Lethal Alleles

Lethal alleles are alleles that cause an organism to die only when present in a homozygous condition

  • The gene involved is considered an essential gene and the lethal allele may be either dominant or recessive

An example of lethal alleles in humans is achondroplasia, a genetic condition which causes dwarfism

  • The lethal allele is dominant and hence causes the death of the organism when present in the homozygous state (AA)
  • Heterozygotes will possess a significantly shorter stature (dwarfism), while homozygous recessive individuals will be normal size

By causing the death of one genotypic variant, lethal alleles create altered genotypic and phenotypic ratios to the norm

  • Normally a cross of two heterozygotes will generate a 3 : 1 ratio (dominant : recessive)
  • In achondroplasia, two dwarf heterozygotes are expected to generate a 2 : 1 ratio (the homozygous dominant doesn’t survive)

Lethal Alleles – Achondroplasia

lethal alleles