Topic 7: Genetics,Populations,Evolution Flashcards
1
Q
define genotype and phenotype
A
genotype: genetic consitution of an organism.
phenotype: the expression of this genetic constitution and its interaction with its environment.
7.1: inheritance
2
Q
What are alleles and how do they arise?
A
- variations of a particular gene (same locus)
- arise by mutation ( changes in DNA base sequence)
3
Q
how many alleles of a gene can be found in diploid organisms?
A
- 2 as diploid organisms have 2 sets of chromosomes ( found in homologous pairs)
- but there be many ( more than 2) alleles of a single gene in a population
4
Q
Describe the different types of alleles
A
- dominant: always expressed (shown in the phenotype)
- recessive allele: only expressed when 2 copies preseent
- codominant allelle: both alleles expressed/contribute to phenotype ( if inherited together).
5
Q
What is meant by the terms homozygous and heterozygous?
A
- homozygous: alleles at a specific locus * are the same
- heterozygous: alleles at a specific locus * are different
* on each homologius chromosome
6
Q
What do monohybrid and dihybrid corsses show?
A
- monohybrid: inheritance of one phenotypic characteristic coded for by a single gene.
- dihybrid cross: inheritance of two phenotypic characteristics coded for by different genes.
7
Q
What is the phenotype and genotype ratio for a heterozygous Monohybrid Cross? (e.g. Tt × Tt)
A
Genotype ratio: 1 : 2 : 1 (TT : Tt : tt)
Phenotype ratio: 3 : 1 (dominant : recessive)
8
Q
What is the expected phenotype ratio for a heterozygous Dihybrid Cross – AaBb × AaBb?
A
- Phenotype ratio: 9 : 3 : 3 : 1
9 = both dominant traits
3 = dominant A, recessive B
3 = recessive A, dominant B
1 = both recessive traits
9
Q
what is a sex-linked gene?
A
- a gene with a locus on a sex chromosome (normally X)
10
Q
Explain why males are more likely to express a recessive X linked allele
A
- females (XX) have 2 allelles: only express recessive allele if homozygous recessive / can be carriers.
- Males (XY) have 1 allele (inherited from mother): recessive allele always expressed.
11
Q
Explain how autosmal linkage affects inheritance of alleles
A
- two genes located on same autosome
- so alleles on same chromosome inherited together
- stay together during independent segregation of homologus chromosomes during meiosis.
- but crossing over between homologous chromosomes can create new combinations of alleles
- if the genes are closer together on an autosome, they are less likely to be split by crossing over.
12
Q
what is epistasis?
A
- interaction of non-linked genes where one masks/supresses the expression of the other
13
Q
Describe when a chi-squared (X2) test can be used
A
- data is categorical (Can be divided into groups e.g. phenotypes)
- when determining if observed results are significantly different from expected results (frequencies)
- i.e phenotypes from genotypes
14
Q
Suggest why in genetic crosses, the observed phenotypic ratios obtained in the offspring are often not the same as the expected ratios.
A
- fusion/fertilisation of gametes is random
- autosmal linkage/epistasis/sex-linkage
- small sample size - not representative of whole population
- some genotypes may be lethal (cause death)
15
Q
Describe how a chi-squared value can be calculated
A
16
Q
Describe how a chi-squared value can be analysed
A
17
Q
(b) Describe briefly how you would use a statistical test to find whether there is a significant correlation between mean March temperature and the date when chaffinches laid their first egg.
A
- Construct null nypothesis;
- Use Spearman rank (and calculate test statistic);
- Look up in table (to find critical value of P = 0.05 / 5 %);
- Use figure (in table) to accept or reject null hypothesis;
18
Q
What is a population?
A
- a group of organisms of the same species in in an ecosystem / habitat / area;
- (at one time that can interbreed).
7.2: populations
19
Q
What is a gene pool?
A
- all the alleles of all the genes in a population at any one time.
20
Q
what is allele frequency?
