Topic 7 - Genetics Flashcards
1
Q
What is a genotype?
A
Genetic constitution of an organism.
2
Q
What is meant by a phenotype?
A
The expression of the genotype and its interaction with the environment in physical traits.
3
Q
What is an allele?
A
Variation of a gene.
4
Q
How many alleles of a gene can be found in diploid organisms?
A
2 as diploid organisms have 2 sets of chromosomes.
5
Q
What is a dominant allele?
A
Always expressed in the phenotype.
6
Q
What is a recessive allele?
A
Only expressed when 2 copies present (homozygous recessive) / NOT expressed when dominant allele present (heterozygous).
7
Q
What are co-dominant alleles?
A
Both alleles expressed / contributed to phenotype (if inherited together).
8
Q
What is meant by homozygous?
A
Alleles at a specific locus are the same.
9
Q
What is meant by heterozygous?
A
Alleles at a specific locus are different.
10
Q
What is a monohybrid cross?
A
Inheritance of one phenotypic characteristic coded for by a single gene.
11
Q
What is meant by dihybrid cross?
A
Inheritance of two phenotypic characteristics coded for by two different genes.
12
Q
What is a sex-linked gene?
A
A gene with a locus on a sex-chromosome (normally X).
13
Q
Explain why males are more likely to express a recessive X-linked allele? Assuming males are XY and females are XX.
A
Females (XX) have 2 alleles → only express recessive allele if homozygous recessive / can be carriers.
Males (XY) have 1 allele (inherited from mother) → recessive allele always expressed.
14
Q
What is an autosome?
A
Non-sex chromosome.
15
Q
Explain how autosomal linkage affects inheritance of alleles.
A
Two genes located on the same autosome, so alleles on the same chromosome are inherited together.
16
Q
What is epistasis?
A
Interaction of products of non-linked genes where one suppresses the expression of the other.
17
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.
- Autosomal linkage / epistasis / sex-linkage.
- Small sample size.
- Some genotypes may be lethal.
18
Q
What is the chi-squared value equation?
A
x^2 = ∑(O – E) 2 / E
19
Q
In the chi-squared equation, what do the O and the E stand for?
A
O = frequencies observed.
E = frequencies expected.
20
Q
Describe how a chi-squared value can be analysed.
A
- Number of degrees of freedom = number of categories - 1 (eg. 4 phenotypes = 3 degrees of freedom)
- Determine critical value at p = 0.05 (5% probability) from a table
- If X2 value is [greater / less] than critical value at p < 0.05
● Difference [is / is not] significant so [reject / accept] null hypothesis
● So there is [less / more] than 5% probability that difference is due to chance
21
Q
What is a population?
A
A group of organisms of the same species in one area at one time that can be interbred.
22
Q
What is a gene pool?
A
All the alleles of all of the genes in a population at any one time.
23
Q
What is allele frequency?
A
Proportion of an allele in a gene pool.
24
Q
What does the Hardy Weinberg principle state and what are the conditions under which the principles 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.
- Mating is random.
25
What is the Hardy-Weinberg equation? Please specify what each letter stands for.
p2 +2pq+q2 =1
This can be used simultaneously with:
p+q=1
● p = frequency of one (usually dominant) allele of the gene
● q = frequency of the other (usually recessive) allele of the gene
● p2 = frequency of homozygous (usually dominant) genotype
● 2pq = frequency of heterozygous genotype
● q2 = frequency of homozygous (usually recessive) genotype
26
Explain why individuals within a population of a species may show a wide range of variation in phenotype.
● Genetic factors
- Mutations = primary source of genetic variation
- Crossing over between homologous chromosomes during meiosis
- Independent segregation of homologous chromosomes during meiosis
- Random fertilisation of gametes during sexual reproduction
● Environmental factors (depends on context - eg. food availability, light intensity)
27
What is evolution?
Change in allele frequency over time / many generations in a population, occurring through the process of natural selection.
28
Describe factors that may drive natural selection.
● Predation, disease and competition for the means of survival.
● These result in differential survival and reproduction, ie. natural selection.
28
Explain the principles of natural selection in the evolution of populations.
1. Mutations
Random gene mutations can result in [named] new alleles of a gene.
2. Advantage
Due to [named] selection pressure, the new allele might benefit its possessor [explain why] → organism has a selective advantage.
3. Reproductive success
Possessors are more likely to survive and have increased reproductive success.
4. Inheritance
Advantageous allele is inherited by members of the next generation (offspring).
