Inheritance Flashcards
What is the definition of “genotype”?
All of the alleles that an organism carries on its chromosomes
What is the definition of “phenotype”?
The observable characteristics that are produced as a result of the expression of the genotype and its interaction with the environment.
What are alleles and how do they arise?
Alleles are variations of a particular gene and they arise by mutations.
How many alleles of a gene can be found in diploid organisms?
2 as diploid organisms have 2 sets of chromosomes, found in homologous pairs (one maternal, one paternal)
What is a sex-linked gene?
A gene with a locus on a sex-chromosome (usually X)
Why are males more likely to express a recessive X-linked allele?
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.
Explain how autosomal linkage affects inheritance of alleles
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
What is epistasis?
Interaction of non-linked genes where one suppresses the expression of the other
What is a population?
A group of interbreeding individuals of the same species
What is a gene pool?
All the alleles of all the genes in a population at any one time
What is allele frequency?
Proportion of an allele of a gene in a gene pool (decimal or percentage)
What does the Hardy-Weinberg principle state and what are the conditions under which the principle applies?
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
What is the Hardy-Weinberg equation?
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
What is evolution?
Change in allele frequency over time through the process of natural selection
Describe factors that may drive natural selection
Predation, disease and competition for the means of survival will result in different survival and reproduction rates
Describe allopatric speciation
- 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
Describe sympatric speciation
- 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
Explain genetic drift and its importance in small populations
- 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
What is a community?
All the populations of different species living in the same habitat at the same time
What is an ecosystem?
A community and the abiotic factors of its environment
What is a niche?
The specific role of a species within its habitat, its interaction with biotic and abiotic factors
What is carrying capacity?
The maximum stable population size of a species that an ecosystem can support
Explain the advantage of species occupying different niches
There will be less competition for food and resources. If two species tried to occupy the same niche, one would outcompete the other
List the factors that influence carrying capacity
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
Explain how abiotic factors may affect population size / carrying capacity
If conditions are favourable, organisms are more likely to survive and reproduce, increasing carrying capacity.
Explain how interspecific competition may affect population size
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
Explain how intraspecific competition may affect population size
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
Explain the changes which occur in populations of predators & prey
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)
Describe how the size of a population of slow-moving or non-motile
organisms can be estimated
- 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
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 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
What assumptions does the mark-release-recapture method make?
- 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
Suggest why the mark-release-recapture method can produce unreliable results in very large areas
It is unlikely that organisms will distribute randomly and evenly. Also, there is less chance of recapturing organisms that were marked initially
Describe and explain how primary succession occurs
- 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)
Describe features of a climax community
- The species will be stable community over a long time
- Abiotic factors remain constant over time