A
- proportion of an allele (of a gene) in a gene pool (decimal or percentage)
21
Q
What does the Hardy-weinberg principle state and what are the conditions under which the principle applies?
A
- allele frequencies will not change from generation to generation , given:
- population is large
- no immigration/emigration ( to introduce/remove alleles)
- no mutations ( to create new alleles)
- no selection for/against particular alleles
- random mating
22
Q
Give the Hardy-Weinberg equation
A
- p²+2pq+q²=1
- p+q=1
- p= frequency of one dominant allelle
- q=frequency of recessive allele of the gene
- p²= frequency of homozygous genotype
- 2pq: frequency of heterozygous genotype
- q²= frequency of homozygous usually recessive genotype.
23
Q
Explain why individuals within a population may show a wide range of variation in phenotype
A
genetic factors:
- mutations: primary source of variation
- crossing over
- independent segregation
- random fertilisation of gametes during sexual reproduction.
- environmental factors
24
Q
what is evolution?
7.33: evolution may lead to speciation
A
- change in the allele frequency over time/many generations in a population
- occuring through the process of natural selection
25
What is evolution?
- change in allele frequency over time/many generations in a population
- occuring through the process of natural selection
26
Describe factors that may drive natural selection
- predation,disease and competition for the means of survival.
- these result in differential survival and reproduction, i.e. natural selection
27
Describe the process of natural selection in the evolution of populations
- random gene mutations can result in new alleles of a gene
- due to a selection pressure - a benefical allele - organism has a selective advantage
- possessors are more likely to survive and have increased reproductive success
- advantageous allele is inherited by members of the next generation (offspring)
- over many generations, **allele increases in frequency in the gene pool**
28
Describe and explain stabilising selection
- occurs in an **unchanging environment**
- organisms with alleles **coding for average variations** of a trait have **a selective advantage.**
- so frequency of those alleles increase and those coding for extreme variations decrease.
- **so range /standard deviation is reduced**
29
Explain what is meant by stabilising selection and describe the circumstances under which it takes place. (5)
1. Occurs in an unchanging environment;
2. Selection against extremes / selection for the mean / mean / median / mode unaltered
**3. Range / S.D is reduced**
4. Increasing proportion of populations/frequency of alleles becomes well adapted to environment;
30
Explain the effects of directional selection
- organisms with **alleles coding for one extreme variation** of a trait have a *selective* *advantage*
- so **frequency of alleles** coding for one extreme variations of the trait *increase* and those coding for the *average* variation of the trait *decrease*
- this can lead to speciation.
31
Describe speciation
- reproductive isolation leads to different selective pressures
- differences in gene pools
- formation of a new species when unable to interbreed and produce fertile offspring
32
Define:
- allopatric speciation
- sympatric speciation
1. New species form when populations are geographically isolated from each other.
2. New species form without geographical isolation. Individuals living in the same area are reproductively isolated.
| *formation of
33
Describe allopatric speciation
- population is split due to geographical isolation (e.g. new river formed)
- this leads to reproductive isolation , separating gene pools by preveting interbreeding/gene flow between populations.
- random mutations cause genetic variation within each population.
- different selection pressures so diff advantageous alleles are inherited
- so allele frequencies within each gene pool change over many generations
- eventually populations cannot interbreed to produce fertile offspring.
34
Describe sympatric speciation
- No geographical isolation; populations live in the same area.
- Mutations cause reproductive isolation, **preventing interbreeding and gene flow** within the population.
- Reproductive isolation may arise from:
Diff breeding seasons ; courtship behaviors; ecological exploitation
- Different selection pressures act on each population.
- Different advantageous alleles are selected in each group.
- Over many generations, allele frequencies change in each gene pool.
- The two populations can no longer interbreed to produce fertile offspring.
