I created these notes throughout y12&13 and used them for my A level 2022 exams. I got a grade 9 in GCSE and am awaiting for my a level biology result. These notes were made using the CGP revision guide, AQA Biology textbook and class notes in order to cover everything in the course. I have also ad...
18.1 POPULATION GENETICS
17 November 2021
18:32
Population - Group of organisms of the same species that occupy a particular space
at a given time.
Gene pool - All the alleles of all the genes in a population at a given time.
Allelic Frequency - Number of times an allele occurs within the gene pool.
Ecosystem - All living and non-living components for a particular area.
Community - All the living organisms present in an ecosystem at a given time.
Habitat - Place where an organism usually lives.
● Characterised by physical conditions and other organisms present.
Genetic diversity - Variety and number of different alleles possessed by the
individuals that make up a population of a species.
The Hardy-Weinberg principle can be used to calculate the frequencies of alleles of a
particular gene in a population.
The principle predicts that the allele frequencies will not change from generation to
generation.
For this to be met, there are 5 conditions:
● No mutations arise.
● Population is isolated, no flow of alleles into or out of the population.
● No selection, and so alleles are equally as likely to be passed on.
● The population is large.
● Mating within the population is random.
, 18.2 VARIATION IN PHENOTYPE
17 November 2021
19:38
Phenotype - Observable or biochemical characteristics due to expression of the
genotype and its interaction with the environment.
Individuals of the same species have different alleles, causing genetic variation within
a species.
Genetic variation arises from:
● Mutations.
● New combinations of gametes are produced from independent segregation
and crossing over in meiosis.
● Random fertilisation of gametes.
Variation within a species can also be caused by differences in the environment.
● Climatic conditions.
● i.e temperature, rainfall, sunlight...
● Soil condition.
● pH
● Food availability.
In most cases, variation is caused by both genetic and environmental factors.
Discontinuous variation has only a few categories.
● Represented as bar or pie charts.
Found in characteristics controlled by one or very few genes.
● i.e tongue rolling, blood group.
Environmental factors have little effect.
Continuous variation is distributed on a continuum.
● Shows the normal distribution range.
Found in polygenetic characteristics.
● i.e height, weight, skin colour, leaf width.
Can be affected by environmental factors.
18.3 NATURAL SELECTION
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