AQA Biology Unit 2 Variation

Some revision cards summarised from my notes of this page:

Enjoy! :)

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Distribution on a graph

  • Mode- most frequent/common 
  • Median- middle value 
  • Mean- average value
  • Bell shape is the usual curve on a graph. It shows even distribution of values above and below the mean.
  • Thinner bell shape = smaller deviation value = less variation
  • Standard error is ±1.96
  • The standard error is how much of a sample mean is likely to vary
  • Greater standard error = greater variation of mean
  • Standard deviation is a measure of a spread of results about the mean of a normal distribution curve
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Causes of Variation (1)

Independent assortment of bivalents at the equator duting Anaphase 1

  • Chromosomes at bivalents pulled to opposite poles randomly
  • 2x no.of chromosomes = number of different combinations of chromosomes in 4 haploid cells produced

Crossing over between non-sister chromatids during prophase 1

  • (tips of two chromatids swap)
  • Homologous chromatids (equivalent pieces of genetic material) break and exchange opposite segments between maternal and paternal chromatids
  • This results in a new combination of genes from the parents


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Causes of variation (2)

Fertilisation or random fusion of gametes

  • Genetic difference within zygote
  • New combinations between alleles

Gene mutation and environmental factors

  • Addition- at least one base added to DNA during replication
  • Deletion- at least one base left uncopied (causes frameshift)
  • Substitution- at least one base is copied wrongly
  • Unusual base pairing- not A-T or C-G
  • Degenerate code- different triplets code for same amino acids
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thank you so much really explained a lot

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