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HBIO4 > Variation
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Variation in the next generation

Discontinuous variation

  • IMG 5-14-2
  • Limited number of distinct phenotypes / categories (eg ABO blood groups)
  • Strong genetic factor controlled by alleles on one gene
  • Frequency histogram has separate bars
  • Unaffected by the environment

Continuous variation

  • Continuous range of values / class intervals (eg human height)
  • Alleles on many genes located on different chromosomes / polygenic inheritance
  • Frequency histogram is a smooth (normal distribution) curve
  • Phenotype is affected by environmental factors
    • Lower skin temperature activates a gene for pigment production
    • Diet affects individual's size and health. Malnourishment results in shorter height
    • Thus, genes + environment → phenotype (continuous variation)

Normal distribution about the mean

  • Mode (most frequent) = median (mid) = mean (average value)
  • Bell-shaped/ even distributions of values above and below mean
  • Standard error (SE)
    • True mean of SE is ±1.96
    • In a number of samples each sample will have its own mean
    • Standard error measures how much the value of a sample mean is likely to vary
    • The greater the standard error, the greater the variation of the mean
  • Standard deviation (σ)
    • Measure of the spread of results about the mean of a normal distribution curve
    • Thinner bell-shape / smaller standard deviation / less variation
    • Same pattern with bigger bell-shape

Where variation comes from

Gene mutation

  • Produces new alleles
  • Addition: at least one base is added during DNA replication
  • Deletion: at least one base is not copied (frameshift)
  • Substitution: at least one base is copied wrongly
  • Interferes with normal base pairing (A-T;C-G)
  • Degenerate code: different triplets can code for same amino acids

Meiosis

  • Causes new combination of alleles
  • Crossing over
    • During prophase I
    • At synapsis, non-sister chromatids of homologous pairs cross over at chiasmata
    • Homologous chromatids (corresponding pieces of genetic material) break and exchange equivalent segments between maternal and paternal chromatids
  • Independent assortment
    • From the equator
      • Homologous chromosomes pulled to opposite poles at random (anaphase I)
      • Chromatids pulled to opposite poles at random (anaphase II)
    • 223 different combinations possible in 4 haploid cells where 23 is the number of chromosomes
  • Fertilization
    • Random fusion of gametes
    • Genetic difference amongst the zygote
    • New combinations of alleles

Twin Studies

  • Phenotype depends on genes and environment
  • Twins are
    • Monozygotic (identical) → one zygote develops into 2 embryos
      • Very high concordance
      • Have identical DNA but phenotype is not exactly the same
      • Different environment in womb and in later life
      • Causes different genes to switch on and off
    • Dizygotic (non-identical) → two zygotes
      • High concordance as they are related (many genes similar)
  • Concordance and discordance
    • Helps to find out whether a disease has a genetic cause
    • Useful to compare identical and non-identical twins
    • High concordance
      • Identical twins only → not influenced by environment (eg blood group)
      • Identical and non-identical twins → environmental factor likely (eg virus)
    • Low concordance
      • Requires strong environmental factor
    • Best results with
      • Identical twins living together → same environment
      • Non-identical twins with same sex → sex affects height and body mass

Epigenetic Imprinting

  • Expression of certain genes by one allele depending on which parent it comes from
  • DNA methylation
    • Occurs during gametogenesis
    • Methyl groups are added to cytosine bases of certain genes (imprinting)
    • Modifies DNA structure rather than base sequence (epigenetic)
    • Methyl groups inhibit transcription (genes are switched off)
  • Methylation is reversible
    • Old imprints are removed and re-formed during gametogenesis
    • New sperm → imprinted gene is re-tagged as paternal
    • New oocyte → imprinted gene is re-tagged as maternal
    • Therefore, father passes on chromosome imprinted as "paternal" → his daughter passes on same chromosome imprinted as "maternal"
  • There are <1% of imprinted genes on autosomal chromosomes

Prader-Willi Syndrome (PWS)

  • Characteristics
    • Obesity
    • Short stature
    • Learning difficulties
    • Small testes
  • Caused by the deletion of imprinted genes on chromosome 15
    • Imprinting prevents expression of maternal genes
    • Maternal allele for the gene is methylated
    • This inhibits transcription of the defective gene and PWS cannot develop
    • Therefore, defective gene only causes PWS when inherited from the father