key questions

• what is a gene mutation


  a change in the base sequence of DNA
• when do gene mutations occur

  during DNA replication
• what types of gene mutation are there

  base deletion, base addition, base substitution
• what is a substitution mutation also knows as

  point mutation because the substitution happens at a single point of the DNA
• what does it mean by genetic code is degenerate

  more than one triplet codes for the same amino acid
• do all substitution mutations alter the protein being coded

  no because the genetic code is degenerate so the amino acid could be coded by the triplet containing the mutation
• what level of protein structure will be affected if a mutation does have an effect

  primary structure - the sequence of amino acid in the polypeptide could be altered
• what is a mutagenic agent

  anything that increases the rate of gene mutation above the normal level
• give some examples of mutagenic agents

  radiation (UV, Gamma, X-rays, alpha and beta particles etc), heavy metals (mercury, lead) and some chemicals
• what is a chromosome mutation 


  a change in the number of chromosomes that should be present in a cell nucleus
• when do chromosome mutations occur

  during meiosis
• what does chromosome non-disjunction mean

  either two chromosomes or two sister chromatids do not split from each other (when they should do) during meiosis
• state examples of inherited disorders that arise from chromosome non disjunction

  downs syndrome (trisomy 21), Edwards syndrome (trisomy 18), and pat syndrome (trisomy 13)
• what does the term diploid mean

  a cell that contains two complete sets of chromosomes, one from each parent (2n)
• what does the term haploid mean


  a cell that contains only one complete set of chromosomes from one parent (n)
• what does n refer to when discussing haploid and diploid cells

  the number of pairs of homologous chromosomes
• what types of cells are haploid

  gametes are always haploid cells - when they fertilise the resulting cell is diploid
• describe daughter cells that arise from meiosis

  genetically different from one and other and haploid
• how many daughter cells arise from a cell undergoing meiosis

  4 haploid daughter cells from each diploid parent cells
• why is it important that meiosis is able to generate haploid cells

  haploid gametes which can then be fertilised by another gamete maintains the chromosome number across generations
• how many times does the nucleus usually divide during meiosis

  twice
• what processes cause the haploid gametes to be genetically different

  during meiosis, crossing over and independent segregation cause daughter cells to be genetically diffferent
• how does independent segregation cause genetic variation

  recombines maternal and paternal chromosomes when they align at the metaphase plate
• how does crossing over cause genetic variation

  recombines maternal and paternal alleles when the chromosomes align at the metaphase plate
• besides processes in meiosis how else is genetic variation increased


  fertilisation is random as any combination of egg and sperm can potentially fertilise and become a zygote
• what is a population

  the number of individuals in the same speceis in a particular habitat
• what does diversity mean

  the number of different individuals
• what does genetic diversity mean

  the number of different alleles of genes in a population
• what is the theory of natural selection 


  speceis change over time die to a combination of their alleles and the pressures from their environment
• what does natural selection result in

  speceis that have become better adapted to their environment
• how does random mutation affect natural selection


  new alleles form as a result of mutations and these new alleles may be advantageous, disadvantageous or have no effect
• what happens if an all is advantageous

  individuals with an advantageous allele will have better survival chances and are more likely to breed and produce offspring with the same allele. this is called reproductive success
• what happens if an allele is passed on due to reproductive success

  more offspring will have the advantageous allele and survive to reproduce successfully themselves, leading to the allele frequency increasing over many generations
• what do we mean by the phrase 'allele frequency'


  the number of individuals in a population that have a particular allele. it is usually represented as a decimal so the whole population then adds up to 1
• what does a normal distribution curve look like

  bell shaped curve
• what is directional selection

  where natural selection occurs to push a characteristic away from normal distribution resulting in a skewed curve
• give an example of directional selection

  antibiotic resistance in bacteria as an allele for resistance is advantageous so more and more bacteria will inherit it when exposed to the antibiotic
• what is stabilising selection

  where the natural selection occurs to keep a characteristic normal distributed so the curve remains bell shaped for many generations
• give an example of stabilising selection

  human birth weight, as extremely low and high birth weight babies historically have the lowest survival chances so average birthweight has remained the same for many generations
• what type of advantageous adaptation could be inherited through natural selection

  the advantages could be anatomical (birthweight), physiological (antibiotic resistance) or behavioural (nocturnal animals less likely to be predated)