theory

• gene

  short length of DNA found on a chromosome that codes for a particular characteristic/protein
• allele


  variations of the same gene
• phenotype


  the observable characteristics of an organism
• genotype

  the combination of alleles that control each characteristic
• dominant allele

  only needs to be inherited from one parent for the characteristic to show up in the phenotype.

  • represented with a capital letter
• recessive allele

  needs to be inherited from both parents for the characteristic to show up in the phenotype
• what happens if there is only 1 recessive allele?

  it will remain hidden and the dominant characteristic will show
• homozygous dominant

  having 2 copies of the dominant allele
• homozygous recessive

  having 2 copies of the recessive allele
• heterozygous

  having 2 different alleles of a gene
• gamete

  sex cells
• what are the sex cells in animals?

  sperm and ovum
• what are the sex cells in plants?

  ovum and pollen
• chromosome

  thread-like structures of DNA, carrying genetic information in the form of genes.

  • located in nucleus
• the entire set of the genetic material of an organism is known as its genome
• what is variation?

  differences between individuals of the same species
• what is phenotypic variation?

  difference in features between individuals of the same species
• what can phenotypic variation result from?

  the genome - genetic variation

  • e.g. blood group, eye colour

  interactions between organism and its environment - environmental variation

  • e.g. language, accents, obesity
• what is continuous variation?

  when there are very many small degrees of difference for a particular characteristic between individuals.

  • the data are arranged in order
  • can be measured on a scale
  • e.g. height, mass
• features of continuous variation

  • no distinct classes/categories exist`
• discontinuous variation

  when there are distinct differences for a characteristic

  • data fits into discrete categories with no crossover between categories
• what is discontinuous variation usually caused by?

  genetic variation alone

  • e.g. people are either blood group A, B, AB or O
  • there are no 'in-betweens'
• features of discontinuous variation

  • distinct classes/categories exist
  • characteristics cannot be measured over a range
  • individuals cannot have features that fall between categories
• why does variation occur?

  as a result of different alleles in the population

  • these allele variants are introduced into the gene pool as a result of mutations
• how can mutation impact the genome?


  1. some make no difference to the resulting protein
  2. others make a difference to the protein but have no effect on the protein function
  3. in some cases, mutations alter the protein significantly enough to impact the function of the protein

  • only this 3rd scenario would have an effect on the phenotype of the organism
• what does coding DNA refer to?

  the genes of an organism which provide the instructions required to synthesize proteins

  • only a small proportion of the genes in the genome may be expressed at any time
  • this is because genes must be activated if they are to be expressed
• what does non-coding DNA refer to?

  non-coding DNA does not code for proteins

  • however, it can influence protein synthesis through interactions with the coding regions of DNA.
  • therefore, non-coding DNA can cause activation or deactivation of genes in the coding DNA.
• what can non-coding DNA do?

  activate or deactivate of genes in the coding DNA
• how do variants that directly impact gene expression work?

  they alter the DNA base sequence within the coding DNA regions of the genome

  • the mutation significantly alters the structure of the protein produced
  • this results in a non-functional protein
• how do variants that indirectly impact gene expression work?

  they alter the DNA base sequence within the non-coding DNA regions of the genome
• what are the 2 effects of variants that indirectly impact gene expression on coding DNA?

  1. it may activate genes in the coding regions

  • resulting in the transcription of a gene which wasnt previously expressed
  • a protein is produced

  2. it may inactivate genes in the coding regions

  • halts transcription of a gene which was previously being expressed
  • a protein isn't produced
• what is sexual reproduction?

  a process involving the fusion of the nuclei of 2 gametes to form a zygote and the production of offspring that are genetically different from each other
• zygote

  fertilised egg cell
• what are the gametes of animals?

  sperm cell and egg cells
• what are the gametes of flowering plants?

  pollen cells and egg cells
• advantages of sexual reproduction?

  • increases genetic variation
  • the species can adapt to new environments due to variation, giving them a survival advantage
  • disease is less likely to affect population due to variation
• disadvantages of sexual reproduction?

  • takes time and energy to find mates
  • difficult for isolated members of the species to reproduce
• haploid nucleus

  have half the number of chromosomes found in other body cells
• does a gamete cell have a haploid nucleus or a diploid nucleus?

  haploid
• in a human, how many chromosomes does a normal body cell have?

  46