35
These two species are thought to have evolved as a result of sympatric speciation. Suggest how this might have occurred. (4)
- The original population (1) has genetic variation. (1)
- Possible mechanism; A barrier (e.g., behavioural, temporal, or ecological) causes reproductive isolation. (1)
- Gene pools become increasingly different; mutations / natural selection
- Until interbreeding does not produce fertile offspring;
36
Describe disruptive selection
- natural selection that maintains high frequencies of two different extreme sets of alleles
- This can occur in an environment that shows variation
37
Explain genetic drift and its importance in small populations
- GD: allele frequencies in a population change over generations due to chance.
- so strongest effects in small population as gene pool is small and change has a greater influence.
- E.g. when a population is sharply reduced in size ; bottleneck effect
- E.g. when a small, new colony forms from a main population ; founder effect.
- This can reduce genetic diversity - some alleles can become fixed or lost entirely
38
What is a community?
| 7.4
- organisms of all species / all populations in an ecosystem / habitat / area; (1)
| 7.4 Populations in ecosystems
39
What is an ecosystem?
- a community and its interaction with the non-living (abiotic) components of its environment.
40
What is a niche?
- the specific role of a species within its habitat,
- goverened by its adaptation to both abiotic and biotic conditions
| e.g. what it eats, where and when it feeds
41
What are abiotic and biotic factors. Give examples.
- abiotic: Non-living physical or chemical elements of the environment
- biotic: living parts of the environment ; predation , competition, disease
42
Name examples of abiotic factors.
- Light intensity – affects photosynthesis in plants
- Temperature
- Soil pH – affects nutrient availability and plant growth
- Water availability
- Humidity – affects transpiration and evaporation
- Oxygen and carbon dioxide levels – important for respiration and photosynthesis
43
# ,
Explain the advantage of species occupying different niches
- less competition for food/resources
- if two species tried to occupt the same niche, one would outcompete the other
44
What is carrying capacity?
- the maximum population size of one species that an ecosystem can support.
45
List the factors that influence carrying capacity
- **abiotic factors:** light intensity, soil ph, and mineral content, humidity.
- **interspecific competition**: between organisms of different species
- **intraspecific competition:** between organisms of the same species
- **predation**
46
Explain how an abiotic factor like light intensity may affect population size/carrying capacity
| do this for two other factors - check with chatgpt
- if conditions favourable, organisms mroe likely to survive and reproduce, increasing carrying capacity
- e.g. increasing light intensity increases rate of photosynthesis in plants.
- this increases CC of a vairety of plant species
- increases the number and variety of habitats, niches, and food sources for animals.
- so increasing CC of a variety of animal species.
47
Explain how interspecific competition may affect population size
- reduces resources available to both species limiting their chances of survival and reproduction so reduces population size of both species
- if one species is better adapted, it will outcompete the other so population size of less adapted species declines.
48
Explain how intraspecific competition may affect population size
- **as population size increases** resource availability per organism decreases , so competition decreases
- as population size decreases, resource availability per organis m increases, so competition decreases so chances of survival and reproduction increase
49
Explain the changes which occur in populations of predators and prey
1. Prey population increases so predators have more food so more predators survive and reproduce
2. Predator population increases so more prey killed & eaten so less prey survive & reproduce
3. Prey population decreases so predators have less food so less predators survive and reproduce.
4. Predator population decreases so less prey killed & eaten so more prey survive and reproduce ( cycle repeats).
50
The figure shows a population growth curve. Describe and Explain the Figure.
1. Lag phase: Slow growth because the population is still small + reproductive rate is still low;
2. Rapid / fast / exponential growth phase due to an increase in number of breeding pairs; + high resource availability
3. Stable stage - plateau phase - external resources limits the growth; birth and death is equal.
4. Ecosystem has reached the carrying capacity;
51
Describe how the size of a population of slow moving or non-mobile organisms can be estimated
| give equation
1. Divide area into a grid/squares.
2. Generate a pair of coordinates using a random number generator (e.g. on a calculator)
3. Place a quadrat here and count number/frequency of species
4. **Repeat** a large number of times and calculate a mean per quadrat
5. ** Population size= total area of habitat/quadrat area) x mean per quadrat**
52
Describe how the mark-release-recapture method can be used to estimate the size of a population of motile organisms
- capture sample of species, mark and release
- Ensure marking is not harmful/does not affect survival
- Allow time for organisms to randomly distribute before collecting second sample
- Population size = (number in sample x number in sample 2)/ number marked in sample 2
53
Explain how the mark-release recapture equation can be derived
number in marked sample 1/ total population size = number in marked sample 2 / total number in sample 2
54
What assumptions does the mark-release-capture method make?