5. Allele frequency
Over many generations, [named] allele increases in frequency in the gene pool.
28
Explain the effects of stabilising selection.
Organisms with alleles coding for average / modal variations of a trait have a selective advantage (eg. babies with an average weight) so frequency of alleles coding for average variations of a trait increase and those coding for extreme variations of a trait decrease so range / standard deviation is reduced.
28
Explain the effects of directional selection.
Organisms with alleles coding for one extreme variation of a trait have a selective advantage (eg. bacteria with high resistance to an antibiotic) so frequency of alleles coding for this extreme variation of the trait increase and those coding for the other extreme variation of the trait decrease.
28
Explain the effects of disruptive selection.
Organisms with alleles coding for either extreme variation of a trait have a selective advantage so frequency of alleles coding for both extreme variations of the trait increase and those coding for the average variation of the trait decrease. This can lead to speciation.
28
Describe speciation (how new species arise from existing species).
1. Reproductive separation of two populations (of the same species).
2. This can result in accumulation of differences in their gene pools.
3. New species arise when these genetic differences lead to an inability of
members of the populations to interbreed and produce fertile offspring.
29
Describe allopatric speciation.
1. Population is split due to geographical isolation (eg. new river formed).
2. This leads to reproductive isolation, separating gene pools by preventing
interbreeding / gene flow between populations
3. Random mutations cause genetic variation within each population.
4. Different selection pressures / environments act on each population.
5. So different advantageous alleles are selected for / passed on in each population.
6. So allele frequencies within each gene pool change over many generations.
7. Eventually different populations cannot interbreed to produce fertile offspring.
29
Describe sympatric speciation.
1. Population is not geographically isolated.
2. Mutations lead to reproductive isolation, separating gene pools by preventing.
interbreeding / gene flow within one population, eg.
● Gamete incompatibility
● Different breeding seasons (eg. different flowering times)
● Different courtship behaviour preventing mating
● Body shape / size changes preventing mating
3. Different selection pressures act on each population.
4. So different advantageous alleles are selected for / passed on in each population.
5. So allele frequencies within each gene pool change over many generations.
6. Eventually different populations cannot interbreed to produce fertile offspring.
30
Explain genetic drift and its importance in small populations.
Genetic drift = a mechanism of evolution in which allele frequencies in a population change over generations due to chance.
As some alleles are passed onto offspring more / less often by chance.
Regardless of selection pressures and whether alleles give a selective advantage.
So strongest effects in small populations as gene pool is small and chance has a greater influence.
Eg. when a population is sharply reduced in size (bottleneck effect).
Eg. 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.
31
What is an ecosystem?
A community and the non-living (abiotic) components of its environment. Ecosystems can range in size from very small to very large. They are dynamic systems (populations rise / fall over time).
32
What is a niche?
The specific role of a species within its habitat, eg. what it eats, where and when it feeds. Governed by its adaptation to both abiotic (non-living) and biotic (living) conditions.
33
Explain the advantage of species occupying different niches.
Less competition for food / resources. If two species tried to occupy the same niche, one would outcompete the other.
33
What is carrying capacity?
The maximum (stable) population size of a species that an ecosystem can support.
33
List the factors that influence carrying capacity.
Abiotic factors:
Eg. light intensity, temperature, soil pH & mineral content, humidity
Interactions between organisms:
a. Interspecific competition - between organisms of different species
b. Intraspecific competition - between organisms of the same species
c. Predation (predators kill and eat other animals, called prey)
33
Explain how abiotic factors may affect population size / carrying capacity.
If conditions favourable, organisms more likely to survive & reproduce → increasing carrying capacity.
Eg. increasing light intensity increases rate of photosynthesis in plants.
This increases carrying capacity of a variety of plant species so increases the number and variety of habitats, niches and food sources for animals so increasing carrying capacity of a variety of animal species.
34
Explain how interspecific competition may affect population size.
Reduces [named resource] available to both species, limiting their chances of survival & reproduction ○ So reduces population size of both species. If one species is better adapted, it will outcompete the other
so population size of less well adapted species declines, potentially leading to extinction.
34
Explain how intraspecific competition may affect population size.
As population size increases, resource availability per organism decreases, so competition increases so chances of survival & reproduction decrease → population size decreases.
As population size decreases, resource availability per organism increases, so competition decreases so chances of survival & reproduction increase → population size increases.
34
Explain the changes which occur in populations of predators & prey.
Prey population increases so predators have more food so more predators survive & 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 & reproduce.
