Lecture 8 - Pedigrees and Allele Variation

• What about traits when there are more than 2 alleles?

  Here there are a number of different types of eye colour.

  • Red eyes
  • Brown eyes
  • Scarlet eyes
  • White eyes

  So now we have more than two alleles. Not so simple.
• • Genes may (and usually) have more than 2 alleles
  • Different alleles may affect the phenotype in different ways
  • Additional factors at a single locus can affect the phenotype
• The most common allele is considered the wild type.

  • designated with a superscript +
• All other alleles are considered as mutants.
• Any allele found at appreciable frequencies (at least 1%) in the population is considered to be a polymorphism
• • Different mutations in a gene can cause the same disorder
  • Different mutations in a gene can cause different phenotypes
• Recessive mutations almost always involve a mutation is a gene that results in at least some loss of protein function
• Mutations can be of different types:

  1. Complete loss of function (null allele)
  2. Partial loss of function (hypomorphic allele)
• Different Types of Dominance:

  • Each version of a gene at a particular locus is defined as an allele
  • Not necessarily as simple as dominant vs recessive
  • 3 different types of dominance include: Complete, incomplete and codominance
• Complete dominance:

  • Phenotype is wild type (genotype is aa recessive)
  • can gain function - something novel happens
  • can lose function - loss of structure or protein function
• Incomplete Dominance:

  • Heterozygous phenotype can be differentiated from those of the two homozygotes and falls in between them
• Codominance:

  • Phenotype is not intermediate between the two homozygotes
  • Phenotype simultaneously expresses the phenotype of both homozygotes
• What is an example of Complete Dominance?


  The Antennapedia mutation is Drosophila
• What is an example of incomplete dominance?

  Snapdragons, where a red flower and a white flower produce different shades of pink offspring.
• What is an example of codominance?


  A black and white chicken produce checkered offspring.
• Differences between the different types of dominance:

  Complete Dominance: The phenotype of the heterozygote is the same as the phenotype of one of the homozygotes

  Incomplete Dominance: The phenotype of the heterozygote is intermediate (falls within the range) between the phenotypes of the two homozygotes.

  Codominance: The phenotype of the heterozygote includes the phenotypes of both homozygotes.
• The ABO different blood types in human is another example of a multi-allelic system and an example of codominance.
• Additional Factors at a single locus can affect the phenotype:

  • Penetrance: the percentage of individuals having a particular genotype that express the expected phenotype. (complete = 100%, incomplete < 100%)
  • Expressivity: the degree to which a character is expressed. Everyone has it, just some more than others.
• Incomplete Penetrance - Polydactyl

  • Individuals do not express a trait even though they have the appropriate genotype.
  • Extra digit
• Variable Expressivity:

  • Not all individuals express a trait at the same level even though they have the same genotype.
  • For example Icabod mutant in zebrafish (missing head)
  • They all have the same genotype but exhibit a different phenotype
• The expression of a genotype may be influenced by maternal age

  • the phenotype of the mutant offspring is less severe in older female parents.
• The expression of a genotype may be influences by environmental effects

  • Temperature sensitive allele: an allele whose product is functional only at a certain temperature
  • A fruit fly can have a mutation in a wing development gene. Low temperatures the protein is non-functional, but is functional in warmer temperatures. This would be a cold-sensitive mutation