Biology a level topic 7 aqa PMT

Define genotype.

The genetic constitution of an organism.

Define phenotype.

The expression of an organism's genetic constitution, combined with its interaction with the environment.

What is an allele?

Different forms of a particular gene, found at the same locus (position) on a chromosome. A single gene could have many alleles.

How many alleles per gene do diploid organisms carry?

Two.

What is meant by a dominant allele?

An allele whose characteristic will always appear in the phenotype, whether one or two are present.

What is meant by a recessive allele?

An allele whose characteristic only appears in the phenotype if no dominant allele is present, meaning two must be present.

What is meant by codominant alleles?

Two dominant alleles that both contribute to the phenotype, either by showing a blend of both characteristics, or the characteristics appearing together.

What is meant by homozygous and heterozygous?

Homozygous= both alleles are dominant, or both alleles are recessive.
Heterozygous= one allele is dominant, the other is recessive.

Define monohybrid inheritance.

Where one phenotypic characteristic is controlled by a single gene.

Draw a Punnett square to show a monohybrid cross with parental

genotypes of GG and gg.
G
G
g
Gg
Gg
g
Gg
Gg

What % of these offspring will express the characteristic determined by allele G?

100% all potential offspring genotypes are Gg meaning none of them will express the recessive allele.

Draw a Punnett square to show a monohybrid cross with parental genotypes of Gg and Gg.

G
g
G
GG
Gg
g
Gg
gg

What % of these offspring should express the characteristic determined by allele G?

75% only one of the combinations results in expression of the recessive allele.

Draw an example of a genetic diagram.

Parental phenotypes
Parental genotypes
Gametes
Offspring genotypes (draw a punnet square)
Offspring phenotypes
Brown eyes Blue eyes Bb bb
Bb bb
Bb, Bb, bb, bb
2:2 brown eyes:blue eyes

Define dihybrid inheritance.

Where two phenotypic characteristics are determined by two different genes present on two different chromosomes at the same time.

Draw a Punnett square to show a dihybrid cross with parental genotypes of RrGg and RrGg.

RG
Rg
rG
rg
RG
RRGG
RRGg
RrGG
RrGg
Rg
RRGg
RRgg
RrGg
Rrgg
rG
RrGG
RrGg
rrGG
rrGg
rg
RrGg
Rrgg
rrGg
rrgg

How many of these offspring should have the same phenotype as their parents?

9 (out of 16)
6 will match their parents on one characteristic, but differ on the other.
1 will differ on both characteristics.

What is meant by sex-linkage?

Where an allele is located on one of the sex chromosomes, meaning its expression depends on the sex of the individual.

Why are males more likely to express a recessive sex-linked allele?

Most sex-linked alleles are located on the X chromosome. Therefore males only get one copy of the allele, so will express this characteristic even if it's recessive. Since
females get two alleles, this is less likely.

Which parent do males inherit sex-linked characteristics from?

Their mother, since the Y chromosome can only come from their father. Therefore if the mother is heterozygous for sex-linked alleles, she is a carrier and may pass on the trait on.

Draw a Punnett square to show a sex-linked cross with parental genotypes of XHXh and XHY.

XH
Xh
XH
XHXH
XHXh
Y
XHY
XhY

Describe the four possible phenotypes of these offspring.

Normal female, carrier female, normal male, affected male.

What is meant by autosomal linkage?

Where two or more genes are located on the same (non-sex) chromosome. In this case, only one homologous pair is needed for all four alleles to be present. For genes that aren't linked, two homologous pairs are needed.

Draw a Punnett square to show a multiple alleles cross with parental genotypes of IAIO and IBIO.

IA
IO
IB
IAIB
IBIO
IO
IAIO
IOIO

If IA and IB are codominant and IO is recessive, which alleles could the offspring express?

AB, A, B, O

What is meant by epistasis?

Where two non-linked genes interact, with one gene either masking or suppressing the other gene.

Define the two types of epistasis.

Recessive epistasis= where two homozygous recessive alleles mask expression of another allele.
Dominant epistasis= where one dominant allele masks expression of multiple other alleles.

Draw a Punnett square to show an epistasis cross with parental genotypes of AABB and aabb.

AB
Ab
aB
ab
AB
AABB
AABb
AaBB
AaBb
Ab
AABb
AAbb
AaBb
Aabb
aB
AaBB
AaBb
aaBB
aaBb
ab
AaBb
Aabb
aaBb
aabb

Genotypes BB or Bb allow expression of gene A, while genotype bb masks gene A. With this in mind, what % of the offspring will have gene A masked?

25%

What is the chi-squared test?

A statistical test to find out whether the difference between observed and expected data is due to chance or a real effect.

What are the criteria for the chi-squared test?

● Data placed in discrete categories
● Large sample size
● Only raw count data allowed i.e. not
percentages
● No data values equal zero

How is a chi-squared test performed?

The formula results in a number, which is then compared to a critical value (for the corresponding degrees of freedom). If the number is greater than or equal to the critical value, we
conclude there is no significant difference and the results occured due to chance.

How can we use a chi-squared test in relation to the content of this topic?

We can compare expected phenotypic ratios with observed ratios to test our understanding of how different genes and alleles are inherited.

7.2


Define species.

A group of organisms that can interbreed to produce fertile offspring.

Define population.

All the organisms of a particular species that live in the same place.

Define gene pool.

The range of different alleles existing for a particular locus within a population.

Define allele frequency.

The proportion of a certain allele within a gene pool, expressed as a decimal or percentage.

What is the Hardy-Weinberg principle?

Allows us to estimate the frequency of alleles in a population, as well as if allele frequency is changing over time.

Give the assumptions made by the Hardy-Weinberg principle.