1. Sufficient time for marked individuals to mix/distribute evenly within the population.
2. Marking not removed and doesn't affect chances of survival/predation.
3. Limited/ no immigration / emigration
4. No / few births/ deaths/ breeding/ change in population size
55
Give four conditions necessary for results from mark-release-recapture investigations to be valid. (4)
- No immigration / migration (Ignore references to emigration);
- No reproduction (Ignore references to death);
Idea of mixing;
- Marking does not influence behaviour / increase vulnerability
to predation;
- Sample / population large enough;
56
Suggest why the mark-release-recapture-method can produce unreliable results in **very** **large** areas
- unlikely that organisms will distribute randomly/evenly
- less chance of recapturing organisms ( that were marked initially)
57
(i) Using mark-release-recapture to estimate the size of a blue tit population **in June** would not give reliable results. Explain why. (2)
ii) (i) Using mark-release-recapture to estimate the size of a blue tit population in **March** would not give reliable results. Explain why. (2)
i) Population changes; As young birds leave nest / join population;
ii) Would be likely to) catch all birds (again) in second sample / sample sizes are the same;
Birds not mixing with population;
| Reject: population decreases
## Footnote
Accept only estimates number of birds in territories sampled / territory sample not representative (of population) for 1 mark
Accept: idea of the population is divided
58
define succession
- : change in a **community** over time due to change in **abiotic** factors/species.
59
describe what is meant by a climax community (1)
stable community / no further succession / final community;
60
Describe and explain how primary succession occurs
1. Colonisation by pioneer species ( first to colonise)
2. Pionner species change abiotic conditions
3. They die and decompose , forming soil which retains water ( humus/organic matter)
4. So environment becomes less hostile/more suitable for other species with different adaptations and less suitable for previous species, so better adapted species outcompete previous species
5. As succession goes on , biodiversity increases
6. Climax community reached - final stable community ( no further succession)
61
Describe features of a climax community
- same species present/stable community over a long time
- abiotic factors ( fairly) constant over time
- Populations stable around carrying capacity
62
Succession occurs in natural ecosystems. Describe and explain how succession occurs (5)
1. (Colonisation by) pioneer (species);
2. Change in environment / example of change caused by organisms present;
3. Enables other species to colonise / survive;
4. Change in diversity / biodiversity;
5. Stability increases / less hostile environment;
6. Climax community;
63
Describe the process of succession (5)
1. (Colonisation by) pioneer species
2. Pioneer species grows / reproduces = increases.
4. Pioneer species changes the environment / makes conditions less hostile / adds humus / nutrients.
6. Enables other / new / named species to colonise / survive.
7. New / named species better competitors. PS die.
8. Previous / named / pioneer species outcompeted.
11. Change in biodiversity / species composition.
12. Stability increases / community becomes more stable. Climax community is reached.
64
Explain how one can conserve habitats through succession management
- stop a climax community forming.
- So early species are not outcompeted by later species and habitats / niches are not lost.
- by removing or preventing growth of species associated with later stages e.g. by allowing grazing
- preserves an ecosystem at a certain point/in its current stage of succession.
65
Describe the conflict between human needs and conservation as well as the importance of managing this
- human demand for natural resources e.g. timber is leading to habitat destruction + biodiversity loss
- conservation protects habitats / niches/ species biodiversity
- this maintains the sustainability of natural resources
- meeting current needs w/o compromising the ability of future generations to meet theirs.
