Genetic Variation (AQA A Level Biology): Revision Note
Exam code: 7402
Genetic variation
Phenotype variation
An organism’s phenotype is its observable characteristics.
Phenotypic variation refers to differences in these traits among individuals of the same species
This variation can arise from:
Genetic factors
e.g. Human blood groups result from combinations of three alleles at the ABO gene
Environmental factors
e.g. Genetically identical plants may grow to different heights in different conditions
Both genetic and environmental factors
e.g. The sickle cell allele is maintained in some populations because heterozygotes are resistant to malaria
Examiner Tips and Tricks
The phenotypic variation of the individuals in a population is determined by the genetic variation within the population and the interaction of the environment on the individuals:
Phenotypic variation = genetic variation + environmental influence
Genetic variation
Although members of the same species have similar genomes, small differences in their DNA base sequences create genetic variation.
Genetic variation is transferred from one generation to the next and it generates phenotypic variation within a species population
This variation arises mainly from mutations, which create new alleles
These alleles may be beneficial, harmful, or have no effect
Some may remain unexpressed for generations
Genetic variation also results from sexual reproduction, through:
Crossing over of non-sister chromatids during prophase I of meiosis
Independent assortment of chromosomes during metaphase I of meiosis
Random fertilisation of gametes during fertilisation
Process | Mechanism | Consequences |
|---|---|---|
Independent assortment of homologous chromosomes during metaphase I (See more detailed notes on independent assortment) | Random alignment of chromosomes results in different combinations of chromosomes and different allele combinations in each gamete | Genetic variation between gametes produced by an individual |
Crossing over of non-sister chromatids during prophase I (See more detailed notes on crossing over) | Exchange of genetic material between non-sister chromatids leads to new combinations of alleles on chromosomes. It can also break the linkage between genes | Genetic variation between gametes produced by an individual |
Random fusion of gametes during fertilisation (See more detailed notes on random fertilisation) | Any male gamete can fuse with any female gamete (random mating in a species population) | Genetic variation between zygotes and resulting individuals |
Mutation (see more detailed notes on mutation) | Random change in the DNA base sequence results in the generation of a new allele. A mutation must occur in gametes to be passed on | Genetic variation between individuals within a species population |
These processes generate new allele combinations in offspring, increasing diversity within a population.
Examiner Tips and Tricks
Some questions in the exam may ask you to explain why the variation in phenotype due to genetics is inherited but the variation in phenotype due to environmental factors is not. This is because genetic variation directly affects the DNA of the gametes but variation in phenotype caused by the environment does not.
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