predator population increases
this cycle repeats over time
NB in natural ecosystems, organisms eat a range of foods so fluctuations in population size are less severe
cyclic fluctuation can also be due to disease & climatic factors
predators & prey evolve alongside each other ‘arms race’ = they match adaptations of each other
why must a population size be estimated how is it done?
it’s not possible to count every organism in a habitat
estimate pop. size/abundance from a sample - a much smaller area
the larger the sample,
the more reproducible - someone else samples & gets similar results
repeatable - you take sample again & get similar results
& valid - can accurately answer the question
what does the sampling method depend on?
the type of organism (motile or non-motile)
& questions being asked
what are the 3 sampling methods?
random quadrat sampling
belt transect systematic sampling
mark-release-recapture
when is random quadrat sampling used?
when sampling a large uniform habitat
measuring abundance - how many individuals in a habitat?
define uniform
same conditions/abiotic factors
describe the method of random quadrat sampling
- split study area into a grid w coordinates using 2 tape measures
- randomly chose co-ordinates to sample using a random number generator
- place quadrat at the co-ordinates & count the # of organisms in it
inclusion criteria decided before sampling & kept constant throughout - REPEAT at least 20 times = more reliable & gives representative mean
- from this data, calculate a mean that can be used to estimate population
why must co-ordinates be selected at random?
to prevent bias & therefore produce data that can be statistically testeed
population in an area =
mean # of individuals/m^2 x area of field
when is transect sampling used?
when environmental conditions change in a predictable way e.g. along a line of succession
describe the method of transect sampling
- place a tape measure across the environment e.g. from sea to cliff
- place quadrats at regular, systematic intervals e.g. every metre
- record the # & abundance of different species in each quadrat
- move along the study area randomly at least 20 times & repeat the whole transect
belt vs line transect sampling
line transect sampling: recording where species touch the line
belt transect sampling: using quadrats at regular intervals along transect
how can abundance be measured in quadrats?
frequency
percentage cover
define frequency & state its pros & cons
the likelihood of a species occuring in a quadrat
e.g. species is in 15/30 quadrats sampled = 50% frequency
pro: good for species that are hard to count e.g. if there are too many
con: does not provide info on density or distribution
define percentage cover & state its pros & cons
the estimate of the area within a quadrat that a plant species covers
pro: useful when plants are v abundant so there are too many to count
con: less useful when species are overlapping
what type of species do & don’t random quadrat sampling & transect sampling work for?
plant species
non-motile or slow-moving animal species
NOT motile organisms bc they will move away or hide when approached
what type of organisms is mark-release-recapture used for?
motile or fast-moving organisms
describe the process of mark-release-recapture
- known # of a species is captured & marked in a way that doesn’t reduce survival chances
- marked organisms are released back into the same area they were caught in
- after some time another known # of organisms is recaptured & the # of these individuals that are marked is recorded
(enough time that marked organisms re-integrate into population but not so long for death or birth)
what is the formula for the estimated population size?
(total # individuals in 1st sample - total # individuals in 2nd sample)/# marked individuals recaptured in 2nd sample
what 6 assumptions does mark-release-recapture rely on?
- the proportion of marked to unmarked individuals in the 2nd sample is the same as the proportion of marked to unmarked in the whole pop.
- the marked individuals released from 1st sample reintegrate back into population & have enough time
- the population has a definite boundary (no immigration/emigration)
- there are few, if any, deaths or births within the pop.
- the method of marking is non-toxic or won’t make the organisms more likely to be predated = does not reduce survival chances
- the mark/label is not removed or rubbed off
define succession
the term used to describe changes over time in species that occupy an area
define primary succession
the establishment of a new community where none existed before
define 7 describe secondary succession
succession when land already sustained life before it is altered e.g. by flooding, forest fires, deforestation
the succession process is much quicker bu same overall process
describe pioneer species
initially colonise barren land
have features suited to this role e.g. asexual reproduction, ability to fix nitrogen, ability to photosynthesise
describe the process of succession
q. colonisation of an area by pioneer species (relate to Q)
2. causes a change in environment (give e.g.: increases nitrate ion conc. in soil/increase food for other species)
3. this enables other species (relate to Q) to colonise & survive due to conditions being less hostile
these other species outcompete pioneer species which decreases pioneer spec. pop.)
4. this causes an increase in biodiversity & increase in species’ populations
5. the stability of the ecosystem increases & environment becomes even less hostile
6. eventually, a climax community forms (name species present)
as succession occurs,
common features emerge
more complex food webs
increase in biomass
what is conservation?
the management of Earth’s natural resources by humans in such a way that max. use of them can be made in the future
this involves active intervention by humans to maintain ecosystems & biodiversity, which requires careful management of existing resources
what are the main reasons for conservation?
personal - to maintain the planet & so our life support system
ethical - other species have occupies Earth longer than humans & should be allowed to coexist w us
economic - living organisms contain large gene pool w the capacity to make useful substances e.g. medicines, food etc.
cultural/aesthetic - habitats & organisms enrich our lives. their variety adds interest & promotes/intrigues artists etc.
how are habitats conserved by managing succession?
succession leads to a climax community
species that existed at earlier stages are lost due to being outcompeted or their habitats disappearing
human intervention can manage succession & prevent this
e.g. heather is grazed by sheep & burnt back by humans to prevent land reaching climax community