66
(d) Suggest four reasons for conserving woodlands. (2)
1. Conserving / protecting habitats / niches;
2. Conserving / protecting (endangered) species / maintains / increases (bio) diversity;
3. Reduces global warming / greenhouse effect / climate change / remove / take up carbon dioxide;
4. Source of medicines / chemicals / wood;
5. Reduces erosion / eutrophication.
| accept tourism
67
Hydrilla (Hydrilla verticillata) is an aquatic plant which has become a major pest of waterways in parts of the USA. (b) The spread of Hydrilla has had economic consequences for commercial activities and for the government’s environmental agency.
Suggest two economic consequences of the spread of Hydrilla. (2)
1. (Cost of) control / removal;
2. (Cost of) restoring habitat / conservation;
3. (Loss of income) from fishing;
4. (Loss of income) from boating / tourism / recreation;
68
Describe how you could investigate the effect of an environmental factor on the distribution of a species in a habitat ( random sampling in two areas)
| RP12
1. Divide two areas into grids / squares eg. place 2 tape measures at right angles
2. Generate a pair of coordinates using a random number generator (eg. on a calculator)
3. Place a quadrat here and count number / frequency of [named species]
4. Standardise this eg. only count it if it is more than half in the quadrat
5. Repeat a large number of times (10 or more) and calculate a mean per quadrat for both areas
6. Measure environmental factor in each area eg. take soil moisture readings with a soil moisture meter
69
Suggest why percentage cover may be used rather than frequency.
- individual organisms are too small to count
70
Explain why random sampling is used and the importance of a large sample size
| - RP12
- to avoid sampling bias
- **minimises the effect of anomialies** + ensures sample is representatiive of the population
71
Describe how to decide the number of quadrats that should be usied in order to colllect representative data.
- calculate a running mean
- when enough qudrats , this shows little change
- enough to carry out a statistical test.
72
Describe how you could investigate the effect of a factor on the distribution of a species in a habitat
| systematic sampling
1. Place a transect line across an area with a environmental gradient
2. Place quadrats at regular intervals e.g. 1m and record the number of organisms of species and environmental factor
3. Repeat in other parallel areas and calculate mean number of plants at each point along the transect.
73
Explain the limitations of using systematic sampling to estimate the population of a species in a field
- not apporpriate unless there is an environmental gradient
- transects run in one direction, but to cover the entire field they would need placing in multiple directions. - not fully representative
74
Which statistical test should be used to determine the relationship between abundance and an environmental factor
- correlation coefficient e.g. Spearman's rank
75
What is the phenotype and genotype ratio for Codominance?
– Heterozygous × Heterozygous (e.g. CRCW × CRCW)?
Genotype ratio: 1 : 2 : 1
Phenotype ratio: 1 : 2 : 1
(All three phenotypes are different)
76
What is the expected outcome for female offspring in a Sex Linkage – Carrier female (XᴺXⁿ) × Normal male (XᴺY)?
| N = not affected
Female offspring: 1 : 1 (XᴺXᴺ : XᴺXⁿ) – none affected
77
What is the expected outcome for male offspring in a Sex Linkage – Carrier female (XᴺXⁿ) × Normal male (XᴺY)?
Male offspring: 1 : 1 (XᴺY : XⁿY) – 50% affected
→ Only males can be affected if recessive X-linked
78
What is the effect on expected ratios like 9:3:3:1 in Autosomal Linkage?
The ratio is skewed (not 9:3:3:1)
→ Linked genes are inherited together
→ Less variation unless crossing over occurs
79
What is the ratio for:
- dominant epistasis
- recessive epistasis
- **Dominant** epistasis ratio: 12 : 3 : 1
→ A dominant allele in one gene masks expression of another gene
→ only when crossed two homozygous dominant
- **Recessive** epistasis ratio: 9 : 3 : 4
→ One recessive gene masks another gene’s effect
→ only when crossing two homozygous recessive.
