AQA A Level Biology

Revision Notes

7.3.1 Genetic Variation

Genetic Variation

Phenotype variation

  • The observable characteristics of an organism are its phenotype
  • Phenotypic variation is the difference in phenotypes between organisms of the same species
  • This variation means that the individuals within a population of a species may show a wide range of variation in phenotype
  • In some cases, phenotypic variation is explained by genetic factors
    • For example, the four different blood groups observed in human populations are due to different individuals within the population having two of three possible alleles for the single ABO gene
  • In other cases, phenotypic variation is explained by environmental factors
    • For example, clones of plants with exactly the same genetic information (DNA) will grow to different heights when grown in different environmental conditions
  • Phenotypic variation can also be explained by a combination of genetic and environmental factors
    • For example, the recessive allele that causes sickle cell anaemia has a high frequency in populations where malaria is prevalent due to heterozygous individuals being resistant to malaria
  • The phenotype 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:

Phenotype variation = Genetic variation + Environment

Genetic variation

  • Organisms of the same species will have very similar genotypes, but two individuals (even twins) will have differences between their DNA base sequences
  • Considering the size of genomes, these differences are small between individuals of the same species
  • The small differences in DNA base sequences between individual organisms within a species population is called genetic variation
  • Genetic variation is transferred from one generation to the next and it generates phenotypic variation within a species population
  • The primary source of genetic variation is mutation (changes in the DNA base sequence)
  • Mutation results in the generation of new alleles
    • The new allele may be advantageous, disadvantageous or have no apparent effect on phenotype
    • New alleles are not always seen in the individual that they first occur in
    • They can remain hidden (not expressed) within a population for several generations before they contribute to phenotypic variation
  • Genetic variation is also caused by the following processes as they result in a new combination of alleles in a gamete or individual:
    • Crossing over of non-sister chromatids during prophase I of meiosis
    • Independent assortment of homologous chromosomes during metaphase I of meiosis
    • Random fusion of gametes during fertilization

Crossing over

  • Crossing over is the process by which non-sister chromatids exchange alleles
  • Process:
    • During meiosis I homologous chromosomes pair up and are in very close proximity to each other
    • The non-sister chromatids can cross over and get entangled
    • These crossing points are called chiasmata
    • The entanglement places stress on the DNA molecules
    • As a result of this a section of chromatid from one chromosome may break and rejoin with the chromatid from the other chromosome
  • This swapping of alleles is significant as it can result in a new combination of alleles on the two chromosomes
  • There is usually at least one, if not more, chiasmata present in each bivalent during meiosis
  • Crossing over is more likely to occur further down the chromosome away from the centromere

Crossing over (1), downloadable AS & A Level Biology revision notesCrossing over (2), downloadable AS & A Level Biology revision notes

Crossing over

Independent assortment

  • Independent assortment is the production of different combinations of alleles in daughter cells due to the random alignment of homologous pairs along the equator of the spindle during metaphase I
  • The different combinations of chromosomes in daughter cells increases genetic variation between gametes
  • In prophase I homologous chromosomes pair up and in metaphase I they are pulled towards the equator of the spindle
    • Each pair can be arranged with either chromosome on top, this is completely random
    • The orientation of one homologous pair is independent / unaffected by the orientation of any other pair
  • The homologous chromosomes are then separated and pulled apart to different poles
  • The combination of alleles that end up in each daughter cell depends on how the pairs of homologous chromosomes were lined up
  • To work out the number of different possible chromosome combinations the formula 2n can be used, where n corresponds to the number of chromosomes in a haploid cell
  • For humans this is 223 which calculates as 8,324,608 different combinations

Independent assortment (1), downloadable AS & A Level Biology revision notesIndependent assortment (2), downloadable AS & A Level Biology revision notes

Independent assortment

Random fertilisation of gametes

  • Meiosis creates genetic variation between the gametes produced by an individual through crossing over and independent assortment
  • This means each gamete carries substantially different alleles
  • During fertilization any male gamete can fuse with any female gamete to form a zygote
  • This random fusion of gametes at fertilization creates genetic variation between zygotes as each will have a unique combination of alleles
  • There is an almost zero chance of individual organisms resulting from successive sexual reproduction being genetically identical

Sources of genetic variation table

Sources of Genetic Variation Table, downloadable AS & A Level Biology revision notes

Exam Tip

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.

Author:

Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.
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