Chapter 8 - Meiosis and Variation

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  • Chapter 8 - Meiosis and Variation
    • Variation
      • Natural selection
        • Stabilising selection is where the environemtn hasn't changed so the same alleles are being selected for
        • Directional selection is where the environment has changed and one of the exrtreme forms of the species is at a selective advantage, so that allele becomes more common in the population
      • Genetic drift
        • Evolutionary change purely due to chance
        • Common on island populations
      • Speciation
        • The formation of a new species
        • Isolating mechanisms
          • Ecological barriers
            • Two species living in the same area at the same time but do not meet (crayfish)
          • Temporal barriers
            • Two species living in the same habitat but are not active at the same time of day or have different breeding seasons (shrubs)
          • Reproductive barriers
            • Unsuccessful breeding, due to any one of of the following mechanisms: different courtship, mechanical problems with mating (e.g. size) gamete incompatibility, zygote inviability or hybrid sterility
        • Species concept
          • Biological species concept requires the species to have reproductive isolation
          • Phylogenetic species concept just requires there to be a clear difference between the two species - but have come from a common ancestor
      • Artificial selection
        • Breeding animals for human's benefit
        • Could be bad for the animal
    • Meiosis
      • The process
        • Meiosis I
          • Halves the no of chromosomes,crossing over also happens in prophase I
        • Meiosis II
          • Similar to mitosis
      • Independent assortment
        • The mixing of genes from the organism's mother and father in diploid cells
        • Total of 8,388,608 different combinations of alleles in a gamete
    • Genes and inheritance
      • Single gene inheritance
        • The inheritance of a single gene (both copies)
        • Whether an individual has CF is determined by this
        • Homozygous vs heterozygous
      • Codominance
        • Blood group
        • Two dominant alleles, both have an effect on the phenotype of the individual
      • Sex linkage
        • Male only diseases (e.g. haemophilia)
        • Recessive alleles more likely to effect mmales (with only one X chromosome) than females
      • Dihybrid inheritance
        • Inheritance of two genes at the same time, but the genes are NOT on the same chromosme
      • Autosomal linkage
        • Inheritance of two genes that ARE on the same chromosome
      • Epistasis
        • One gene overridng another
      • Equations and tests
        • Hardy-Weinberg equation
        • Chi (x2) test
    • Genotype = the alleles an organism inherited, phenotype= the physical characteristics of the organism (e.g. coat colour)

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