Lethal Genes

A lethal allele is one capable of causing the death of the organism that carries it. E. Baur reported the first such case in 1907, in the snapdragon (Antirrhinum sp.), where it behaves as a recessive lethal. Snapdragon plants come in three kinds based on chlorophyll content: green plants with normal chlorophyll (genotype CC), pale/yellowish-green 'aurea' or golden plants that are heterozygous for green and white (Cc), and white plants that lack chlorophyll entirely (cc). Crossing two aurea (Cc) plants together gives an F2 with the expected 1:2:1 genotypic and phenotypic ratio - 1 green (CC) : 2 aurea (Cc) : 1 white (cc) - but because white seedlings have no chlorophyll at all, they cannot photosynthesise and fail to survive, so the observed F2 ratio collapses from 1:2:1 down to a 1:2 ratio of green to aurea among the survivors. More generally, a fully dominant or fully recessive lethal allele kills its carrier only when homozygous, so depending on which allele is lethal, the surviving F2 genotypic ratio is modified to either 2 …