Genetic diagrams

• Alleles

  Different versions of the same gene
• Most of the time you have two copies of each gene (i.e. two alleles)
• Alleles are different


  Instructions for two different versions of a characteristic
• Alleles are different

  Show one version of the two
• Dominant allele


  The version of the characteristic that appears
• Recessive allele


  The other allele that is said to be recessive
• Characteristic caused by the recessive allele


  Only appears if both alleles are recessive
• Genetic diagrams

  Letters are used to represent genes
• Dominant alleles


  Shown with a capital letter
• Recessive alleles


  Shown with a small letter
• Homozugous for a trait

  Two alleles the same for that particular gene
• Heterozygous for a trait

  Two different alleles for that particular gene
• Genotype


  The genes and alleles an organism has
• Phenotype


  The characteristics that an organism displays
• Some characteristics are controlled by a single gene, e.g. blood group - this is called simple gene inheritance
• Genetic diagrams help to predict the phenotype of the offspring when you know the genotype of the parents
• Genotype


  The genetic makeup of an individual
• Phenotype

  The physical characteristics of an individual
• Simple gene inheritance

  A single gene controls the characteristic
• Allele


  A variant of a gene
• Homozygous


  An individual has two identical alleles for a particular gene
• Heterozygous


  An individual has two different alleles for a particular gene
• Dominant allele


  An allele that is expressed in the phenotype when present in either a homozygous or heterozygous state
• Recessive allele


  An allele that is only expressed in the phenotype when present in the homozygous state
• Superpowered hamster

  A hamster with superpowers
• Boring hamster

  A hamster without superpowers
• bb

  Homozygous recessive genotype
• BB

  Homozygous dominant genotype
• Bb

  Heterozygous genotype
• A superpowered hamster must have the genotype bb (i.e. it must be homozygous for this trait)
• A boring hamster could have two possible genotypes - BB (homozygous) or Bb (heterozygous), because the dominant allele (B) overrules the recessive one (b)
• If you breed from two heterozygous hamsters, there's a 75% chance of having a boring hamster, and a 25% chance of a superpowered one. To put that another way... you'd expect a 3:1 ratio of boring: superpowered hamsters
• If you breed two homozygous hamsters, there's only one possible offspring you can end up with. E.g. breeding BB and bb hamsters can only give offspring with a Bb genotype - and they'd all have a boring phenotype