4. Predator population decreases so less prey killed & eaten so more prey survive & reproduce (cycle repeats).
34
Describe how the size of a population of slow-moving or non-motile organisms can be estimated.
1. Divide area into a grid / 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. Repeat a large number of times (10 or more) and calculate a mean per quadrat.
5. Population size = (total area of habitat / quadrat area) x mean per quadrat
34
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 = (number in sample 1 x number in sample 2) / number marked in sample 2
35
Explain how the mark release recapture equation can be derived.
Number (marked) in sample 1 / Total population size.
=
Number marked in sample 2 / Total number in sample 2.
35
What assumptions does the mark-release-recapture 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 (or birth & death rate are equal).
35
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).
36
Describe and explain how primary succession occurs.
1. Colonisation by pioneer species (first to colonise).
2. Pioneer species (and other species at each stage in succession) change abiotic conditions
● Eg. they die and decompose, forming soil which retains water (humus / organic matter).
3. 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.
4. As succession goes on, biodiversity increases.
5. Climax community reached - final stable community (no further succession).
36
What is succession?
A change in a community over time due to change in abiotic factors / species.
36
Describe features of a climax community.
Same species present / stable community over a long time.
Abiotic factors (fairly) constant over time.
Populations (fairly) stable (around carrying capacity).
36
Explain how conservation of habitats involves management of succession.
Further succession can be prevented to stop a climax community forming.
By removing or preventing growth of species associated with later stages eg. by allowing grazing.
This preserves an ecosystem at a certain point / in its current stage of succession (plagioclimax).
So early species are not outcompeted by later species and habitats / niches are not lost.
37
Describe the conflict between human needs and conservation as well as the importance of managing this.
Human demand for natural resources (eg. timber) is leading to habitat destruction / biodiversity loss.
Conservation is needed to protect habitats / niches / species / biodiversity.
Management of this conflict maintains the sustainability of natural resources.
Meeting current needs without compromising the ability of future generations to meet theirs.
37
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).
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]
○ Standardise this eg. only count it if it is more than half in the quadrat.
4. Repeat a large number of times (10 or more) and calculate a mean per quadrat for both areas.
5. Measure environmental factor in each area eg. take soil moisture readings with a soil moisture meter.
38
Suggest why percentage cover may be used rather than frequency.
Too difficult to count individual organisms / individual organisms are too small to count.
38
Explain why random sampling is used.
To avoid sampling bias.
39
Explain the importance of a large sample size.
Minimises the effect of anomalies.
Ensures sample is representative of the population.
40
Describe how you could decide the number of quadrats that should be used in order to collect representative data.
Calculate a running mean.
When enough quadrats, this shows little change.
Enough to carry out a statistical test.
41
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 (tape measure) across an area with an environmental gradient eg. tree to full sun.
2. Place quadrats at regular intervals eg. 1m (until end of transect) and record the number of organisms of
[named species] and [named environmental factor] eg. light intensity using a light meter.
3. Repeat in other parallel areas and calculate mean number of plants at each point along the transect.
42
Explain the limitations of using systematic sampling to estimate the population of a species in a field.
Not appropriate unless there is an environmental gradient.
Transects run in one direction, but to cover the entire field,
they would need placing in multiple directions
43
Which statistical test should be used to determine the relationship between abundance and an environmental factor?
Correlation coefficient eg. Spearman’s rank.
44
What is a gene?
A sequence of bases on a DNA molecule that codes for a protein resulting in a characteristic.
45
What is an allele?
Different versions of a gene.
46
What is the loci?
Fixed position of a gene on a chromosome.
47
What is a dominant allele?
An allele that is always expressed in the phenotype even when there's only one copy of it.
48
What are co-dominant alleles?
Both alleles are expressed in the phenotype because neither is recessive.
49
What is Monohybrid inheritance.
The inheritance of a characteristic controlled by a single gene.
50
What is dihybrid inheritance?
The inheritance of two characteristics which are controlled by different genes.
51
What does sex-linked mean?
The alleles code for characteristics are located on a sex chromosome.
52
What is a recessive epistatic allele?
Two copies of it will mask the expression of the other gene.
53
What is a dominant epistatic alleles?
Having at least one copy of it will mask the expression of the other gene.
54
What does chi-squared test, test?
A statistical test that is used to see if the results of an experiment support a theory.
55
What is a species?
A group of similar organisms that can reproduce to give fertile offspring.
56
What is a gene pool?
Complete range of alleles present in a population.
