Inheritance

Question 1

What is the definition of “genotype”?

Answer 1

All of the alleles that an organism carries on its chromosomes

Question 2

What is the definition of “phenotype”?

Answer 2

The observable characteristics that are produced as a result of the expression of the genotype and its interaction with the environment.

Question 3

What are alleles and how do they arise?

Answer 3

Alleles are variations of a particular gene and they arise by mutations.

Question 4

How many alleles of a gene can be found in diploid organisms?

Answer 4

2 as diploid organisms have 2 sets of chromosomes, found in homologous pairs (one maternal, one paternal)

Question 5

What is a sex-linked gene?

Answer 5

A gene with a locus on a sex-chromosome (usually X)

Question 6

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

Answer 6

Females (XX) have 2 alleles, so they will only express the recessive allele if they are homozygous recessive. Males (XY) have 1 allele, so the recessive allele will always be expressed if present.

Question 7

Explain how autosomal linkage affects inheritance of alleles

Answer 7

Two genes are located on the same autosome, so the alleles are inherited together. They will stay together during independent segregation of homologous chromosomes during meiosis. Crossing over between homologous chromosomes can create new combinations of alleles but if the genes are closer together on an autosome, they are less likely to be split by crossing over

Question 8

What is epistasis?

Answer 8

Interaction of non-linked genes where one suppresses the expression of the other

Question 9

What is a population?

Answer 9

A group of interbreeding individuals of the same species

Question 10

What is a gene pool?

Answer 10

All the alleles of all the genes in a population at any one time

Question 11

What is allele frequency?

Answer 11

Proportion of an allele of a gene in a gene pool (decimal or percentage)

Question 12

What does the Hardy-Weinberg principle state and what are the conditions under which the principle applies?

Answer 12

It states that allele frequency will not change over generations. The conditions are:
* 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

Question 13

What is the Hardy-Weinberg equation?

Answer 13

p² + 2pq + q² = 1 and p + q = 1
* p = frequency of dominant allele
* q = frequency of recessive allele
* p² = frequency of homozygous dominant genotype
* 2pq = frequency of heterozygous genotype
* q² = frequency of homozygous recessive genotype

Question 14

What is evolution?

Answer 14

Change in allele frequency over time through the process of natural selection

Question 15

Describe factors that may drive natural selection

Answer 15

Predation, disease and competition for the means of survival will result in different survival and reproduction rates

Question 16

Describe allopatric speciation

Answer 16

• Population is separated due to geographical isolation
• This leads to reproductive isolation, separating gene pools by preventing interbreeding
• Random mutations cause genetic variation within each population
• Due to different environmental factors, there will be different selection pressures on the 2 populations
• Different advantageous alleles are selected for and therefore passed on in each population
• Over many generations, allele frequencies within each population change
• Eventually the different populations cannot interbreed to produce fertile offspring, therefore becoming separate species

Question 17

Describe sympatric speciation

Answer 17

• Population is not geographically isolated
• Instead, mutations lead to reproductive isolation, separating gene pools by preventing interbreeding. For example, gamete incompatibility, different breeding seasons, different courtship behaviour preventing mating, body shape/size changes preventing mating.
• There will be different selection pressures on the 2 populations
• Different advantageous alleles are selected for and therefore passed on in each population
• Over many generations, allele frequencies within each population change
• Eventually the different populations cannot interbreed to produce fertile offspring, therefore becoming separate species

Question 18

Explain genetic drift and its importance in small populations

Answer 18

• Genetic drift is when allele frequencies in a population change over generations due to chance (NOT due to natural selection)
• 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 with no interbreeding with other populations (no gene
  flow), as gene pool is small and chance has a greater influence
• This can reduce genetic diversity - some alleles have much higher frequencies, others are lost

Question 19

What is a community?

Answer 19

All the populations of different species living in the same habitat at the same time

Question 20

What is an ecosystem?

Answer 20

A community and the abiotic factors of its environment

Question 21

What is a niche?

Answer 21

The specific role of a species within its habitat, its interaction with biotic and abiotic factors

Question 22

What is carrying capacity?

Answer 22

The maximum stable population size of a species that an ecosystem can support

Question 23

Explain the advantage of species occupying different niches

Answer 23

There will be less competition for food and resources. If two species tried to occupy the same niche, one would outcompete the other

Question 24

List the factors that influence carrying capacity

Answer 24

Abiotic factors such as light intensity, temperature, soil pH & mineral content, humidity. Also, interactions between organisms such as Interspecific competition (between organisms of different species), Intraspecific competition (between organisms of the same species) and predation

Question 25

Explain how abiotic factors may affect population size / carrying capacity

Answer 25

If conditions are favourable, organisms are more likely to survive and reproduce, increasing carrying capacity.

Question 26

Explain how interspecific competition may affect population size

Answer 26

Reduces resources available to both species, limiting their chances of survival & reproduction. Therefore 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

Question 27

Explain how intraspecific competition may affect population size

Answer 27

As population size increases, resource availability per organism decreases, so intraspecific competition increases. This means chances of survival & reproduction decrease so population size decreases. As population size decreases, resource availability per organism increases, so competition decreases, so chances of survival & reproduction increase and population size increases

Question 28

Explain the changes which occur in populations of predators & prey

Answer 28

Populations fluctuate in cycles, the predator population peaking after the prey. * Prey population increases so predators have more food, so more predators survive & reproduce * Predator population increases so more prey are killed & eaten, so less prey survive & reproduce * Prey population decreases so predators have less food, so less predators survive & reproduce * Predator population decreases so less prey are killed & eaten, so more prey survive & reproduce (cycle repeats)

Question 29

Describe how the size of a population of slow-moving or non-motile organisms can be estimated

Answer 29

* Divide area into a grid * Generate a pair of coordinates using a random number generator * Place a quadrat here and count number / frequency of organism * Repeat a large number of times (10 or more) and calculate a mean per quadrat * Population size = (total area of habitat / quadrat area) x mean per quadrat

Question 30

Describe how the mark-release-recapture method can be used to estimate the size of a population of motile organisms

Answer 30

* Capture sample of species, mark and release * Ensure marking is not harmful / does not affect rate of 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

Question 31

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

Answer 31

* Sufficient time for marked individuals to mix / distribute evenly within the population * Marking is not removed or marking doesn’t affect chances of survival * No immigration / emigration 4. Birth & death rate are equal

Question 32

Suggest why the mark-release-recapture method can produce unreliable results in very large areas

Answer 32

It is unlikely that organisms will distribute randomly and evenly. Also, there is less chance of recapturing organisms that were marked initially

Question 33

Describe and explain how primary succession occurs

Answer 33

* Colonisation by a pioneer species * This results in a change abiotic conditions * So the 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 * As succession continues,, biodiversity increases until climax community reached (no further succession)

Question 34

Describe features of a climax community

Answer 34

* The species will be stable community over a long time * Abiotic factors remain constant over time