● No mutations occur to create new alleles.
● No migration in or out of the population.
● No selection, so alleles are all equally passed
on to the next generation.
● Random mating.
● Large population.

Explain the Hardy-Weinberg equation for calculating allele frequency.

The frequencies of each allele for a characteristic must add up to 1.0. The equation is therefore p + q = 1
Where p= frequency of the dominant allele, and q= frequency of the recessive allele.

Explain the Hardy-Weinberg equation for calculating genotype frequency.

The frequencies of each genotype for a characteristics must add up to 1.0. The equation is therefore p2 + 2pq + q2 = 1
Where p2= frequency of homozygous dominant, 2pq= frequency of heterozygous, and q2= frequency of homozygous recessive.

7.3




Give genetic factors that cause phenotypic variation within a species.

1. Mutation of alleles.
2. Random fertilisation by gametes. 3. Random assortment of genetic
material during meiosis.

Other than genetic factors, why else may phenotype vary within a species?

Environmental influences.

Why does natural selection occur?

1. Predation 2. Disease
3. Competition
All resulting in differential survival and reproduction.

How does natural selection cause a change in a population's gene pool over generations?

Organisms with advantageous characteristics are more likely to survive and pass their favourable alleles to offspring. Frequency of unfavourable
alleles decreases.

What is stabilising selection?

Occurs when environmental conditions stay the same. Individuals closest to the mean are favoured, and any new characteristics are selected against. Results in low diversity.

What is directional selection?

Occurs when environmental conditions change. Individuals with phenotypes suited to the new conditions will survive and pass on their genes. Over time the mean of the population will move towards these characteristics.

What is disruptive selection?

The opposite of stabilising selection, in that both extremes of the normal distribution are favoured over the mean. Over time, the population becomes phenotypically Divided

Define speciation.

Where a population is split and isolated, there are different selective pressures on the two groups. If the genetic makeup changes to the extent the two groups can not longer interbreed, they have become separate species.

What is meant by allopatric speciation?

Speciation resulting from a physical barrier e.g. river, mountain range. The environments occupied by the two groups are different, and therefore different alleles are favoured.

What is meant by sympatric speciation?

Speciation resulting from a non-physical barrier e.g. a mutation that no longer allows two organisms to produce fertile offspring. Any changes in anatomy or behaviour may also prevent breeding.

Define genetic drift.

A change in a population's allele frequencies that occurs due to chance rather than selective pressures. In other words, it is caused by 'sampling error' during reproduction.

Why does genetic drift affect small populations more than large ones?

The gene pool is smaller, so there are less alleles available and any change in frequency becomes pronounced very quickly.

7.4




Define community.

All the different species that live in one area and interact with each other.

Define ecosystem.

All the living organisms found in one area, combined with non-living aspects of their environment. Can vary from very large to very small.

Describe biotic and abiotic factors, giving examples.

Biotic= living features of an ecosystem e.g. predators, disease.
Abiotic= non-living features of an ecosystem e.g. light, temperature.

Define habitat.

The place where an organism lives within an ecosystem.

Define a niche.

The role of a species within its habitat, consisting of both its biotic interactions e.g. what it eats, and abiotic interactions e.g. time of day it is active.

What is meant by carrying capacity?

The maximum size of population an ecosystem can support.

Name four abiotic factors that affect population growth.

1. Temperature 2. Light
3. pH
4. Water/humidity

What is meant by intraspecific and interspecific competition?

Intraspecific = competition between organisms of the same species.
Interspecific = competition between organisms of different species.

What resources might organisms compete for?

Food, water, shelter, minerals, light, mates (intraspecific only).

Describe the pattern of a typical predator-prey relationship in terms of population change.

● Prey is eaten by predator, resulting in predator population increasing and prey population decreasing.
● Fewer prey means increased competition for food, so predator population decreases.
● Fewer predators means more prey survives, and the cycle begins again.

How are quadrats used for estimating population size?

Can be placed on grid coordinates, or at intervals along a belt transect. Results reported as either percentage cover or frequency. For slow-moving or non-motile organisms.

How is mark-release-recapture used for estimating population size?

● A sample of a species is captured, marked, then released back into the same area they were caught.
● After a certain period of time another sample of the is captured, and the number of marked organisms are counted.
For motile organisms.

What is the equation for mark-release-recapture?

Estimated population size =
total number of individuals in the first sample X total number of individuals in the second sample
number of marked individuals recaptured

What assumptions does the mark-release-recapture method make?

● Marked individuals distribute evenly.
● No migration in or out of the population.
● Few births or deaths.
● Method of marking does not affect survival. ● Mark does not come off.

Why are ecosystems described as being dynamic?

● Populations constantly rise and fall.
● Any small change can have a large
effect.
● Biotic and abiotic factors may alter the
conditions of the environment

What is meant by primary succession?

Where an area previously devoid of life is colonised by a community of organisms.

Summarise the process of primary succession.

● Pioneer species can survive harsh conditions & colonise the area.
● They change abiotic factors of their environment e.g. decomposition adds nutrients to ground.
● Over time, this allows more complex organisms to survive.

What is the climax community and how is it reached?

The final stage of succession, where the ecosystem is balanced and stable. It is reached when the soil is rich enough to support large trees or shrubs, and the
environment is no longer changing.

How might a species alter the environment that develops during succession?

A species may improve the environment to make it more suitable for other species. Alternatively, a species may worsen the environment by making it less suitable for other species.

Define conservation.

The protection and management of species and habitats, in order to maintain biodiversity. Methods need to be adapted to the ecosystem in question.

How might succession be managed in order to aid conservation?

Sometimes succession needs to be prevented in order to preserve an ecosystem at a certain point, e.g. stopping moorland from progressing into spruce forest. This is called a plagioclimax.