Alleles (DP IB Biology: HL)

• A gene codes for a specific polypeptide that can affect a specific trait or characteristic in an organism
  • Eg. blood type

• Alternative forms of a gene can exist, these various specific forms are called alleles
  • Note that although alleles are different forms of the same gene, they all still occupy the same locus on the chromosome
  • New alleles occur through mutations

• Multiple alleles can exist for a gene that determines a specific trait
  • Each allele results in a different variation of that trait
    • Eg. blood types A, B, AB and O

Chromosomes showing genes, loci and alleles

• Alleles differ from each other by one or only a few bases
• Even a very small change in base sequence can bring about a large effect in gene function, with a large knock-on effect on the phenotype
• Even though different alleles of a gene have slightly different base sequences, they still occupy the same locus on the chromosome
• Since the Human Genome Project, sophisticated techniques can analyse different alleles
• These techniques are becoming faster, more accurate and more accessible to individuals
  • Comparable sequences can be analysed down to individual bases to determine evolutionary relationships
  • The more differences in base sequence, the further apart two species are in evolutionary terms
• The exact positions where bases differ between alleles are called SNPs or snips (Single Nucleotide Polymorphisms)
  • An allele can have several snips but still only differ by a few bases from its other allele